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What visual information is used by riders in jumping?
Human Movement North-Holland
Science 8 (1989) 481-501
WHAT VISUAL INFORMATION IN JUMPING? * Michel LAURENT UniversitC &Am-Marserlle
Hubert Lahorutoire
and RCmi DINH
481
IS USED
BY RIDERS
PHUNG
II, Frunce
RIPOLL de Neurosciences
du Sport, I.N.S. E.P., Pans. France
Laurent, M., R. Dinh Phung and H. Ripoll, 1989. What visual information is used by riders in jumping? Human Movement Science 8, 481-501.
The aim of the present study was to assess the use of visual information by horse-riders approaching an obstacle to be jumped. Two experiments are described: the first involved decreasing peripheral visual information during the obstacle approach phase, and the second consisted of analysing the visual exploratory strategies observed in subjects performing this part of the jumping sequence, in which vision plays a leading role. It was attempted to elucidate two ma.in questions: does a horseman’s assessment of his distance from an obstacle depend on his visual perception of the speed? And what visual exploratory strategies and more generally. what oculo-cephalic coordination did the subjects use and did these strategies vary depending on the characteristics of the obstacle? The results show that peripheral visual information plays only a minor role in a rider’s control of his horse’s locomotion, whereas temporal factors such as the horse’s gait and the time-to-contact (Tc) with the obstacle were found to play an important, and possibly decisive role in this kind of activity involving severe spatio-temporal constraints. The visual exploratory strategies used were not found to differ significantly with changes in the obstacle characteristics. The riders consistently focused their gaze centrally towards the top of the obstacle and kept their heads in a fixed position. These patterns are compatible with the possibility that the rider’s visual information processing may have been based on the retinal expansion rate of the obstacle, from which they may have deduced the Tc. * The authors wish to thank the Director of the Ecole Nationale d’Equitation (Saumur, France) for kindly providing expert riders, staff and horses. Their competence and helpful collaboration contributed greatly to the efficiency of the experiment. Requests for reprints should be sent to M. Laurent, Centre de Recherche de L’UFR STAPS, Universite d’Aix-Marseille II, Case Postale 910, 13288 Marseille Cedex 9, France.
0167.9457/89/$3.50
0 1989, Elsevier Science Publishers
B.V. (North-Holland)
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ofgait in jumping
The purpose of the present study was to determine the role played by the visual information available to a horseback rider as he approaches an obstacle to be jumped. Two experiments are described. The first involved a restriction of peripheral vision during the obstacle approach phase and the second was designed to analyse the visual exploration strategies used during this stage in the jumping sequence, where vision is crucial. The results are discussed with a view to identifying the processes underlying distance control in locomotor space. Vision plays a primary role in all sports. Numerous studies have shown this to be true, in the case of ball catching (Whiting 1969; Whiting and Sharp 1974; Fishman and Schneider 1985) and in visual exploration strategies (Bard and Fleury 1981; Rip011 et al. 1986, 1988a). Unlike situations where the subject remains relatively stationary, tasks requiring the displacement of one’s own body, which give rise to dynamic visual stimulation or an optic flow field (Gibson 1979; Lee 1976), have not been sufficiently investigated so far. In studies taking the ecological approach to visual perception (Gibson 1966, 1979), a few researchers such as Lee (1976) and Lee and Young (1985) have focused on this problem. In all tasks requiring a high degree of spatio-temporal coordination, crucial information can be obtained by the subject by estimating time-to-contact (Tc) with a target-object (see in particular Lee 1976). Tc is the time it will take an observer moving at a constant speed to reach a specific point on his path (or the time taken by a moving object to reach the observer). In fact, it has emerged that Tc is not determined by calculation, but by direct perception of the radial flow on the retina. The tau (7) ratio or the optic variable 7, which is the inverse of the retinal expansion of any two points of the target over time, defines Tc with any displacement occurring at a constant speed (Lee 1976). On the basis of this model, McLeod and Ross (1983) have proposed the following formula:
where 0, and e2 are the angular separation between any two target image points on the retina at times t, and t, respectively. Thus (8, - 0,) represents the apparent target enlargement during the observation time (t, - t,). The optic variable 7 has turned out to be a
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useful means of investigating the timing of an action, especially in activities involving heavy spatio-temporal constraints. Various studies based on the ‘ecological approach’ have shown that in tasks such as stride regulation in long jumping (Lee et al. 1982), intersegmentary coordination in ball hitting (Lee et al. 1983), and in other activities requiring precise action timing (Solomon et al. 1984; Lee and Young 1985), the results obtained are compatible with the use of the optic variable r by the subjects. This method of determining Tc is not the only one, however, and in theory, Tc can be obtained from the distance/speed ratio. McLeod and Ross (1983) called these two methods the ‘optic flow method’ and the ‘cognitive method’, respectively. To test this alternative, we chose to decrease the number of available perceptual cues indicating speed by limiting the riders’ visual field while they performed the task. Previous psycho-physiological data on movement and posture control (see Paillard (1982) and Paillard and Amblard (1985) for example) have shown how important the peripheral zones of the retina are to kinetic vision, which is tuned for velocity coding and direction selectivity. Static vision is the second mode of spatial relationship transcoding. The foveate and para-foveate regions of the retina (up to 10 or 15”) participate in processing the stable properties of the environment. This involves locating the relevant stimuli and detecting their shape and pattern and/or their contours. These two visual sub-systems can be said to be complementary so that speed information, for example, can be either directly encoded by the kinetic channel or indirectly deduced from the amplitude of the changes of position with respect to time. Studies on vection (see for instance Berthoz et al. 1975) have stressed how important the peripheral field can be for assessing the movement and speed of one’s own body. Salvatore’s research (1968) on the estimation of driving speed on a circuit led this author to a similar conclusion, that visual estimation of speed is better in the peripheral visual field (between 155” and 180”) than in the central visual field (25 o ) (see also Denton 1980). In the field of sports, few data are available on this topic. Graybiel et al. (1955), however, have reported that restricting the use of the peripheral visual field in various sports (javelin throwing, skating, etc.) leads to disturbances in movement control. This loss of information has an even more detrimental effect on performance than occlusion of the central visual field.
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Our study deals with jumping in horseback riding. This situation was chosen for three main reasons. First of all, because of the real problems riders are confronted with as they approach an obstacle: one of the difficulties encountered even by expert riders is that of regulating the horse’s stride so that its hindlegs will land at a given spot, i.e. the take-off point. The position of the last push-off, which is not spatially predefined by a board as in long jumping, is determined here by the height and shape of the obstacle. With each type of obstacle, there exists one ideal ‘frame’ that ensures an efficient and sure jump. With these constraints, a visual stride regulation in relation to the obstacle is required. We propose here to analyse the contribution of vision to gait regulation. The second reason for choosing horseback riding was that in this locomotor situation, the rider is passively transported. Unlike long-jumpers, who themselves produce the energy required to run and to jump and who therefore receive a great deal of proprioceptive feedback information about the ongoing movement (active locomotion), horse-riders are carried by the horse and therefore do not receive the same kind of information (passive locomotion). The forces exerted upon the ground seem to provide a direct means of controlling locomotion (see in particular Warren et al. 1986). A rider can use other available sources of information to control the locomotion of his mount. First there is proprioceptive information relating to the horse’ pace, which can be conveyed to the rider from the leg proprioceptors. When galloping, which is a leaping type of gait, the rider has to counteract the horse’s vertical thrusts by flexing and extending the ankle, knee and hip joints. Other kinds of information about the horse’s state of balance are provided by various contacts and pressures which are exchanged between horse and rider, thus constituting a kind of ‘kinesthetic dialogue’ between the two partners. Lastly, in this kind of ‘carried subject’ situation, exproprioceptive cues, according to Lee’s definition of the term, are likely to play a particularly privileged role; these cues have been defined as ‘information about the position, orientation and movement of the body as a whole relative to the environment’ (Lee 1978). Within this context, we feel that it may be possible for a rider to assess the speed of his horse’s locomotion by means of exproprioceptive visual cues, and to use this information to control the movements of the latter in such a way as to fulfill the spatial and temporal requirements of the jumping task. Experimental variations in the
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amount of visual information available can therefore be expected to have a significant effect on the subjects’ performance levels. The last point of interest to us here concerns the rider-to-horse interaction. In horseback riding as a sport, expert riders ‘pilot’ the horse like a vehicle. They modulate the various gait parameters of their mount depending on the required effect. In dressage, ‘prancing’ (or trotting on the spot) is a good illustration of this. We can be sure in this case that any changes in the horse’s stride are in fact the result of actions on the rider’s part. This is why the variations in the horse’s locomotion parameters observed in this study can be said to be attributable to the control exerted by the rider.
Experiment
1
Purpose The first experiment was designed to determine the role played by the visual information available to a rider as he controls the horse’s gait in the obstacle-approach zone. We hypothesized that if visual information about speed is used by the rider, then stride regulation will be affected by limiting such information. Taking this further, this would mean that the rider uses a method based on the distance/speed ratio to determine Tc. If this is not the case, then a more ‘direct’ processing method, based on the optic variable r, may be used. Method Subjects Five 23- to 35-year-old male subjects took part in the experiment. They all had between 10 and 16 years of experience in horseback riding, and were competing at national level. Five horses were used, four of which were 9 years old; the other one was 13. All the horses were trained, and had previously participated in class-A competitions. Experimental apparatus The experiment took place in a covered ring belonging to the Ecole Nationale d’Equitation (National Riding School) in Saumur, France. Each rider was asked to jump several times over an upright obstacle of
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varying height after approaching at a gallop along a rectilinear path. The gallop was preceded by a preparation phase on a curved path (walk-gallop pace transition). The rectilinear phase was begun after going through a 5 m wide gate located 25 m from the obstacle (fig. 1). Performance was recorded in terms of the spatial and temporal parameters of the horse’s stride. The values of spatial parameters were obtained by measuring the horse’s footprints on the track. The time interval between each left foreleg contact with the ground was recorded; since the horses always galloped with their left foot in the lead, all their strides could be measured. For horses, galloping is a natural gait, involving leaping, rocking, diagonal three-time movements followed by a flight time. When the horse gallops with the left foot in the lead, the three parts of the movements are as follows: (a) stance on the right posterior limb, (b) stance on the opposite diagonal pair (right anterior and left posterior limbs simultaneously), (c) stance on the anterior left limb. In our experimental situation, the horse had to turn left in order to reach the run-up. In horse riding it is more natural for the left foot to be in the lead when the horse turns to the left, and vice-versa. To simplify the data recording, we therefore asked the riders to gallop with the left foot in the lead. After each trial, the track was harrowed and rolled so that all jumps would take place under the same conditions, and above all, so that each footprint would be perfectly visible on the track. The accuracy of the measurements made was to within 3 cm. The temporal values were deduced from frame-by-frame analysis on a videotape fitted with a 0.04 set timer. The camera was placed at a 90” angle to the left of the track. The accuracy here was to within kO.04 sec. Procedure Each subject was tested at one session. The riders were not aware of the purpose of the experiment. After a warm-up period, glasses that limited the visual field were fitted to each rider individually. The riders were instructed to jump the obstacle as if they were on a normal course. They were thus supposed to make an efficient and sure jump, and to ride at the same pace as that used in competitions (350 m/mm). Independent
variables
The three experimental conditions were as follows: Obstacle height. Hl = 1.30 m, H2 = 1.40 m, H3 = 1.50 m. The obstacle height was
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varied for two reasons: (1) to avoid excessive standardization of the task, and (2) to evaluate any modifications in approach speed as a function of this parameter. The height most commonly used for upright obstacles in national competitions is 1.50 m. Although the obstacle heights were not nearly the maximum height which these horses were capable of jumping, which is around 2 m, they clearly required stride length adjustment to be made before the jump. The subjects were told before each trial how high the obstacle would be.
with either normal vision (NV) or Vision. Jumps were performed restricted vision (RV): With RV, the rider’s visual field was reduced by opaque glasses that had a 2 mm opening in the center, allowing him a visual field of only 15 O. This angle enabled the rider to see the entire obstacle when 23 m away from it. Six jumping situation were thus obtained. Each subject was tested 3 times in each situation, making a total of 18 trials per subject. The trials were performed in random order across subjects. Variables studied The spatio-temporal defined as follows:
~ _
_ _
parameters
analysed
with
each
stride,
were
Stride duration was the time separating two consecutive left-foreleg strikes on the ground. Stride length was the distance between two consecutive left foreleg footprints. Date was the sum of the stride durations going back from time T,, where T, was the moment when the left foreleg left the ground at the take off. Distance was the sum of the stride lengths going back from position DO where DO was the obstacle position. Time-to-contact (Tc) was the theoretical time it took the rider + horse to reach the obstacle from a reference point situated x strides from this obstacle. Tc was calculated at each stride by the ratio D/S where D was the distance to the obstacle and S the speed of displacement at the previous stride.
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Results General characteristics of horse locomotion in the obstacle-approach zone Stride F-l was the stride onto the take-off place, F-2 was the previous stride, etc. The characteristics of the run-up between F-l and F-7 were studied. All subjects and conditions combined, the mean oistance covered by these last 7 strides was 22.05 m (standard deviation (s.d) = 1.12 m) and the duration was 3.72 set (sd= 0.25 set). The mean distance between the take-off and the vertical plane of the obstacle bars, called beat distance, was 2.10 m (sd = 0.14). The analysis of variance did not show either the rider’s vision or the obstacle height to significantly influence the distance of the push-off place from the obstacle: we obtained F, 4 = 0.12, n.s. and F2,* = 1.33, n.s. for vision and obstacle height, respectively. The mean stride length was 3.12 m, and the mean stride duration was 0.55 sec. The mean displacement speed was 5.50 m set-‘, which is approximately equal to the normal competition pace. The jumps were successful in all the situations (obstacle height x vision). The effects of the experimental conditions will thus be studied by examining the patterns of the various spatiotemporal parameters as a function of the stride number in the jumping sequence. The following question arises here: at what point in the sequence do modifications occur in these spatio-temporal patterns, and more precisely, are these changes ascribable to ,adjustments based on visual cues?
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/15m
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Fig. 1. Diagram
Timer
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apparatus.
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-7
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-5 STRIDE
-4
-3
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-1
NUMBER
Fig. 2. Means and standard deviations of stride duration (A) in order of rank in the stride sequence, and intra-subject stride variations (B). NV and RV represent the normal and restricted (15 o ) visual condition, respectively. The stride labelled - 1 was the last one before the jump.
Stride duration and length An initial three-way analysis of variance (ANOVA 3R2) with vision, obstacle height and stride number as factors was conducted on the absolute variations in stride duration and length. Only the stride number was found to have a significant effect on the stride duration ( F6,28= 6.01, p -c 0.001). The vision and obstacle height were not significant factors, nor were the various interactions. Similar results were obtained with the stride length, since only the stride number was found to be a significant factor ( F6,28= 6.85, p < 0.001). The obstacle height was thus eliminated in subsequent analyses, as was F-l, which is a jump preparation stride (see figs. 2a and 3a).
The stride number did not significantly affect the (a) Stride duration. duration under either of the visual conditions (NV: F5,24=0.8,,8; RV: F5,24 = 0.990). No significant effect was observed on intra-subject variations (mean of the standard deviations) for NV ( F5,24= 2.54) but a significant effect was observed for RV ( F5,24= 5.99; p < 0.001) (see fig. 2).
(6) Stride length. The stride number was found to have a significant effect. Under NV, we obtained F, 24 = 5.24, p < 0.001, and under RV,
M. Laurent er al. / Reguhon
490
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of gait in jumping
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of stride lengths (A) and their intra-subject
variations
(B).
F5,24= 6.24, p < 0.001. In intra-subject variations, the stride number also had a significant effect under both NV ( F5,24= 7,19, p < 0.001) = 2.95, p < 0.05) (fig. 3). Paired comparisons-between and RV (& strides (F-7/F-6, F-6/F-5, etc.) using the paired t-test showed the existence of significant variations under condition NV starting with F-4. In the comparison F-4/F-3 we obtained t = 4.91, p -C0.01. These results indicate that changes in locomotion occurred in the obstacle approach phase. The following effects were observed: although the duration remained relatively stable, the stride length increased gradually up to F-l, in preparation for the jump. The intra-subject variation increases were significant only in the case of the length parameter. This increase in variation as the obstacle is approached can be attributed either to difficulty in preparing for the jump or to the effect of visual regulation with respect to the obstacle, or both. To elucidate this point, variations in the time and distance at each stride were analysed. Date and distance Absolute variations in date and distance were not studied since their effects are obvious. It seemed more relevant to examine the consistency
M. Laurent et al. / Regulation ofgait in jumping
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deviations
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-3
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-1
and date with each stride number.
with which subjects adopted a given pattern over a number of runs. The standard deviation was therefore chosen as a measure of performance. We again analysed the results by analysis of variance looking for any significant effects of stride number in the sequence. With the distance parameter, the stride number was found to have a significant effect both in situation NV ( F5,24= 6.55, p < 0.001) and in situation RV (C&24 = 3.39, p < 0.025). With the date parameter, the stride number was significant neither in NV ( F5,24= 0.608, n.s.) nor in RV ( F5,24= 0.599, n.s.). Fig. 4 shows the pattern of these two parameters for each stride in the jumping sequence. Note the decrease in the standard deviation of the distance starting at F-4. The decrease in date at that point was not significant. In horseback riding, the rider enters the obstacle-approach phase somewhat haphazardly. He cannot train to develop a consistent run-up from a measured mark to the board as a long jumper does. This particularity of horseback riding leads us to interpret the decrease in the standard deviation of the distance parameter as resulting from the rider’s regulation of the horse’s stride length. This regulation appears to be of a visual nature, and is conducted in relation to the obstacle. Time-to-contact with the obstacle (Tc) As with distance and date, the absolute values of Tc have little meaning. On the other hand, if we consider the Tc spread at each
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STRIDE
Fig. 5. Mean of the rank standard
of gcril
inJumping
NUMBER
deviations in the time-to-contact rank in the stride sequence.
of the obstacle
in order
of
stride, we can see (fig. 5) that the standard deviation of Tc was relatively stable from F-7 to F-4, and then dropped significantly at F-3 ( F,,4 = 2.60, p < 0.001). Neither of the visual conditions had a significant effect on Tc variation. The decrease in the Tc spread as a function of stride number indicates that the closer the subject came to the obstacle, the less variation occurred across trials in the time remaining before ‘contact’ with it. As stated above, this regulation effect started three strides before the obstacle. Discussion
The first important result to emerge from the above analysis is that vision does not significantly affect the regulation of the horse’s stride as the obstacle is approached. One might have expected deprivation of most of the peripheral visual information to severely alter the spatio-temporal characteristics of the horse’s stride. In the situation studied here, impairment of performance was expected to occur at two levels: during the stabilized part of the obstacle-approach phase (where pace control is very important), and in the jump preparation phase, which requires distance assessment in order to place the horse’s hind-
hf. Laurent et ul. / Reguhon
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legs at the proper distance. Yet, neither of these phases was significantly affected. How can this be explained? The stabilized phase was characterized in the NV situation by the setting up of a very stable gait. The absence of, or decrease in visual movement information in RV did not affect the gait, which remained stable (see fig. 2). It is worth recalling here that the horses and riders used in our experiments formed expert partnerships. One of the features of a horse’s expertise is the ability to maintain a very steady pace. The rider’s task is to assist and maintain this steadiness. It is probably due to the fact that a rider uses mainly proprioceptive cues that his ability to control the horse’s pace was not significantly disturbed when his own vision was restricted. In partnerships between expert horses and riders, the various aids whereby the two partners communicate form a kind of language, which in academic riding makes it possible for the rider to maintain perfect control of his mount. The three natural aids with which a rider controls his mount’s locomotion are the legs, the hands acting via the reins, and the body weight on the seat. Artificial aids such as the spurs and riding crop are also usually available if required, although in our experiments this was not the case. Aids are the functional support to the communication between the rider and the horse. The rider sends messages to the horse, which must be able to decode them and to respond appropriately by turning, going faster, lengthening its pace, etc. The horse in turn sends out information about its ongoing locomotion. These signals which are decoded by the rider can be used like indirect feedback in controlling the horse’s gait efficiently and precisely. It seems likely that the pace and the horse’s stride length are particularly susceptible of control via the aids we have just mentioned, which would explain why restricting the rider’s visual field had little effect: the requisite information was being conveyed via the proprioceptive channel (pressure, muscle tension, etc.). In the regulation phase, the results show that visual speed information provided by the peripheral field was not essential to distance encoding, as the performance levels were similar in situations RV and NV. These findings are compatible with the idea that the Tc parameter involved in distance control is obtained directly by mean of the optic variable 7, as suggested by Lee et al. (1982) in the case of long jumping. It is known that this process does not a priori require all the available visual information but only the retinal expansion of the target-object.
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Experiment
M. Laurenr et al. / Regulation
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2
The second experiment can be said to be a small pilot study. Its aim was to identify the visual strategies and the eyeehead coordination involved in jumping in order to complete the discussion on experiment 1. This study breaks new ground because no previous experiments have been reported in similar situations using a gaze video recorder carried by a moving subject approaching an obstacle. From the results obtained in the first experiment, certain assumptions can be drawn about the nature of the visual behavior involved in jumping. There, the absence of effect in the restricted visual condition indicated that the operational visual field (i.e. the non-occluded visual field) was within 15 o and that the useful visual information was necessarily situated within this angle. This agrees with the optic flow hypothesis, which suggests that riders might use the optical variable 7 to obtain time-tocontact information. In order to collect this information, it might seem preferable for the rider to look at the obstacle during the whole approach phase rather than to make sweeping eye movements from the top to the bottom of the obstacle or from the obstacle to the expected take off point. If the rider does fix his gaze on the obstacle, it would be interesting to know which part of it he focuses on. Actually, riding coaches, on the basis of their empirical experience, usually advise riders to adapt their visual strategies to the type of obstacle to be jumped. In order to perform the jump as economically as possible, coaches ask the rider to fix their gaze onto the top of the obstacle in a DROIT for example and towards the bottom in a SPA. These two strategies apparently cause the take off place to be farther from and nearer to the obstacle, respectively. Do skilled riders actually use these strategies? A video oculographic method was used to examine this question. Method
A video-oculographic gaze recorder, (Nat Eye Mark Recorder IV) adapted to sports situations (Rip011 1988b) was used. The eye was illuminated by a point light source. The bright spot reflected by the cornea was video recorded in the shape of a bright ‘V’. This signal was coupled with the visual field image recorded by a camera placed on the subject’s forehead. The final picture observed on the monitor showed the shifts and positions of the ‘V’ in relation to the external world. The
M. Laureni et al. / Regulation of gait in jumping
video image taken from the mounted head axis orientation.
camera served as a measure
495
of the
Subjects Two out of the five riders who participated in the first experiment were selected because of the high competition standards they had achieved. They were of national standard (A class). Task The same procedure was used as in experiment 1. Each rider, equipped with an oculometric device, had to jump three times over each of three types of obstacle (DROIT, SPA and OXER). Recording
technique
Data collection. Each parameter was collected on a transparency film superimposed on the video screen, as described by Rip011 (1988b). Various parameters were analysed, in connection with the following: (1) Gaze direction in relation to the obstacle. The direction of the gaze corresponded to the cornea1 reflection in relation to the obstacle and the gap between the cornea1 reflection and the obstacle was recorded (fig. 6A). (2) Head orientation in relation to the external environment. The relationship between the head and the environment was measured by analysing the head orientation in relation to a fixed point located far away in the background (fig. 6B). (3) Head orientation in relation to the obstacle. The direction of the head corresponded to the centre of the image delivered by the forehead camera and the gap between this virtual point and the obstacle was recorded (fig. 6C). (4) Eye-head coordination. The relationship between the gaze and the head was measured by analysing the gap between the cornea1 reflection and the head direction (fig. 6D). The eye was taken to be immobile in relation to the head when the cornea1 reflection was itself immobile on the image delivered by the forehead camera. The movement of the eye on the video screen showed the movement of the eye in relation to the head. These movements were measured in degrees.
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Samples were collected frame by frame at the rate of one every 20 msec on the transparency support superimposed on the video screen, then analysed. A second camera located sideways on profile simultaneously filmed the exercise. A time basis simultaneously superimposed on the two images (the oculographic and the external images) made it possible to relate the eye and head behaviour to the characteristic parameters of the approach as mentioned in experiment 1. Results
The data collected on two riders with three different types of obstacles (DROIT, SPA and OXER) revealed the absence of any change in the visual scanning patterns whatever the obstacle.
Fig. 6. Relationship between eye, head, and environment during the approach, between t-2 set up to take off: Gaze orientation in relation to the fence (A): Head orientation in relation to the environment (B); Head orientation in relation to the fence (C); Eye-head coordination (D) (see comments in the text). Curves A, B and C are based on the video screen data. They correspond to relative values expressed in arbitrary units. D data are absolute values (in degrees) of the eye-head alignment. Subject’s eye movement up (+) and down (- ) in relation to the head.
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Fig. 6 shows an example of a visual pattern produced by a rider in a DROZT. The following behaviour was described. (1) The gaze was accurately directed towards the upper part of the fence throughout the approach phase (fig. 6A). (2) The head was fixed straight ahead and was displaced onto the antero-posterior axis without any oscillation (lateral or up and down movement). This stability was possible because hip, knee and ankle flexion absorb the swinging of the horse in order to maintain the upper part of the body immobile (fig. 6B). Consequently, head stabilization produced an apparent shift of the fence into the lower part of the rider’s visual field (fig. 6C). The gaze direction then moved progressively further away from the center of the orbit so as to keep a stable retinal picture of the fence during the whole approach (fig. 6D). Discussion Our results, contrary to empirical analyses, show that riders exhibited similar visual behaviour whatever the obstacle profile. Particularly interesting features were the head stabilization with respect to the environment and the fact that the gaze was maintained constantly towards the obstacle despite the horse’s swinging. These results are consistent with those obtained in experiment 1. Indeed, the RV conditions made visual saccade amplitude lower than the possible 15 o visual window. If these saccades had been necessary, they would have involved a rapid orientation of the head onto the visual target axis. Given the head inertia, this improbable manoeuver would have disturbed the rider’s balance, which was not the case. A second remark concerns the calculations of the distance in approaching the obstacle. We assumed that Tc estimation would be directly given by the optic flow field (variable 7). This process is consistent with the idea of a visual anchorage onto the looming target until the last stride occurs. When Tc information is of major importance (which was the case with F-4), visual saccades might greatly disturb the collecting of information necessary to the stride regulation. Furthermore, we noted that eye movement, in relation to the persistent head stability, began at about F-4, that is at the beginning of the stride regulation phase. This might correspond to a Tc value being extracted about 1.5 set before the take-off. Visual anchorage onto the obstacle
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led to a shift of the gaze with respect to the head, which remained consolidated with the trunk, in a fixed position straight ahead. It is difficult to say whether this shift merely resulted from the head being immobilised, or whether the eye-head coordination exhibited by expert riders might play a role in the information processing. If the iatter were the case, this parameter, which is a type of extraretinal information, might play a role in the encoding of the location of the obstacle in relation to the body. Further investigations are necessary to confirm this hypothesis.
General
conclusion
(1) Analysis of the spatio-temporal parameters of the locomotion of horses approaching an obstacle revealed the existence of a so-called stubilized phase during which the gait imposed by the riders was very stable, both within and across trials. Here the rider was probably attempting to control the displacement speed. On F-4, substantial variations in the gait parameters were recorded. Changes in the patterns of various parameters (time, distance, and obstacle Tc) led us to ascribe these effects to visual adjustements made with respect to the obstacle. The stride regulation phase requires careful control of the distance to the obstacle. During this phase, the significant variations in speed observed indicate that control of this distance is based on information about the time-to-contact with the obstacle. The variations in Tc decreased significantly three strides before take-off. Thus Tc might seem to be the factor that is actually controlled by the rider to modify the distance at which he will place the horse for the jump. Lee et al. (1982) have developed this theoretical problem in connection with a similar motor task, the regulation of gait in long jumping by highly skilled athletes. During the zeroing in phase, subjects adjust their stride pattern, varying their flight time by regulating the vertical impulse of their strides. These authors have described the relationships between the optic variable 7 and the duration of flight time (and hence of the stride length). A recent study by Warren et al. (1986) confirms the fact that the vertical impulse is directly modified by the parameter 7. (2) Limitation of the visual information to the 15 o field provided to expert riders did not significantly alter gait control as these riders approached the obstacle. In the preparatory phase, where the speed is
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stabilized, the rider must do all he can to ensure a steady pace. Here he controls the horse without taking the restricted peripheral cues much into account. The most objective explanation for this was that these cues are not indispensible here, since proprioceptive cues can probably be used to provide any missing exteroceptive information. This aspect of movement control in skilled subjects has classically been reported in many studies on motor learning. During the regulation phase, the simple perception of the obstacle appears to be sufficient for extracting this high-order invariant represented by the optic variable 7 (Lee 1976; McLeod and Ross 1983) from which Tc can be directly obtained. Consequently the hypothesis concerning the use of a cognitive method for obtaining Tc in this task is not confirmed by our results. Of course, this does not mean that riders do not use speed information when it is available; it only indicates that this information is not indispensable for skilled riders. This study on horseback riding emphasizes the importance of the role played by temporal factors (not just rhythm, but also Tc) in actions involving severe spatio-temporal constraints. It can thus be said that it is on temporal factors that the control is actually based. The visual exploration strategies used during the obstacle approach phase were not found to be significantly related to the characteristics of the obstacles. The patterns observed indicate that the subject’s gaze focused on the upper part of the obstacle with a fixed head position. These findings are compatible with the processes discussed above, in which the retinal expansion of the size of the obstacle is taken into account in extracting the Tc. When a subject moves towards an obstacle many visual cues are available. To identify precisely the cues actually used by a subject in a particular task is not easy and probably the various control processes work in a complementary fashion (Laurent et al. 1988; Cavallo and Laurent 1988). In order to determine under what conditions various kinds of information are used further experimental studies are necessary.
References Bard, C. and M. Flew-y, 1981. ‘Considering eye movement as a predictor of attainment’. In: I.M. Cockerill and W.W. Mac Gillivary (eds.). Vision and sport. Cheltenham: Stanley Thomes. pp. 28-41.
Berthoz, A., B. Pavard and L.R. Young, 1975. Perception of linear horizontal self motion induced by peripheral vision (linear vection). Experimental Brain Research 43, 471-489. Cavallo, V. and M. Laurent, 1988. Visual information and skill level in time to collision estimation. Perception 17, 623-632. Denton, G.G., 1980. The influence of visual pattern on perceived speed. Perception 9. 393-402. Fishman, M.G. and T. Schneider, 7985. Ski11 level, vision, and proprioception in simple one-hand catching. Journal of Motor Behavior 17. 219-229. Gibson, J.J.. 1966. The senses considered as perceptual systems. Boston, MA: Houghton Mifflin. Gibson, J.J., 1979. The ecological approach to visual perception. Boston. MA: Houghton Mifflin. Graybiel, A., E. Jokl and C. Trapp, 1955. Russian studies of viston in relation to physical activity. Research Quarterly 26, 480-485. Laurent, M., 1981. Problemes poses par I’Ctude du pointage locomoteur d’une cible visuelle. Cahiers de Psychologie Cognitive I, 1733197. Laurent. M., P. Paul and V. Cavallo, 1988. How is gait visually regulated when the head goes faster than the legs? Journal of Motor Behavior 20, 301-316. Lee, D.N., 1976. A theory of visual control of braking based on information about time-to-collision. Perception 5, 4733459. Lee, D.N., 1978. ‘The functions of vision’. In: H. Pick and E. Salzmann (eds.). Modes of perceiving and processing information. Hillsdale, NJ: Erlbaum. Lee, D.N., 1980a. ‘Visuo-motor coordination in space-time’. In: G.E. Stelmach and J. Requin (eds.), Tutorials in motor behavior. Amsterdam: North-Holland. pp. 281-295. Lee, D.N., 1980b. The optic flow field: The foundation of vision. Philosophical Transactions of the Royal Society of London, B 290. 169-179. Lee, D.N. and D.S. Young, 1985. ‘Visual timing of interceptive action’. In: D.J. Ingle. M. Jeannerod and D.N. Lee (eds.), Brain mechanisms and spatial vision. Dordrecht: Martinus Nijhoff. Lee. D.N., J. Lishman and J.A. Thomson, 1982. Regulation of gait in long jumping. Journal of Experimental Psychology: Human Perception and Performance 8, 4488459. Lee, D.N., D.S. Young, P.E. Reddish, S. Lough and T.M.H. Clayton, 1983. Visual timing in hitting an accelerating ball. Quarterly Journal of Experimental Psychology 35 A, 333-340. McLeod. R.W. and H.E. Ross, 1983. Optic-flow and cognitive factors in time-to-collision estimates. Perception 12, 417-423. Paillard, J., 1982. ‘The contribution of peripheral and central vision to visually guided reaching’. In: D.J. Ingle, M.A. Goodale and R. Mansfield (eds.), Advances in the analysis of visual behavior. Cambridge, MA: The MIT Press. Paillard, J. and B. Amblard, 1985. ‘Static versus kinetic visual cues for the processing of spatial relationship’. In: D.J. Ingle. M. Jeannerod and D.N. Lee (eds.), Brain mechanisms and spatial vision. Series D: no. 21. Dordrecht: Martinus Nijhoff. Ripoll. H., 1988a. ‘La resolution du conflit semantique sensori-moteur en sport’. In: H. Ripoll and G. Aremar (eds.), Neurosciences du sport. Traitement des informations, prise de decisions et realisation de l’action en sport. Paris: INSEP. Ripoll, H., 1988b. Utilisation d’un dispositif video-oculographique d’enregistrement de la direction du regard en sport. Science et Motricite 4. 26-31. Ripoll, H., C. Bard and J. Paillard, 1986. Stabilization of head and eyes on target as a factor in successful basket-ball shooting. Human Movement Science 5, 47758. Salvatore. S., 1968. The estimation of angular velocity as a function of visual stimulation. Human Factors 10, 27-32. Solomon, J., C. Carello and M.T. Turvey, 1984. ‘Optical flow: The visual support for skilled activity’. In: W. Straub and J. Williams (eds.), Cognitive sport psychology. Lansing. NY: Sport Science Associates.
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Warren, W.H., D.S. Young and D.N. Lee, 1986. Visual control of step length during running over irregular terrain. Journal of Experimental Psychology: Human Perception and Performance 12. 259-266. Whiting, H.T.A., 1969. Acquiring ball skill: A psychological interpretation. London: G. Bell. Whiting, H.T.A. and R.H. Sharp, 1974. Visual occlusion factors in a discrete ball-catching task. Journal of Motor Behavior 6, 11-16.
Science 8 (1989) 481-501
WHAT VISUAL INFORMATION IN JUMPING? * Michel LAURENT UniversitC &Am-Marserlle
Hubert Lahorutoire
and RCmi DINH
481
IS USED
BY RIDERS
PHUNG
II, Frunce
RIPOLL de Neurosciences
du Sport, I.N.S. E.P., Pans. France
Laurent, M., R. Dinh Phung and H. Ripoll, 1989. What visual information is used by riders in jumping? Human Movement Science 8, 481-501.
The aim of the present study was to assess the use of visual information by horse-riders approaching an obstacle to be jumped. Two experiments are described: the first involved decreasing peripheral visual information during the obstacle approach phase, and the second consisted of analysing the visual exploratory strategies observed in subjects performing this part of the jumping sequence, in which vision plays a leading role. It was attempted to elucidate two ma.in questions: does a horseman’s assessment of his distance from an obstacle depend on his visual perception of the speed? And what visual exploratory strategies and more generally. what oculo-cephalic coordination did the subjects use and did these strategies vary depending on the characteristics of the obstacle? The results show that peripheral visual information plays only a minor role in a rider’s control of his horse’s locomotion, whereas temporal factors such as the horse’s gait and the time-to-contact (Tc) with the obstacle were found to play an important, and possibly decisive role in this kind of activity involving severe spatio-temporal constraints. The visual exploratory strategies used were not found to differ significantly with changes in the obstacle characteristics. The riders consistently focused their gaze centrally towards the top of the obstacle and kept their heads in a fixed position. These patterns are compatible with the possibility that the rider’s visual information processing may have been based on the retinal expansion rate of the obstacle, from which they may have deduced the Tc. * The authors wish to thank the Director of the Ecole Nationale d’Equitation (Saumur, France) for kindly providing expert riders, staff and horses. Their competence and helpful collaboration contributed greatly to the efficiency of the experiment. Requests for reprints should be sent to M. Laurent, Centre de Recherche de L’UFR STAPS, Universite d’Aix-Marseille II, Case Postale 910, 13288 Marseille Cedex 9, France.
0167.9457/89/$3.50
0 1989, Elsevier Science Publishers
B.V. (North-Holland)
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ofgait in jumping
The purpose of the present study was to determine the role played by the visual information available to a horseback rider as he approaches an obstacle to be jumped. Two experiments are described. The first involved a restriction of peripheral vision during the obstacle approach phase and the second was designed to analyse the visual exploration strategies used during this stage in the jumping sequence, where vision is crucial. The results are discussed with a view to identifying the processes underlying distance control in locomotor space. Vision plays a primary role in all sports. Numerous studies have shown this to be true, in the case of ball catching (Whiting 1969; Whiting and Sharp 1974; Fishman and Schneider 1985) and in visual exploration strategies (Bard and Fleury 1981; Rip011 et al. 1986, 1988a). Unlike situations where the subject remains relatively stationary, tasks requiring the displacement of one’s own body, which give rise to dynamic visual stimulation or an optic flow field (Gibson 1979; Lee 1976), have not been sufficiently investigated so far. In studies taking the ecological approach to visual perception (Gibson 1966, 1979), a few researchers such as Lee (1976) and Lee and Young (1985) have focused on this problem. In all tasks requiring a high degree of spatio-temporal coordination, crucial information can be obtained by the subject by estimating time-to-contact (Tc) with a target-object (see in particular Lee 1976). Tc is the time it will take an observer moving at a constant speed to reach a specific point on his path (or the time taken by a moving object to reach the observer). In fact, it has emerged that Tc is not determined by calculation, but by direct perception of the radial flow on the retina. The tau (7) ratio or the optic variable 7, which is the inverse of the retinal expansion of any two points of the target over time, defines Tc with any displacement occurring at a constant speed (Lee 1976). On the basis of this model, McLeod and Ross (1983) have proposed the following formula:
where 0, and e2 are the angular separation between any two target image points on the retina at times t, and t, respectively. Thus (8, - 0,) represents the apparent target enlargement during the observation time (t, - t,). The optic variable 7 has turned out to be a
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useful means of investigating the timing of an action, especially in activities involving heavy spatio-temporal constraints. Various studies based on the ‘ecological approach’ have shown that in tasks such as stride regulation in long jumping (Lee et al. 1982), intersegmentary coordination in ball hitting (Lee et al. 1983), and in other activities requiring precise action timing (Solomon et al. 1984; Lee and Young 1985), the results obtained are compatible with the use of the optic variable r by the subjects. This method of determining Tc is not the only one, however, and in theory, Tc can be obtained from the distance/speed ratio. McLeod and Ross (1983) called these two methods the ‘optic flow method’ and the ‘cognitive method’, respectively. To test this alternative, we chose to decrease the number of available perceptual cues indicating speed by limiting the riders’ visual field while they performed the task. Previous psycho-physiological data on movement and posture control (see Paillard (1982) and Paillard and Amblard (1985) for example) have shown how important the peripheral zones of the retina are to kinetic vision, which is tuned for velocity coding and direction selectivity. Static vision is the second mode of spatial relationship transcoding. The foveate and para-foveate regions of the retina (up to 10 or 15”) participate in processing the stable properties of the environment. This involves locating the relevant stimuli and detecting their shape and pattern and/or their contours. These two visual sub-systems can be said to be complementary so that speed information, for example, can be either directly encoded by the kinetic channel or indirectly deduced from the amplitude of the changes of position with respect to time. Studies on vection (see for instance Berthoz et al. 1975) have stressed how important the peripheral field can be for assessing the movement and speed of one’s own body. Salvatore’s research (1968) on the estimation of driving speed on a circuit led this author to a similar conclusion, that visual estimation of speed is better in the peripheral visual field (between 155” and 180”) than in the central visual field (25 o ) (see also Denton 1980). In the field of sports, few data are available on this topic. Graybiel et al. (1955), however, have reported that restricting the use of the peripheral visual field in various sports (javelin throwing, skating, etc.) leads to disturbances in movement control. This loss of information has an even more detrimental effect on performance than occlusion of the central visual field.
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Our study deals with jumping in horseback riding. This situation was chosen for three main reasons. First of all, because of the real problems riders are confronted with as they approach an obstacle: one of the difficulties encountered even by expert riders is that of regulating the horse’s stride so that its hindlegs will land at a given spot, i.e. the take-off point. The position of the last push-off, which is not spatially predefined by a board as in long jumping, is determined here by the height and shape of the obstacle. With each type of obstacle, there exists one ideal ‘frame’ that ensures an efficient and sure jump. With these constraints, a visual stride regulation in relation to the obstacle is required. We propose here to analyse the contribution of vision to gait regulation. The second reason for choosing horseback riding was that in this locomotor situation, the rider is passively transported. Unlike long-jumpers, who themselves produce the energy required to run and to jump and who therefore receive a great deal of proprioceptive feedback information about the ongoing movement (active locomotion), horse-riders are carried by the horse and therefore do not receive the same kind of information (passive locomotion). The forces exerted upon the ground seem to provide a direct means of controlling locomotion (see in particular Warren et al. 1986). A rider can use other available sources of information to control the locomotion of his mount. First there is proprioceptive information relating to the horse’ pace, which can be conveyed to the rider from the leg proprioceptors. When galloping, which is a leaping type of gait, the rider has to counteract the horse’s vertical thrusts by flexing and extending the ankle, knee and hip joints. Other kinds of information about the horse’s state of balance are provided by various contacts and pressures which are exchanged between horse and rider, thus constituting a kind of ‘kinesthetic dialogue’ between the two partners. Lastly, in this kind of ‘carried subject’ situation, exproprioceptive cues, according to Lee’s definition of the term, are likely to play a particularly privileged role; these cues have been defined as ‘information about the position, orientation and movement of the body as a whole relative to the environment’ (Lee 1978). Within this context, we feel that it may be possible for a rider to assess the speed of his horse’s locomotion by means of exproprioceptive visual cues, and to use this information to control the movements of the latter in such a way as to fulfill the spatial and temporal requirements of the jumping task. Experimental variations in the
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amount of visual information available can therefore be expected to have a significant effect on the subjects’ performance levels. The last point of interest to us here concerns the rider-to-horse interaction. In horseback riding as a sport, expert riders ‘pilot’ the horse like a vehicle. They modulate the various gait parameters of their mount depending on the required effect. In dressage, ‘prancing’ (or trotting on the spot) is a good illustration of this. We can be sure in this case that any changes in the horse’s stride are in fact the result of actions on the rider’s part. This is why the variations in the horse’s locomotion parameters observed in this study can be said to be attributable to the control exerted by the rider.
Experiment
1
Purpose The first experiment was designed to determine the role played by the visual information available to a rider as he controls the horse’s gait in the obstacle-approach zone. We hypothesized that if visual information about speed is used by the rider, then stride regulation will be affected by limiting such information. Taking this further, this would mean that the rider uses a method based on the distance/speed ratio to determine Tc. If this is not the case, then a more ‘direct’ processing method, based on the optic variable r, may be used. Method Subjects Five 23- to 35-year-old male subjects took part in the experiment. They all had between 10 and 16 years of experience in horseback riding, and were competing at national level. Five horses were used, four of which were 9 years old; the other one was 13. All the horses were trained, and had previously participated in class-A competitions. Experimental apparatus The experiment took place in a covered ring belonging to the Ecole Nationale d’Equitation (National Riding School) in Saumur, France. Each rider was asked to jump several times over an upright obstacle of
486
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varying height after approaching at a gallop along a rectilinear path. The gallop was preceded by a preparation phase on a curved path (walk-gallop pace transition). The rectilinear phase was begun after going through a 5 m wide gate located 25 m from the obstacle (fig. 1). Performance was recorded in terms of the spatial and temporal parameters of the horse’s stride. The values of spatial parameters were obtained by measuring the horse’s footprints on the track. The time interval between each left foreleg contact with the ground was recorded; since the horses always galloped with their left foot in the lead, all their strides could be measured. For horses, galloping is a natural gait, involving leaping, rocking, diagonal three-time movements followed by a flight time. When the horse gallops with the left foot in the lead, the three parts of the movements are as follows: (a) stance on the right posterior limb, (b) stance on the opposite diagonal pair (right anterior and left posterior limbs simultaneously), (c) stance on the anterior left limb. In our experimental situation, the horse had to turn left in order to reach the run-up. In horse riding it is more natural for the left foot to be in the lead when the horse turns to the left, and vice-versa. To simplify the data recording, we therefore asked the riders to gallop with the left foot in the lead. After each trial, the track was harrowed and rolled so that all jumps would take place under the same conditions, and above all, so that each footprint would be perfectly visible on the track. The accuracy of the measurements made was to within 3 cm. The temporal values were deduced from frame-by-frame analysis on a videotape fitted with a 0.04 set timer. The camera was placed at a 90” angle to the left of the track. The accuracy here was to within kO.04 sec. Procedure Each subject was tested at one session. The riders were not aware of the purpose of the experiment. After a warm-up period, glasses that limited the visual field were fitted to each rider individually. The riders were instructed to jump the obstacle as if they were on a normal course. They were thus supposed to make an efficient and sure jump, and to ride at the same pace as that used in competitions (350 m/mm). Independent
variables
The three experimental conditions were as follows: Obstacle height. Hl = 1.30 m, H2 = 1.40 m, H3 = 1.50 m. The obstacle height was
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varied for two reasons: (1) to avoid excessive standardization of the task, and (2) to evaluate any modifications in approach speed as a function of this parameter. The height most commonly used for upright obstacles in national competitions is 1.50 m. Although the obstacle heights were not nearly the maximum height which these horses were capable of jumping, which is around 2 m, they clearly required stride length adjustment to be made before the jump. The subjects were told before each trial how high the obstacle would be.
with either normal vision (NV) or Vision. Jumps were performed restricted vision (RV): With RV, the rider’s visual field was reduced by opaque glasses that had a 2 mm opening in the center, allowing him a visual field of only 15 O. This angle enabled the rider to see the entire obstacle when 23 m away from it. Six jumping situation were thus obtained. Each subject was tested 3 times in each situation, making a total of 18 trials per subject. The trials were performed in random order across subjects. Variables studied The spatio-temporal defined as follows:
~ _
_ _
parameters
analysed
with
each
stride,
were
Stride duration was the time separating two consecutive left-foreleg strikes on the ground. Stride length was the distance between two consecutive left foreleg footprints. Date was the sum of the stride durations going back from time T,, where T, was the moment when the left foreleg left the ground at the take off. Distance was the sum of the stride lengths going back from position DO where DO was the obstacle position. Time-to-contact (Tc) was the theoretical time it took the rider + horse to reach the obstacle from a reference point situated x strides from this obstacle. Tc was calculated at each stride by the ratio D/S where D was the distance to the obstacle and S the speed of displacement at the previous stride.
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in/umpinl:
Results General characteristics of horse locomotion in the obstacle-approach zone Stride F-l was the stride onto the take-off place, F-2 was the previous stride, etc. The characteristics of the run-up between F-l and F-7 were studied. All subjects and conditions combined, the mean oistance covered by these last 7 strides was 22.05 m (standard deviation (s.d) = 1.12 m) and the duration was 3.72 set (sd= 0.25 set). The mean distance between the take-off and the vertical plane of the obstacle bars, called beat distance, was 2.10 m (sd = 0.14). The analysis of variance did not show either the rider’s vision or the obstacle height to significantly influence the distance of the push-off place from the obstacle: we obtained F, 4 = 0.12, n.s. and F2,* = 1.33, n.s. for vision and obstacle height, respectively. The mean stride length was 3.12 m, and the mean stride duration was 0.55 sec. The mean displacement speed was 5.50 m set-‘, which is approximately equal to the normal competition pace. The jumps were successful in all the situations (obstacle height x vision). The effects of the experimental conditions will thus be studied by examining the patterns of the various spatiotemporal parameters as a function of the stride number in the jumping sequence. The following question arises here: at what point in the sequence do modifications occur in these spatio-temporal patterns, and more precisely, are these changes ascribable to ,adjustments based on visual cues?
f
/15m
Yf Video IL
Fig. 1. Diagram
Timer
of the experimental
apparatus.
489
-7
-6
-5 STRIDE
-4
-3
-2
-1
NUMBER
Fig. 2. Means and standard deviations of stride duration (A) in order of rank in the stride sequence, and intra-subject stride variations (B). NV and RV represent the normal and restricted (15 o ) visual condition, respectively. The stride labelled - 1 was the last one before the jump.
Stride duration and length An initial three-way analysis of variance (ANOVA 3R2) with vision, obstacle height and stride number as factors was conducted on the absolute variations in stride duration and length. Only the stride number was found to have a significant effect on the stride duration ( F6,28= 6.01, p -c 0.001). The vision and obstacle height were not significant factors, nor were the various interactions. Similar results were obtained with the stride length, since only the stride number was found to be a significant factor ( F6,28= 6.85, p < 0.001). The obstacle height was thus eliminated in subsequent analyses, as was F-l, which is a jump preparation stride (see figs. 2a and 3a).
The stride number did not significantly affect the (a) Stride duration. duration under either of the visual conditions (NV: F5,24=0.8,,8; RV: F5,24 = 0.990). No significant effect was observed on intra-subject variations (mean of the standard deviations) for NV ( F5,24= 2.54) but a significant effect was observed for RV ( F5,24= 5.99; p < 0.001) (see fig. 2).
(6) Stride length. The stride number was found to have a significant effect. Under NV, we obtained F, 24 = 5.24, p < 0.001, and under RV,
M. Laurent er al. / Reguhon
490
I G
of gait in jumping
3-
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W A W P CT
*_
TT
T
i-
-6
-5
T
T
T
7
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7
-7
’
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deviations
-4
1
-3
I
-2
I
-1
NUMBER
of stride lengths (A) and their intra-subject
variations
(B).
F5,24= 6.24, p < 0.001. In intra-subject variations, the stride number also had a significant effect under both NV ( F5,24= 7,19, p < 0.001) = 2.95, p < 0.05) (fig. 3). Paired comparisons-between and RV (& strides (F-7/F-6, F-6/F-5, etc.) using the paired t-test showed the existence of significant variations under condition NV starting with F-4. In the comparison F-4/F-3 we obtained t = 4.91, p -C0.01. These results indicate that changes in locomotion occurred in the obstacle approach phase. The following effects were observed: although the duration remained relatively stable, the stride length increased gradually up to F-l, in preparation for the jump. The intra-subject variation increases were significant only in the case of the length parameter. This increase in variation as the obstacle is approached can be attributed either to difficulty in preparing for the jump or to the effect of visual regulation with respect to the obstacle, or both. To elucidate this point, variations in the time and distance at each stride were analysed. Date and distance Absolute variations in date and distance were not studied since their effects are obvious. It seemed more relevant to examine the consistency
M. Laurent et al. / Regulation ofgait in jumping
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Fig. 4
491
of the
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-5
-4
STRIDE
NUMBER
deviations
in the distance
-3
-2
, t.02
-1
and date with each stride number.
with which subjects adopted a given pattern over a number of runs. The standard deviation was therefore chosen as a measure of performance. We again analysed the results by analysis of variance looking for any significant effects of stride number in the sequence. With the distance parameter, the stride number was found to have a significant effect both in situation NV ( F5,24= 6.55, p < 0.001) and in situation RV (C&24 = 3.39, p < 0.025). With the date parameter, the stride number was significant neither in NV ( F5,24= 0.608, n.s.) nor in RV ( F5,24= 0.599, n.s.). Fig. 4 shows the pattern of these two parameters for each stride in the jumping sequence. Note the decrease in the standard deviation of the distance starting at F-4. The decrease in date at that point was not significant. In horseback riding, the rider enters the obstacle-approach phase somewhat haphazardly. He cannot train to develop a consistent run-up from a measured mark to the board as a long jumper does. This particularity of horseback riding leads us to interpret the decrease in the standard deviation of the distance parameter as resulting from the rider’s regulation of the horse’s stride length. This regulation appears to be of a visual nature, and is conducted in relation to the obstacle. Time-to-contact with the obstacle (Tc) As with distance and date, the absolute values of Tc have little meaning. On the other hand, if we consider the Tc spread at each
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STRIDE
Fig. 5. Mean of the rank standard
of gcril
inJumping
NUMBER
deviations in the time-to-contact rank in the stride sequence.
of the obstacle
in order
of
stride, we can see (fig. 5) that the standard deviation of Tc was relatively stable from F-7 to F-4, and then dropped significantly at F-3 ( F,,4 = 2.60, p < 0.001). Neither of the visual conditions had a significant effect on Tc variation. The decrease in the Tc spread as a function of stride number indicates that the closer the subject came to the obstacle, the less variation occurred across trials in the time remaining before ‘contact’ with it. As stated above, this regulation effect started three strides before the obstacle. Discussion
The first important result to emerge from the above analysis is that vision does not significantly affect the regulation of the horse’s stride as the obstacle is approached. One might have expected deprivation of most of the peripheral visual information to severely alter the spatio-temporal characteristics of the horse’s stride. In the situation studied here, impairment of performance was expected to occur at two levels: during the stabilized part of the obstacle-approach phase (where pace control is very important), and in the jump preparation phase, which requires distance assessment in order to place the horse’s hind-
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legs at the proper distance. Yet, neither of these phases was significantly affected. How can this be explained? The stabilized phase was characterized in the NV situation by the setting up of a very stable gait. The absence of, or decrease in visual movement information in RV did not affect the gait, which remained stable (see fig. 2). It is worth recalling here that the horses and riders used in our experiments formed expert partnerships. One of the features of a horse’s expertise is the ability to maintain a very steady pace. The rider’s task is to assist and maintain this steadiness. It is probably due to the fact that a rider uses mainly proprioceptive cues that his ability to control the horse’s pace was not significantly disturbed when his own vision was restricted. In partnerships between expert horses and riders, the various aids whereby the two partners communicate form a kind of language, which in academic riding makes it possible for the rider to maintain perfect control of his mount. The three natural aids with which a rider controls his mount’s locomotion are the legs, the hands acting via the reins, and the body weight on the seat. Artificial aids such as the spurs and riding crop are also usually available if required, although in our experiments this was not the case. Aids are the functional support to the communication between the rider and the horse. The rider sends messages to the horse, which must be able to decode them and to respond appropriately by turning, going faster, lengthening its pace, etc. The horse in turn sends out information about its ongoing locomotion. These signals which are decoded by the rider can be used like indirect feedback in controlling the horse’s gait efficiently and precisely. It seems likely that the pace and the horse’s stride length are particularly susceptible of control via the aids we have just mentioned, which would explain why restricting the rider’s visual field had little effect: the requisite information was being conveyed via the proprioceptive channel (pressure, muscle tension, etc.). In the regulation phase, the results show that visual speed information provided by the peripheral field was not essential to distance encoding, as the performance levels were similar in situations RV and NV. These findings are compatible with the idea that the Tc parameter involved in distance control is obtained directly by mean of the optic variable 7, as suggested by Lee et al. (1982) in the case of long jumping. It is known that this process does not a priori require all the available visual information but only the retinal expansion of the target-object.
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2
The second experiment can be said to be a small pilot study. Its aim was to identify the visual strategies and the eyeehead coordination involved in jumping in order to complete the discussion on experiment 1. This study breaks new ground because no previous experiments have been reported in similar situations using a gaze video recorder carried by a moving subject approaching an obstacle. From the results obtained in the first experiment, certain assumptions can be drawn about the nature of the visual behavior involved in jumping. There, the absence of effect in the restricted visual condition indicated that the operational visual field (i.e. the non-occluded visual field) was within 15 o and that the useful visual information was necessarily situated within this angle. This agrees with the optic flow hypothesis, which suggests that riders might use the optical variable 7 to obtain time-tocontact information. In order to collect this information, it might seem preferable for the rider to look at the obstacle during the whole approach phase rather than to make sweeping eye movements from the top to the bottom of the obstacle or from the obstacle to the expected take off point. If the rider does fix his gaze on the obstacle, it would be interesting to know which part of it he focuses on. Actually, riding coaches, on the basis of their empirical experience, usually advise riders to adapt their visual strategies to the type of obstacle to be jumped. In order to perform the jump as economically as possible, coaches ask the rider to fix their gaze onto the top of the obstacle in a DROIT for example and towards the bottom in a SPA. These two strategies apparently cause the take off place to be farther from and nearer to the obstacle, respectively. Do skilled riders actually use these strategies? A video oculographic method was used to examine this question. Method
A video-oculographic gaze recorder, (Nat Eye Mark Recorder IV) adapted to sports situations (Rip011 1988b) was used. The eye was illuminated by a point light source. The bright spot reflected by the cornea was video recorded in the shape of a bright ‘V’. This signal was coupled with the visual field image recorded by a camera placed on the subject’s forehead. The final picture observed on the monitor showed the shifts and positions of the ‘V’ in relation to the external world. The
M. Laureni et al. / Regulation of gait in jumping
video image taken from the mounted head axis orientation.
camera served as a measure
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of the
Subjects Two out of the five riders who participated in the first experiment were selected because of the high competition standards they had achieved. They were of national standard (A class). Task The same procedure was used as in experiment 1. Each rider, equipped with an oculometric device, had to jump three times over each of three types of obstacle (DROIT, SPA and OXER). Recording
technique
Data collection. Each parameter was collected on a transparency film superimposed on the video screen, as described by Rip011 (1988b). Various parameters were analysed, in connection with the following: (1) Gaze direction in relation to the obstacle. The direction of the gaze corresponded to the cornea1 reflection in relation to the obstacle and the gap between the cornea1 reflection and the obstacle was recorded (fig. 6A). (2) Head orientation in relation to the external environment. The relationship between the head and the environment was measured by analysing the head orientation in relation to a fixed point located far away in the background (fig. 6B). (3) Head orientation in relation to the obstacle. The direction of the head corresponded to the centre of the image delivered by the forehead camera and the gap between this virtual point and the obstacle was recorded (fig. 6C). (4) Eye-head coordination. The relationship between the gaze and the head was measured by analysing the gap between the cornea1 reflection and the head direction (fig. 6D). The eye was taken to be immobile in relation to the head when the cornea1 reflection was itself immobile on the image delivered by the forehead camera. The movement of the eye on the video screen showed the movement of the eye in relation to the head. These movements were measured in degrees.
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Samples were collected frame by frame at the rate of one every 20 msec on the transparency support superimposed on the video screen, then analysed. A second camera located sideways on profile simultaneously filmed the exercise. A time basis simultaneously superimposed on the two images (the oculographic and the external images) made it possible to relate the eye and head behaviour to the characteristic parameters of the approach as mentioned in experiment 1. Results
The data collected on two riders with three different types of obstacles (DROIT, SPA and OXER) revealed the absence of any change in the visual scanning patterns whatever the obstacle.
Fig. 6. Relationship between eye, head, and environment during the approach, between t-2 set up to take off: Gaze orientation in relation to the fence (A): Head orientation in relation to the environment (B); Head orientation in relation to the fence (C); Eye-head coordination (D) (see comments in the text). Curves A, B and C are based on the video screen data. They correspond to relative values expressed in arbitrary units. D data are absolute values (in degrees) of the eye-head alignment. Subject’s eye movement up (+) and down (- ) in relation to the head.
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Fig. 6 shows an example of a visual pattern produced by a rider in a DROZT. The following behaviour was described. (1) The gaze was accurately directed towards the upper part of the fence throughout the approach phase (fig. 6A). (2) The head was fixed straight ahead and was displaced onto the antero-posterior axis without any oscillation (lateral or up and down movement). This stability was possible because hip, knee and ankle flexion absorb the swinging of the horse in order to maintain the upper part of the body immobile (fig. 6B). Consequently, head stabilization produced an apparent shift of the fence into the lower part of the rider’s visual field (fig. 6C). The gaze direction then moved progressively further away from the center of the orbit so as to keep a stable retinal picture of the fence during the whole approach (fig. 6D). Discussion Our results, contrary to empirical analyses, show that riders exhibited similar visual behaviour whatever the obstacle profile. Particularly interesting features were the head stabilization with respect to the environment and the fact that the gaze was maintained constantly towards the obstacle despite the horse’s swinging. These results are consistent with those obtained in experiment 1. Indeed, the RV conditions made visual saccade amplitude lower than the possible 15 o visual window. If these saccades had been necessary, they would have involved a rapid orientation of the head onto the visual target axis. Given the head inertia, this improbable manoeuver would have disturbed the rider’s balance, which was not the case. A second remark concerns the calculations of the distance in approaching the obstacle. We assumed that Tc estimation would be directly given by the optic flow field (variable 7). This process is consistent with the idea of a visual anchorage onto the looming target until the last stride occurs. When Tc information is of major importance (which was the case with F-4), visual saccades might greatly disturb the collecting of information necessary to the stride regulation. Furthermore, we noted that eye movement, in relation to the persistent head stability, began at about F-4, that is at the beginning of the stride regulation phase. This might correspond to a Tc value being extracted about 1.5 set before the take-off. Visual anchorage onto the obstacle
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led to a shift of the gaze with respect to the head, which remained consolidated with the trunk, in a fixed position straight ahead. It is difficult to say whether this shift merely resulted from the head being immobilised, or whether the eye-head coordination exhibited by expert riders might play a role in the information processing. If the iatter were the case, this parameter, which is a type of extraretinal information, might play a role in the encoding of the location of the obstacle in relation to the body. Further investigations are necessary to confirm this hypothesis.
General
conclusion
(1) Analysis of the spatio-temporal parameters of the locomotion of horses approaching an obstacle revealed the existence of a so-called stubilized phase during which the gait imposed by the riders was very stable, both within and across trials. Here the rider was probably attempting to control the displacement speed. On F-4, substantial variations in the gait parameters were recorded. Changes in the patterns of various parameters (time, distance, and obstacle Tc) led us to ascribe these effects to visual adjustements made with respect to the obstacle. The stride regulation phase requires careful control of the distance to the obstacle. During this phase, the significant variations in speed observed indicate that control of this distance is based on information about the time-to-contact with the obstacle. The variations in Tc decreased significantly three strides before take-off. Thus Tc might seem to be the factor that is actually controlled by the rider to modify the distance at which he will place the horse for the jump. Lee et al. (1982) have developed this theoretical problem in connection with a similar motor task, the regulation of gait in long jumping by highly skilled athletes. During the zeroing in phase, subjects adjust their stride pattern, varying their flight time by regulating the vertical impulse of their strides. These authors have described the relationships between the optic variable 7 and the duration of flight time (and hence of the stride length). A recent study by Warren et al. (1986) confirms the fact that the vertical impulse is directly modified by the parameter 7. (2) Limitation of the visual information to the 15 o field provided to expert riders did not significantly alter gait control as these riders approached the obstacle. In the preparatory phase, where the speed is
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stabilized, the rider must do all he can to ensure a steady pace. Here he controls the horse without taking the restricted peripheral cues much into account. The most objective explanation for this was that these cues are not indispensible here, since proprioceptive cues can probably be used to provide any missing exteroceptive information. This aspect of movement control in skilled subjects has classically been reported in many studies on motor learning. During the regulation phase, the simple perception of the obstacle appears to be sufficient for extracting this high-order invariant represented by the optic variable 7 (Lee 1976; McLeod and Ross 1983) from which Tc can be directly obtained. Consequently the hypothesis concerning the use of a cognitive method for obtaining Tc in this task is not confirmed by our results. Of course, this does not mean that riders do not use speed information when it is available; it only indicates that this information is not indispensable for skilled riders. This study on horseback riding emphasizes the importance of the role played by temporal factors (not just rhythm, but also Tc) in actions involving severe spatio-temporal constraints. It can thus be said that it is on temporal factors that the control is actually based. The visual exploration strategies used during the obstacle approach phase were not found to be significantly related to the characteristics of the obstacles. The patterns observed indicate that the subject’s gaze focused on the upper part of the obstacle with a fixed head position. These findings are compatible with the processes discussed above, in which the retinal expansion of the size of the obstacle is taken into account in extracting the Tc. When a subject moves towards an obstacle many visual cues are available. To identify precisely the cues actually used by a subject in a particular task is not easy and probably the various control processes work in a complementary fashion (Laurent et al. 1988; Cavallo and Laurent 1988). In order to determine under what conditions various kinds of information are used further experimental studies are necessary.
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