The effect of movement speed on upper-limb coupling strength

Human Movement North-Holland

Science

11 (19921615-636

615

The effect of movement speed on upper-limb coupling strength * Stephan P. Swinnen a, Charles B. Walter b, Deborah J. Serrien a and Carla Vandendriessche a Catholic Unic~er.sity of Leucen, ’ University of Illinois, Chicago,

a

LeuL:en, Belgium USA

Abstract Swinnen, S.P., C.B. Walter, D.J. Serrien and C. Vandendriessche. 1992. The effect of movement speed on upper-limb coupling strength. Human Movement Science 11, 615-636. The effect of movement speed on upper-limb coupling strength during the execution of discrete movements with different spatiotemporal features was investigated. Subjects performed an elbow flexion movement in the left limb together with a flexion-extension-flexion movement in the right limb. Higher movement speeds were found to result in increased structural and metrical coupling, i.e., the movement topologies and the limbs’ intensity specifications became more synchronized. The more complex flexion-extension-flexion movement constituted the dominant pole of attraction, although there were some signs of mutual interaction effects. In general, the findings exemplify the nature of difficulties that human performers experience when learning to produce incompatible spatiotemporal patterns simultaneously.

Following the pioneering research of von Hoist (1936) on the coordination of fin movements in fish, a number of recent studies have shown that limbs interact with each other when moving concurrently. These interactions are often characterized as (mutual) synchronization effects, indicative of a simplified organization and control of multi-limb action by the central nervous system (Kelso et al. 1979). * Support for the present study was provided through a grant from the Research Council of K.U. Leuven, Belgium (Contract No. OT/89/26). Further support was made available through a Collaborative Research Grant from the NATO Scientific Affairs Division (Contract No. 86/7321. The authors are indebted to Dr. C. Worringham for directing their attention to earlier studies on the role of speed adaptations in skill acquisition. Correspondence to: S.P. Swinnen, Motor Control Laboratory, ILO, K.U. Leuven, Tervuurse Vest 101, 3001 Heverlee, Belgium.

0167.9457/92/$05.00

0 1992 - Elsevier

Science

Publishers

B.V. All rights reserved

616

S.P. Swim-tenet al. / Speed and upper-limb coupling

Interlimb interactions become particularly evident when simultaneously performing ‘different’ movements with the arms (Franz et al. 1991; Kelso et al. 1979; Marteniuk et al. 1984; Sherwood 1991; Swinnen 1991; Swinnen et al. 1988). Under these circumstances, various degrees of synchronization tendencies (or interference to view it from a negative perspective) can be observed at various levels of movement description. Whereas different intensity specifications for both limbs are relatively easy to generate simultaneously - although you will often see an initial bias towards symmetry - different movement forms or topologies are much more difficult to produce (Swinnen 1992). We have termed these capabilities as metrical and structural dissociation, respectively (Swinnen et al. 1991b). Although interlimb interactions have been described for discrete as well as for continuous tasks, the variables that directly determine coupling strength have received limited attention. Their identification may be of importance in furthering our understanding of interlimb coordination and the characteristic constraints inherent in multi-limb action, as well as in providing guidelines for setting up learning environments where such constraints seem to play a dominant (often limiting) role (Walter and Swinnen 1992). Concerning the coordination of rhythmic finger or hand movements, studies have shown that anti-phase coordination (homologous muscles contracting in an alternating fashion) switches to the more stable in-phase coordination (homologous muscles contracting simultaneously) when the frequency of cycling is increased (Kelso 1984; Kelso et al. 1986; Schiiner and Kelso 1988; Walter et al. 1991). The existence of these two phaselocked modes has been demonstrated quantitatively in experiments where the relative phase between the limbs was manipulated and its variability determined. As a result, frequency has been identified as a control parameter that affects the relative phasing between the limbs (an order parameter). Although it remains unclear how well findings from continuous tasks generalize to discrete tasks (but see Schiiner, 1990, for application of a dynamic theory to discrete movement production), there is mounting evidence in favor of a tendency towards interlimb synchronization during discrete task performance (Walter et al. in press). It appears from these observations that there may exist universal modes of movement coordination to which the system is normally drawn. Preferred patterns of coordination can also be observed in

S.P. Swinnen et al. / Speed and upper-limb coupling

617

tapping rhythms with the fingers of both hands. It is relatively easy to produce simple rhythms where the frequency of tapping in one hand is an integer multiple of tapping rate in the other hand (1: 1, 2: 1, 3 : 1). This distinguishes them from the more difficult polyrhythms in which no common divisor exists (3 : 2, 4 : 3 etc). There is considerable evidence that polyrhythms are produced with less consistency than simple rhythms although practice results in performance improvements (Deutsch 1983; Heuer 1991; Peper et al. 1991; Shaffer 1982). How such polyrhythms can be acquired most successfully awaits further investigation as well as the identification of those variables that affect performance directly. These multifrequency tasks may bear similarities with the presently used bimanual coordination task in that both require the hands to do different things simultaneously. ’ The present study addressed the effect of movement speed on upper-limb coupling strength during the performance of discrete movements with widely different spatiotemporal features. Movement speed was manipulated in seven discrete steps from low to high or high to low, depending on the experimental condition. The goal of this stepwise experimental approach was to uncover and characterize the nature of the relation between movement speed and interlimb attraction for a discrete action with respect to metrical as well as structural specifications. Method Subjects

Subjects were 20 right-handed, l&year-old the Catholic University of Leuven.

female students

from

Apparatus and task

The apparatus consisted of two horizontal metal levers (43 cm long), attached to virtually frictionless vertical axles that were mounted ’ In spite of these similarities, there are also differences between producing discrete or continuous bimanual arm movements and finger tapping tasks. In tapping, the finger that is assigned the slower tapping rate usually waits for the faster finger after which they move simultaneously. In contrast, when making continuous (uninterrupted) elbow oscillations, the relative phase is constantly shifting and this provides extra problems for beginners (Swinnen et al. 1991~). This explains why tapping a 2: 1 rhythm is easy for novices to produce in contrast to performing cyclical 2: 1 elbow oscillations in a continuous fashion.

618

S.P. Swinnen et al. / Speed and upper-limb coupling

on a table (see fig. 1). An adjustable handle was located at the distal end of each lever. Shaft encoders (4096 bits per revolution) were affixed to the base of each axle to determine the angular position of the arm-lever system. Data were sampled at 500 Hz. The subject was seated behind the apparatus such that the front of the body was aligned between the lever axles: the locations of the axles were adjusted laterally and the chair was adjusted vertically to accommodate subjects with different shoulder widths and torso lengths. When the arms rested upon the levers, the elbows were nearly extended at the starting position (elbow angle = 155O) The end position was located in front of the subjects (at 90” from the starting position), just lateral to the body’s midline. The elbow was positioned just above the axis of rotation of the lever. A 350 g load was attached to the distal end of each lever. Subjects performed two movements that differed in spatiotemporal features: an elbow flexion in the nondominant arm (hereafter referred to as the unidirectional movement), and an elbow flexion-extensionflexion movement in the dominant arm (hereafter referred to as the reversal movement). The reversal movement was generally more forceful than the unidirectional movement because changes in direction were to be made between the start and final endpoint while maintaining the same overall movement time for both tasks. The reversals in direction were completed within each of two 6” wide vertical targets. Thus, the subject was to move the lever towards the second target zone (angular displacement = 60”), reverse direction to the first target zone (angular displacement = 30”), and then reverse direction again to move to the endpoint (angular displacement = 60”). To determine the end of movement unambiguously, a reversal was also made at the movement’s endpoint. Subjects then moved the lever back to the starting position and they were told that this was not part of the overall task goal. The time at which peak displacement was reached served as the end of movement. The subjects were instructed to initiate the movement shortly after the ‘go’ signal, but reaction time was neither stressed nor measured. Procedure

The experiment was divided into 3 practice phases, administered in one experimental session. The first two phases consisted of unimanual

S.P. Swinnen et al. / Speed and upper-limb coupling

60

619

30 0

Start

Start

Fig. 1. Overhead

perspective

of subject

and apparatus.

practice: subjects first performed the reversal movement (35 trials) in the right limb, followed by practice of the unidirectional movement (35 trials) in the left limb. In both phases, movement speed was varied in 7 steps, consisting of 5 trials each: movement times for Speed Block 1 to 7 were, in order, < 650, 650-800,800-950, 950-1100, 1100-1250, 1250-1400, and > 1400 ms. Movement time was controlled by provision of qualitative knowledge of results (i.e., speed up, slow down). Following unimanual practice, subjects performed both limb movements simultaneously (35 trials). The unidirectional movement was made in the left limb and the reversal movement in the right. Movement speed was again manipulated in seven steps, as indicated above for unimanual practice: subjects of Group 1 started at low speed, then gradually sped up the movement until maximal speed was reached (Slow + Fast). Conversely, subjects of Group 2 started at maximal speed followed by a gradual decrease, also in seven stages (Fast + Slow). Counterbalancing was applied to control for a possible order effect, not to examine two different training techniques. When subjects did not meet the imposed temporal requirements within each speed block, they were encouraged to speed up or slow down the movement accordingly. No displacement-time profiles were displayed. Movement time feedback was only given regarding the reversal limb to reduce task demands. This seems justified given the natural tendency to start and finish bimanual movements simultaneously (Kelso et al. 1979). Timing of the movement was initiated as soon as the subject left the starting position and stopped when peak

620

S.P. Swinnen et al. / Speed and upper-limb coupling

displacement was reached at the end position. Only the first five trials that satisfied the required speed bandwidth at each of the seven levels were stored for further analysis. Additional trials were performed when subjects did not comply with the speed bandwidths until five correct movements were available. Subjects were instructed to initiate and terminate the limb movements simultaneously at all times. In addition, they were told to perform the limb movements independently of each other to obtain the same patterns as produced during unimanual practice. Data analysis The present experiment was mainly focused on the degree of dissociation or decoupling between the limbs. Interlimb interactions were studied at the structural as well as the metrical level of movement specification. A structural prescription refers to the dynamic topography of a biokinematic chain whose links have been constrained to act as a unit, whereas a metrical prescription refers to movement parametrization. Parametrization allows the performer to make movements in a different overall size or in a different time or speed while preserving the essential aspects (Kelso and Tuller 1984; Turvey et al. 1978). Applied to the present bimanual task, the structural coefficient was assumed to represent the relation between the organizational structure of both limb movements. The metrical coefficient, on the other hand, referred to the relation between the intensity specifications of both movements. As the goal of the task was to make both limb movements independent of each other, we have coined the terms structural and metrical dissociation (see Swinnen et al., in press, for a more detailed description). Structural interactions Between-limb changes: Cross-correlation of the left and right limb acceleration pattern. To determine the degree of functional coupling/ decoupling (structural dissociation), the time-evolution profiles of angular acceleration were cross-correlated (Swinnen et al. 1988). Correlations were calculated on each trial, converted to Fisher’s Z coefficients, and then averaged across 5trial blocks. On each trial, data points were taken from both acceleration traces every third percentile as long as both limbs were being moved simultaneously.

S.P. Swinnen et al. / Speed and upper-limb coupling

621

Previous research with a similar movement has shown these correlations to be low when the limb movements are performed independently of each other (Swinnen et al. 1991b). Consequently, the coefficient (Y) obtained through this technique can range from nearly zero (for perfect decoupling) to 1.0 (for perfect coupling). No onset synchronization of the acceleration signals was applied prior to their cross-correlation because it was intended to assess the degree of coupling during actual motion of both limbs. Within-limb changes: Cross-correlation of the acceleration pattern as generated under unimanual and bimanual conditions. A second type

of cross-correlation was computed to evaluate the changes that occurred within a limb between unimanual and bimanual practice. This technique provides information about changes in the acceleration profile of each movement when performed concurrently with the other limb in comparison to when performed alone. This analysis was done on the unidirectional and the reversal movement separately. The acceleration pattern of each individual bimanual trial was cross-correlated with an averaged acceleration profile of five unimanual practice trials, performed under the same speed conditions. Averaging was justified in the latter condition as the acceleration profiles were very stable across trials. The correlations were computed following onset synchronization and temporal normalization of both profiles. The total time value was treated as 100%; the acceleration values were extracted from each movement at every third percentile of movement time, and subsequently cross-correlated (see also Swinnen et al. 1990; and Swinnen et al. 1991b). Metrical interactions

Both limb movements clearly differed from each other in terms of their absolute force requirements, i.e., more muscle activity was to be generated in the right limb’s elbow flexors and extensors to generate the reversal movement in comparison to that of the left limb (Swinnen et al. 1991d). The capability to produce different intensity specifications simultaneously was assessed by estimating the amount of net mechanical work produced in each limb (for an elaboration of this technique, see Swinnen et al. 1991b). Work was calculated from the time integral of the power curve. Power, defined as the rate of doing work (Enoka 19881, was determined by multiplying acceleration and

622

S.P. Swinnen et al. / Speed and upper-limb coupling

velocity at each instant along the movement trajectory. 2 Due to the reciprocity of positive and negative work, only the total amount of positive relative work (when the muscle moment acts in the same direction as the angular velocity) was computed for each of the limb movements. Subsequently, the degree of differentiation of mechanical work output between the limbs (metrical dissociation or decoupling) was determined through division of the amount of work required for the production of the reversal movement by that for the unidirectional movement. This was done for both unimanual and bimanual performance conditions. This metrical dissociation ratio is to be interpreted as follows: The higher the score, the higher the differentiation of the amount of work generated in both limbs. The ratio found under unimanual performance conditions hereby serves as the optimal score, simulating perfect differentiation. Conversely, the lower the ratio, the more similar the intensity specifications generated in both limbs, and thus the lower the metrical dissociation. Work was used to assess intensity differentiation because it is a scalar with a close relation to acceleration (force) under the present circumstances, thereby serving as a useful analog to the structural dissociation measure in which acceleration (force&time profiles were used as well. In addition to the analysis of the metrical dissociation coefficients, the amount of relative work generated in each limb was analyzed separately, as well as the changes it underwent across speed levels. All data were grouped into 5-trial blocks and analyzed by means of ANOVA’s with repeated measures. The data of Group 2 were reordered to conform to those of Group 1; in other words, across Speed Levels 1 through 7, speed gradually increased for both groups.

’ Muscle power is the product of the net muscle moment and angular velocity. Due to the proportionality between acceleration and joint torque under the present task conditions, the power-time profile associated with the movement was obtained by multiplication of angular respectively. Accordingly, the velocity and acceleration, expressed in deg/sec and deg/sec2, variable so obtained in the present study is proportional to power and its time integral is proportional to work. Therefore, we use the term ‘relative’ work. This analysis is only a first approximation because not all the power that is observed during the production phase is produced by the muscle. Part of it is probably due to elastic stretch applied to the muscles and tendons (Enoka 1988). Nevertheless, this does not preclude the present technique to reveal valuable information about central interactions between the limb’s intensity specifications during simultaneous movements.

S.P. Swinnen et al. / Speed and upper-limb coupling

623

Results Structural interactions Cross-correlation of the left and right limb acceleration pattern

Cross-correlations, averaged across both groups, are presented in fig. 2A. Correlations increased with increasing movement speed and this effect reached significance, F(6, 108) = 4.76, p < 0.01. Although the trend for correlations to increase with higher speed levels was more prevalent for group 1 (Slow -+ Fast) than for group 2 (Fast + Slow), this effect was not significant, F(1, 18) < 1, nor was the Group x Speed Level interaction, F(6, 108) < 1. Cross-correlation between the unidirectional acceleration pattern as performed under unimanual and bimanual conditions

As shown in fig. 2B, the correlations dropped significantly across speed levels, F(6, 108) = 3.21, p < 0.01. This is indicative of an increased deviation between the unidirectional acceleration trace as performed under bimanual and unimanual circumstances, indirectly suggesting a gradual coupling of the unidirectional acceleration pattern with the reversal pattern under increased movement speed conditions. The differences between groups were too small to reach significance, F(1, 18) < 1. The Group X Speed Level interaction was not significant either, F(6, 108) = 1.39, p > 0.05. Cross-correlation between the reversal acceleration pattern as performed during unimanual and bimanual conditions

The correlations did not undergo major changes across speed levels for both groups taken together (fig. 2C). The effect for speed level was not significant, F(6, 108) < 1, neither was the group effect, F(1, 18) < 1. However, the Group X Speed Level interaction was significant, F(6, 108) = 4.37, p < 0.01. For Speed Levels 1 to 7, crosscorrelations were, in order, 0.77, 0.79, 0.89, 0.88, 0.89, 0.90, 0.85, for Group 1 and 0.89, 0.85, 0.85, 0.82, 0.81, 0.76, and 0.79 for Group 2. Thus, whereas cross-correlations in Group 1 increased across the seven blocks, cross-correlations in Group 2 decreased. Note, however, that performance order was reversed for analysis purposes in the latter group. Therefore, it can be concluded that the single underlying mechanism responsible for this effect is the role of experience that

624

S.P. Swinnen et al. / Speed and upper-limb coupling

High

LOW

High

SPEED Fig. 2. Cross-correlations between the limbs’ acceleration profiles across bimanual practice (A) and cross-correlations within each limb movement (B, C) under unimanual versus bimanual practice conditions across seven speed levels.

S.P. Swinnen et al. / Speed and upper-limb

appears to enhance the accordance manual reversal acceleration pattern.

between

coupling

the bimanual

625

and uni-

Metrical interactions Metrical dissociation The metrical dissociation coefficient was calculated during unimanual and bimanual practice conditions. Fig. 3A shows the ratios averaged across groups. The ratios found under unimanual practice conditions simulate optimal differentiation of intensity specifications. It is apparent that the bimanual ratios were much lower than the unimanual ratios and this effect was significant, F(1, 18) = 128.59, p < 0.01. A separate analysis was conducted on the bimanual practice ratios but no significant group effect was found F(1, 18) < 1. It is also apparent that the ratios decreased significantly as movement speed was increased, resulting in a significant effect for speed levels, F(6, 108) = 3.71, p < 0.01. Thus, differentiation of the amount of work in both limbs gradually decreased as speed was increased. The Speed Level X Group effect was not significant, F(6, 108) < 1.

Amount of relative work produced in each limb Unidirectional movement. Fig. 3B shows the scores for relative

work during bimanual practice as compared to unimanual practice conditions. Bimanual scores were higher across all seven speed levels, indicative of an excess of work generated for production of the unidirectional movement when performed simultaneously with the reversal movement. The increase was linear across Speed Levels 1 to 5, followed by a much steeper linear increase in work during the remaining trial blocks. This pattern was similar for both groups, The group effect was not significant, F(1, 18) < 1. The score differences between unimanual and bimanual practice were significant, F(1, 18) = 28.49, p < 0.01. The effect for speed level was also significant, F(6, 108) = 60.09, p < 0.01. Of the interactions, only the Practice Mode (unimanual versus bimanual) x Speed Level effect reached significance, F(6, 108) = 16.07, p < 0.01. There was an increased divergence of unimanual and bimanual work scores as speed was increased.

Reversal movement.

A different pattern was observed for the relative work scores of the reversal movement (fig. 30. Here, the data pattern

626

S.P. Swinnen et al. / Speed and upper-limb coupling

High

SPEED

4

0. 0

1

2

3

1

5

6

7

High

LOW

SPEED Fig. 3. Metrical dissociation and relative work produced with respect to the unidirectional and reversal movement during unimanual and bimanual practice conditions and across seven speed levels.

S.P. Swinnen et al. / Speed and upper-limb

coupling

627

across speed levels was essentially the same during unimanual as compared to bimanual practice. The group effect was not significant, F(1, 18) < 1. No significant differences were found between unimanual and bimanual practice conditions, F(1, 18) = 1.36, p > 0.05, although bimanual values were slightly lower. The increase in work across speed levels was significant, F(6, 108) = 187.47, p < 0.01. Of the interactions, only the Practice Mode X Speed Level effect was significant, F(6, 108) = 3.29, p < 0.01. Towards the highest speed conditions, the work scores for bimanual practice became smaller than those for unimanual practice. The relation between structural and metrical variables as reuealed through correlation analysis Correlation analyses were done on the total group of 20 subjects for each of the seven speed conditions of bimanual performance to investigate the possible relations between the structural and metrical variables mentioned above (see table 1). With respect to the variables reflecting structural interactions, it is apparent that the unidirectional-reversal correlations (correlation of the unidirectional acceleration profile with the reversal acceleration profile) were negatively related with the unidirectional-unidirectional correlations across all trial blocks but not with the reversal-reversal correlations, except for the first trial block. In other words: the higher the coupling between the limbs, the more the bimanual unidirectional acceleration profile became impaired, i.e., the more the unidirectional bimanual profile deviated from that observed under unimanual performance conditions. Thus, the high degree of coupling between both limb movements was mainly a consequence of the left acceleration pattern assimilating the features of the right limb pattern when they were performed together. Conversely, the reversal pattern produced in the right limb was less affected and did not show any strong relations to the unidirectional-reversal correlations. Furthermore, the unidirectional-unidirectional and reversal-reversal correlations were not related to each other, i.e., the way the left limb movement was performed did not appear to have any clear and consistent relation to right limb performance. Some relations were also found between the structural measures and work. Subjects showing higher degrees of structural coupling as

628 Table I Correlations total group

S.P.Swinnen

between metrical of subjects.

et al. / Speed and upper-limb

and structural

variables

coupling

across the seven speed conditions

for the

Speed LOW

High

1 car unidir-reversal car reversal-reversal car unidir-reversal car unidir-unidir car unidir-unidir car reversal-reversal work unidir work reversal work unidir car unidir-reversal work reversal car unidir-reversal work unidir car unidir-unidir work reversal car reversal-reversal ‘I p < 0.05:

2 0.55 “

-0.62

h

3 0.29

-0.66

4

- 0.07 h

-0.88

- 0.24

- 0.22

0.20

0.24

0.35

0.34

0.86 ”

0.84 h 0.15

- 0.06 ~ 0.55 ‘I

- 0.54 iI

0.08

0.20

0.16 ”

-0.91

h

6

7

-0.11

-0.19

-0.10

-0.89

h

-0.76

h

-0.73

”

0.24

0.20

0.22

0.22

0.40

0.66 ”

0.42

0.77 h

0.80 h

0.74 h

0.58 h

0.69 ”

0.06

0.08

0.14

0.17

0.08

-0.63 0.42

-0.15

5

h

-0.76h 0.14

-0.69’

-055“

- 0.59 h

-0.01

- 0.06

- 0.21

hp < 0.01.

revealed by the unidirectional-reversal correlations also generated more work for producing the unidirectional movement in the left limb. Conversely, relationships between work produced in the right limb and unidirectional-reversal correlations were low. There was also a negative relationship across all speed levels between relative work generated for production of the unidirectional movement (left limb) and unidirectional-unidirectional correlations. Thus, the more work was being generated in the left limb, the more the bimanual unidirectional acceleration profile deviated from that produced under unimanual circumstances. Altogether, the predominant concentration of significant correlations with respect to the unidirectional movement indicates that it underwent major changes from unimanual to bimanual practice whereas the reversal movement was less affected. More generally, it appears that interlimb assimilation effects during the concurrent performance of different movements are asymmetrical whereby the less complex of the movement patterns undergoes major

S.P. Swinnen et al. / Speed und upper-limb coupling

changes that movement.

are

reflective

of the

features

of the

629

more

complex

Discussion The effect of movement

speed on metrical and structural

dissociation

The present study demonstrated increases in structural and metrical coupling as movement speed was increased. With respect to structural coupling, it was found that the cross-correlation between the acceleration patterns of the right and left limb movement significantly increased with increasing movement speed, irrespective of whether subjects went from low to high or from high to low speeds. For both groups taken together, the variance shared between both profiles increased from 43 to about 69% for Speed Levels 1 to 7, with the largest increase found between de fifth and sixth speed level. Within-movement correlations indicated that the right limb (reversal) pattern did not undergo profound changes during bimanual practice whereas the left limb (unidirectional) pattern became gradually more disparate from its unimanual counterpart. Thus, the unidirectional acceleration pattern was drawn more to the reversal pattern than vice versa. Although the former movement was always made in the left limb and the latter in the right, we believe this effect is not produced by differences in manual specialization (see also Swinnen and Walter 1991a; Walter and Swinnen 1990a). Rather, it is a consequence of an asymmetrical synchronization tendency in which the more complex of the limb movements forms the persistent and dominant pole of attraction. This was also confirmed by the correlational analysis conducted on the total group of subjects: the unidirectionalunidirectional correlations were negatively related with the unidirectional-reversal correlations whereas such an association was not found with respect to the acceleration profile of the reversal movement. Metrical coupling, as determined through the division of the amount of work generated in the right and left limb, was also increased with increasing movement speed. Under increased speed conditions, it became gradually more difficult to differentiate the intensity specifications required for the production of both limb movements. Again, this was mostly due to an excess of work generated in the left limb for

630

S.P. Swinnen et al. / Speed and upper-limb coupling

production of the unidirectional movement. In contrast, work generated in the right limb for production of the reversal movement under bimanual performance conditions was slightly lower than that generated under unimanual conditions. Overall, the excess of work in the left limb was in the order of 200 to 300% and was clearly greatest under the highest speed conditions (> 300% for the highest speed level). The group going from high to low speeds displayed higher degrees of excess than the group who had undergone the converse arrangement although the effect was not strong enough to reach significance. These findings provide additional support for the idea that the reversal movement formed the persistent pole of attraction. The particular relation between speed and work in the left limb can be characterized as follows: for both groups together, the increase in produced relative work for the unidirectional movement was found relatively shallow up to 70% of maximal speed (Speed Levels 1 to 5); beyond this landmark, the slope of the function was much steeper. Thus, the graphs for work under bimanual as compared to unimanual practice conditions gradually diverged. Inspection of the amount of relative work generated for production of the reversal movement provides additional insights (fig. 30: A very similar function across both performance conditions was found between speed and work generated for the reversal movement. We suggest, therefore, that the excess of work found in the left limb for production of the unidirectional movement was to a large extent a function of the amount of work generated for the reversal movement, thereby being less determined by the net requirements for the unidirectional task itself. This points to neural crosstalk or overflow of activity between the limbs’ control centers with the direction of overflow mainly determined by the most energetically demanding task. Neural overflow has been observed experimentally in the electrical activity of the left elbow flexors and extensors in a study where a related task combination was used (Swinnen et al. 1988; Swinnen et al. 1991d). Consequently, a crucial requirement for successful movement dissociation may be to confine the neural crosstalk between the cerebral loci responsible for limb control (Marteniuk et al. 1984; Swinnen 1992; Swinnen and Walter 1988). This capability is also denoted as ‘segregation’ and may be an important requirement for simultaneously producing actions with different spatiotemporal features, as in producing polyrhythms.

S.P. Swinnen et al. / Speed and upper-limb coupling

631

The present findings converge with other studies that have shown high degrees of motor overflow to the contralateral limb during isometric force production tasks, especially beyond 75% of maximal volitional force (Todor and Lazarus 1986a, 1986b). Whereas most of these studies were conducted with children performing simple one-degree of freedom tasks, the present findings show that such overflow phenomena are also prevalent during the simultaneous performance of discrete limb movements in normal adults. It is important to note here that it may not be speed as such but the higher torque requirements associated with producing high speed movements that induces the greater synchronization effects (Walter and Swinnen 1990a, 1990b). Torque or force-time histories may be important variables under control of the central nervous system. However, we cannot exclude the possibility that speed affects interlimb coupling independent of torque, because the range of speeds that were manipulated independent of torque in the Walter and Swinnen (1990b) study was fairly limited. Inspection of the relations between work and the between- and within-movement correlations suggests that the metrics are not totally independent from the movement structure. Higher degrees of work in the left limb are associated with (a) stronger coupling of the acceleration patterns generated in both limb movements and (b) less successful production of the left limb acceleration pattern under bimanual as compared to unimanual performance conditions. It remains unclear at this point whether one variable can be assigned priority over the other. The typical interactions at the metrical and structural level of movement specification that were observed in the present study are not a consequence of the order in which the tasks were practiced. Similar interactions have been observed in related studies where subjects started with bimanual practice without any previous unimanual practice (Swinnen and Walter 1991b; Swinnen et al. 1991b) Moreover, the observed data not only hold for the group as a whole but reflect the majority of the individual subjects’ control strategies. For example, the metrical dissociation ratio during bimanual practice was lower than during unimanual practice in all subjects. With one exception, all subjects showed increases in work generated for production of the unidirectional movement under bimanual as compared to unimanual conditions. The effects for the reversal movement were less

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uniform: thirteen subjects displayed a tendency for decreased work scores under bimanual conditions, two subjects showed similar scores, whereas the five remaining subjects showed an small increase in work. Whereas the goal of the present study was mainly to provide a detailed description of interlimb interactions during discrete dual-task performance, it is conceivable that the observed interaction effects are mediated by attention and limitations in the division of attention. Insights into these phenomena, however, can only be gained in subsequent studies where the effect of directing the subject’s attention to the subtasks on intertask interactions is addressed. Learning to dissociate actions through speed adjustments The present findings clearly demonstrate that simultaneously producing movements with different spatiotemporal patterns becomes more difficult at higher movement speeds. Stated differently: during the initial stages of practice, differentiation of action can be more successfully accomplished at decreased speed requirements. Therefore, when learning new movement topologies involving incompatible limb patterns, it may be preferable to practice these movements initially at low speeds (Walter and Swinnen 1992). However, the present findings also indicate that merely proceeding from low to high speeds will not prevent the gradual emergence of the pull of attraction towards the preferred in-phase coordination mode. Rather, practice should be accompanied with the appropriate instructional techniques and feedback sources. Once the correct pattern becomes established under these circumstances, movement speed could be scaled up gradually towards the required target speed. A similar gradual ‘speed up strategy’, also known as Poppelreuter’s law of practice, was debated in the forties and fifties, but for different reasons (Fulton 1942; Solley 1952). Basically, the law fits with the speed-accuracy trade-off in stating that speed should be reduced in the early stages of practice in favor of accuracy. More recently, specialists in human factors training have investigated a related technique, called adaptive training, mostly with respect to the acquisition of tracking tasks (Lintern and Gopher 1978; Mane 1984; Wickens 1989). Broadly defined, adaptive training refers to the adjustment of task difficulty in relation to the subject’s level of performance. The adaptive variable or task feature being adjusted to change difficulty

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level can relate to perceptual, response, and/or feedback variables. It is noteworthy here that the biophysical properties of movement have hardly been addressed in this respect. Walter and Swinnen (1992) found that subjects practicing a bimanual task (similar to the present one) at gradually increasing speeds were more successful in acquiring the correct movement topologies than a group that always practiced the movements at the target speed. In other words: ‘adaptive tuning’ of the task enhanced movement dissociation. It is important to add here that subjects did not regress back towards interlimb synchronization as movement speed was gradually increased, possibly through the aid of augmented information feedback. In addition, during the acquisition of various bimanual continuous and discrete tasks, we have found that learners initially slow down the movement spontaneously (Swinnen et al. 1991b,c; Swinnen and Walter 1991b). Apparently, it aids them in moving away from preferred modes (i.e., in-phase and anti-phase coordination), perhaps by giving them the benefit of coping more successfully with contralateral overflow and/or providing them with more time for processing the information associated with task execution. Task difficulty has traditionally been defined in relation to the speed-accuracy trade-off. However, task difficulty in multi-limb coordination might be considered in relation to preferred movement states to which the system is normally attracted. Tasks that are functionally close to these preferred modes are hypothesized to be much easier to learn than tasks that are more distant. The main goal of learning is then to move away from these preferred patterns, or, to reduce their pull of attraction. Skilled pianists show conclusive evidence that preferred movement states can indeed be abandoned, as demonstrated in the performance of polyrhythms (Shaffer 1982). In acquiring discrete bimanual tasks, we have demonstrated that knowledge of results and various forms of kinematic feedback aid subjects in moving from full interlimb synchronization to desynchronization of the limbs, while at the same time speeding up the movements to meet the required target time (Swinnen and Walter 1991b; Swinnen et al. 1992; Swinnen et al. 1990). Nevertheless, it remains to be investigated whether the strong interlimb synchronization effect under high speed (force) requirements is still evident after extensive practice. In summary, the present study has shown that interlimb coupling becomes stronger at higher movement speeds. The findings lead us to

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suggest that the strategy of initially reducing speed requirements may enhance the acquisition of new movement topologies. Slowing down performance reduces the force requirements of movement, resulting in lower degrees of neural crosstalk and better motor output differentiation 3 (Swinnen 1992; Swinnen and Walter 1991b). It serves to reduce or eliminate unwanted interlimb interactions, thereby alleviating the pull of attraction to preferred but incorrect interlimb coordination patterns. Perhaps, it provides learners the opportunity to gradually integrate inhibitory neural networks with the construction of facilitatory pathways as part of building the new plan of action. References Deutsch, D., 1983. The generation of two isochronous sequences in parallel. Perception and Psychophysics 34, 331-337. Enoka, R.M., 1988. Neuromechanical basis of kinesiology. Champaign, IL: Human Kinetics. Franz, L., H.N. Zelaznik and G. McCabe, 1991. Spatial topological constraints in a bimanual task. Acta Psychologica 77, 137-151. Fulton, R.E., 1942. Speed and accuracy in learning a ballistic movement. Research Quarterly 13, 30-36. Heuer, H., 1991. ‘Motor constraints in dual-task performance’. In: D.L. Damos (ed.), Multipletask performance. London: Taylor and Francis, Kelso, J.A.S., 1984. Phase transitions and critical behavior in human bimanual coordination. American Journal of Physiology: Regulatory, Integrative, and Comparative Physiology 15, 1000-1004. Kelso, J.A.S., J.P. Scholz and G. Schoner, 1986. Nonequilibrium phase transitions in coordinated biological motion: Critical fluctuations. Physics Letters 118, 279-284. Kelso, J.A.S., D.L. Southard and D. Goodman, 1979. On the coordination of two-handed movements. Journal of Experimental Psychology: Human Perception and Performance 2, 229-238. Kelso, J.A.S. and B. Tuller, 1984. ‘A dynamical basis for action systems’. In: MS. Gazzaniga (ed.), Handbook of cognitive neuroscience. New York: Plenum Press. Lintern, G. and D. Gopher, 1978. Adaptive training of perceptual-motor skills: Issues, results, and future directions. International Journal of Man-Machine Studies 10, 521-551. Mane, A.M., 1984. ‘Acquisition of perceptual-motor skill: Adaptive and part-whole training’. In: Proceedings of the Human Factors Society 28th Annual Meeting. Santa Monica, CA: Human Factors Society. pp 522-526. 3 At first sight, this conclusion may appear to contradict our findings in that the group who progressed from low to high speeds was not significantly more successful in interlimb dissociation than the group who progressed from high to low speeds. However, it should be clear that the present experiment was only intended to describe spontaneously emerging coupling tendencies at initiation of practice and under the imposed experimental conditions, without the deliberate intention to learn the task. Under optimized learning conditions with provision of appropriate information feedback sources (e.g., kinematic feedback), subjects do benefit from a practice schedule with progressively increasing speeds, as shown by Walter and Swinnen (1992).

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