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Organization of local anticipatory movements in single step initiation
EISEVIER
Human
Organization
Movement
Science
13 (1994) 195-210
of local anticipatory movements in single step initiation
Gilles Dietrich *, Yvon Breniere, Manh Cuong Do Laboratoire de Physiologic du Mouvement, U&l.
CNRS 631,
Universite' Paris-&d, 91405 France
Abstract The study deals with the segmental organization of single- and multi-step walking. Local (segmental) and global (center of gravity) anticipatory movements associated with the initiation of the stepping movement were investigated in 5 normal subjects. Each subject performed a single-step walking task under three conditions of step velocity. Global anticipatory phenomena were indicated by the acceleration of the center of gravity and displacement of the center of foot pressure. Local anticipatory movements, i.e. the accelerations of trunk, shoulders, hips and shanks were recorded simultaneously. The biomechanical data suggest that there are two distinct parts in the anticipatory phase. The earlier correspond to the static postural changes, which are correlated to the velocity of the forthcoming movements. As in multi-step walking, global and local accelerations are specific to the forthcoming movement. The latter part aims to counterbalance the disruption induced by heel-off and is related to postural necessities. These results suggest that the same parameters are programmed in the execution of the single- or multi-step walking process.
1. Introduction
Human voluntary nomena (Belenkii et limb movements was Nashner (1982). The
* Corresponding
movements are preceded by dynamic postural pheal., 1967). The posturo-kinetic organization of upper studied by Bouisset and Zattara (1981) and Cordo and anticipatory movements depend on the initial condi-
author.
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tions (Lipshits et al., 1981; Cordo and Nashner, 1982) and are specific to their parameters (Lee, 1980; Bouisset and Zattara, 1981; Zattara and Bouisset, 1986). Anticipatory movements similar to those which precede mono-articular movement also accompany complex movements, such as gait (Carlsoii, 1966; Herman et al., 1973; Mann et al., 1979; Breniere et al., 1981). Biomechanical studies of the gait initiation process (Breniere et al., 1987) have shown that the anticipatory movements along the progression axis are correlated with the parameters of gait, principally its velocity. However, the duration of the anticipatory movements along the lateral axis is independent of velocity. The aim of this work was to study the anticipatory phenomena in single-step execution and to compare these changes with the initiation of multi-step walking. If the initiation process for a single step is similar to the initiation of multi-step walking it could constitute a simple paradigm for the studies of gait initiation in disabled people. The anticipatory movements were studied with a force plate and complementary kinematic data were obtained from accelerometers. The force plate data indicated changes in the center of gravity during the initiation of single-step and multi-step walking, while the accelerometer data allowed a description the chronology of the anticipatory phenomena. This technique was used by Bouisset and Zattara (1981) to study local anticipatory movements.
2. Methods To relate anticipatory phenomena to movement execution, a relevant parameter must be considered. The instantaneous velocity of the center of gravity has been used to study multi-step walking (Breniere et al., 1987; 1988). The same parameter was used to compare single-step and multi-step walking. To initiate walking, the subject was instructed to perform the movement at three different speeds: slow (S), normal (N) and fast (F). 2.1. Experimental
device
Accelerations of the center of gravity were recorded from a force plate and the kinematics of body segments during the anticipatory phase with accelerometers. The force platform (fully described by Breniere et al., 1981) is an equilateral triangle (2 meters on each side) suspended by its apexes and immobilised by cables linked to a system of nine unidirectional
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extensiometry gauges (SEDEME, DC O-1000 N type). Three of these force transducers were vertical and six were horizontal. This type of suspension provides the mechanical system with six degrees of freedom. The mechanical characteristics of its constitutive parts give the loaded platform a natural frequency of 50 Hz. The frequency range of movements was l10 Hz. Six mono-axial accelerometers (ENTRAN, ECG-240-5D, +5 g> were mounted on supports so as to constitute two triaxial accelerometers and attached to different body segments on appropriately shaped splints. The active axis of each set of accelerometers and the axis of the force plate were initially parallel. Thus, measurements of local acceleration during this phase were not distorted by gravitational acceleration. The locations of the accelerometers were chosen after preliminary experiments, so as to record the accelerations of the shank, thigh, forearm, trunk, wrist and head accurately. The displacement of the axes of the accelerometer was checked using an opto-electronic system (SELSPOT II). This experiment showed that there was no displacement of the accelerometers axes during the anticipation phase. The accelerometers were finally placed at the shoulders (Sh), hips (H), thorax (Th) and shank (S) (Fig. 11, corresponding to five groupings (five sessions): Session 1 (ipsilateral Hip (Hi), contralateral Hip (Hc)), Session 2 (Hi, ipsilateral Shoulder (Shi)), Session 3 (Hi, contralateral Shoulder (She)), Session 4 (Hi, Thorax (Th)) and Session 5 (She, Shi). 2.2. Data acquisition Analog data (sampling interval, 5 ms) from the force plate and the accelerometers was acquired, processed and stored on a PDP 11/34. Data collection was triggered at the same time as stepping foot heel-off (HO). This was sensed by force transducers placed under both feet. The system made it possible to record data over a three-second interval: one second before heel-off, and two seconds after. The center of gravity (CG) acceleration (?G) can be calculated, from Newton’s first law, as:
where m is the subject’s mass, l? the subject’s weight and R’ the ground reaction force, which is calculated from the force plate (Fig. 1). The three components of the acceleration of the CG (X’G, Y”G and Z”G) were
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0
Fig. 1. Experimental procedure. Location of the accelerometers 6, Shanks; H, hips; Th, trunk; and Sh, shoulders); R’ is the resultant of the ground reaction on the force plate; V? is the subject’s weight and G is the location of the center of gravity (03).
calculated from this first equation. The antero-posterior component of the velocity of the CG (X’G) was calculated from the acceleration of the CG. The position of the center of foot pressure (XP and W) was calculated as the barycenter of the vertical forces. The force was determined with a precision of 0.5 Newton. The accuracy of the position of the center of foot pressure was 2 millimeters and acceleration was measured with an accuracy of 5 mm.sK2. 2.3. Experimental procedure Each of the five subjects took part in five experimental sessions, corresponding to five different triaxial accelerometer groupings. The subjects stood upright, barefoot and motionless on the force plate and performed multi-step and single-step walking at three speeds (N, F, S). The signal to
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start walking was given by turning off a light. The subject always started from the same marks on the force plate. He/she performed the movement (multi-step and single-step walking) in each series seven times, corresponding to the three experimental conditions. Data was analyzed by analysis of variance (ANOVA) plus Newman-Keuls test.
3. Results 3.1. Global anticipatory phenomena Anticipatory
movements
Anticipatory changes in the center of gravity (CG) along the progression axis were analyzed from the onset (tx) of CG acceleration (X”G) to heel-off time (HO). The antero-posterior CG acceleration (X”G) was positive during this anticipatory phase (Fig. 21, i.e. directed forward, reversed during the execution phase, after HO, and was zero at time tv (time to peak velocity). The CG acceleration along the lateral axis (Y”G) started at time ty before HO. This acceleration was positive and directed to the forthcoming stance foot during the anticipatory phase, i.e., from ty to HO (@-HO). The duration of global anticipatory movement (HO-& and HO-@) was measured along the progression axis and lateral axis between the first global dynamic events (tx and ty> and HO in both single-step and multi-step walking. In single-step walking, the mean duration of the anticipatory phase, along the progression axis (HO-tx) increased from 0.335 + 0.058 s (mean + sd) to 0.439 + 0.048 s (Table 1) and the influence of the experimental condition was highly significant (F(2,8) = 84.5, p < 0.001). In multi-step walking, the duration of the anticipatory phase ranged from 0.492 s f 0.058 to 0.600 s f 0.048 s and the differences between experimental conditions were highly significant as well (F(2,8) = 112.5, p < 0.001). Along the lateral axis, the duration of anticipatory phenomena (HO-ty) in single step ranged from 0.334 + 0.051 s to 0.397 + 0.033 s. The F value, (F(2,8) = 8.5), indicated no significant differences. The initial coordinates of foot pressure (XPO, WO) before the start signal indicated the initial posture of the subject. The results (Table 2) show that the antero-posterior coordinate of the initial position depended on the experimental conditions in a significant way, between conditions S
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I I
0.2
m/s
0.2
m
1 IILs-z I
tx
I HO
500
ms
Fig. 2. Force plate records (‘N’ condition trial). X”G: CG acceleration, forward (f) and backward (b), along the antero-posterior axis (OX); tx: onset of change in acceleration along OX; Y”G: CC acceleration left (I) and right (r), along the lateral axis COY); fy: onset of acceleration change along the OY axis; Z”G: acceleration of G up (u) and down cd), along the vertical axis (OZ); XP: displacement of the foot pressure along the OX axis; YP: displacement of center of foot pressure along the OY axis; X’G: instantaneous velocity of the center of gravity along the OX axis; VI instantaneous peak velocity; HO: heel-off time.
(t = 3.16, p < 0.01) and between conditions F and N (t = 1.98, p < 0.05). The displacement of the center of foot pressure (XP and YP) started at time tx. The center of foot pressure moved backwards and towards the forthcoming stepping foot during the anticipatory phase.
Velocity of the center of gravity
The velocity of the CG (X’G) along the progression axis was calculated from the acceleration by Euler’s algorithm of integration. This progression velocity reached a maximum value ‘V’ at time ‘tv’. The progression velocity has an initial value ‘~0’ at HO. The mean values of the peak progression velocity (V) in single step ranged from 0.617 f 0.101 m/s to 1.448 L-0.110 m/s depending on experimental conditions (Table 3). It varied significantly (F(2,8) = 159.0,
G. Dietrich et al. /Human Table 1 Duration of ‘global’ anticipatory experimental conditions Subject
movements.
Single-Step
Movement
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Mean values ( f sd) are given for 35 trials in each of the 3
walking
&-HO(s) Mean + sd Condition
Multi-step
walking
tr-HO(s) Mean + sd S
Sl s2 s3 s4 s5
0.401 0.397 0.290 0.332 0.253
f 0.035 * 0.022 f 0.018 + 0.03 1 f 0.037
Group
0.335 f 0.058 Condition
201
0.502 0.455 0.525 0.490 0.489
k * k + f
0.035 0.022 0.018 0.03 1 0.037
0.492 f 0.058
N
Sl s2 s3 s4 S5
0.429 0.436 0.394 0.394 0.328
Group
0.389 f 0.041
0.619 0.523 0.455 0.523 0.483
+ 0.040 f 0.026 f 0.035 f 0.020 f 0.020
+ f + + f
0.039 0.058 0.025 0.041 0.025
0.521+ 0.061
Condition F Sl s2 s3 s4 s5
0.478 0.502 0.407 0.447 0.368
Group
0.439 * 0.048
tx-HO:
duration
of global
0.620 0.669 0.594 0.550 0.569
+ 0.023 f 0.034 f 0.030 + 0.037 k 0.021
anticipatory
+_0.026 + 0.030 f 0.043 f 0.070 f 0.011
0.600 k 0.048 movements.
p < 0.001). The mean I/ values in multi-step
walking varied from 1.070 + 0.070 m/s to 1.730 + 0.110 m/s (Table 3) and the mean values were also significantly different (p < 0.001) according to the experimental conditions.
Relationship between CG progression velocity and the duration of anticipatory movements
The duration of global anticipatory movements (time tx to HO) in single-step and multi-step walking was linearly related to the progression velocity of the CG (Fig. 3). The correlation coefficients were respectively r = 0.75 - (p < 0.01) and r = 0.79 - ( p < 0.01) and the two slope values of the linear regression were close, respectively 0.138 m/s and 0.140 m/s.
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Table 2 Initial location of the center of of pressure (XPO YPO) Subject
XPO (m) Mean + sd Condition
Sl s2 s3 s4 s5
ITO (ml Mean + sd S
0.428 + 0.002 0.396 f 0.005 0.432+ 0.010 0.411+ 0.008 0.414 & 0.009
1.026 + 1.006 + 1.017 + 1.009 f 1.016 f
0.035 0.035 0.035 0.035 0.025
1.026 f 1.008 f 1.017 f 1.009 f 1.014 f
0.030 0.020 0.030 0.025 0.025
1.027 f 1.007 f 1.017 f 1.011+ 1.013 f
0.035 0.030 0.035 0.035 0.020
Condition N Sl s2 s3 S4 s5
0.429 f 0.005 0.397 f 0.007 0.440 + 0.007 0.410 f 0.008 0.414 + 0.009 Condition F
Sl s2 s3 s4 s5
0.430 +_0.005 0.398 + 0.006 0.451 f 0.010 0.417 + 0.008 0.418 + 0.009
3.2. Local anticipatory phenomena For a given experimental condition, the amplitude and the time of each acceleration were reproducible for both individuals and between individuals (Fig. 4). Progression axis
The local accelerations along the antero-posterior axis (OX) started before HO. The mean duration of local anticipatory accelerations ranged from 0.198 + 0.044 s to 0.517 &-0.050 s depending on the experimental conditions (Table 4). The shanks accelerated along the antero-posterior axis before tx (3090 ms for the contralateral shank SC). The initial acceleration of all the body segments and the center of gravity was positive, i.e. directed forward. The onsets of shoulder acceleration (Shix and Shcx) coincided with the
G. Dietrich et al. /Human Table 3 Peak of velocity. The velocity was calculated values (* sd) are given for 35 trials Subject
Movement
by integrating
Science 13 (1994) 195-210
the antero-posterior
Single step
Multi-step walking
Peak velocity
Peak velocity (m/s) Mean + sd
(m/s) Mean + sd
CG acceleration.
203
Mean
Condition S Sl s2 s3 s4 SS
0.624+0.116 0.794 f 0.017 0.615 f 0.032 0.489 f 0.020 0.564 f 0.018
1.120*0.050 0.880 f 0.035 1.140+0.060 1.100~0.080 1.020 f 0.060
Group
0.617+0.101
1.070~0.070
Condition N Sl s2 s3 s4 s5
0.917 1.046 0.892 0.962 0.923
f 0.033 & 0.027 f 0.030 f 0.008 k 0.023
Group
0.948 f 0.054
1.460 f 0.040 1.210+0.050 1.380+0.010 1.440 f 0.040 1.260 + 0.020 1.350 + 0.06
Condition F Sl s2 s3 s4 s5
1.467+0.038 1.586 f 0.021 1.274+0.100 1.531~0.031 1.382~0.008
1.540+ 0.060 1.900 f 0.070 1.800f0.120 1.700 + 0.080 1.720+0.140
Group
1.448+0.110
1.730f0.110
onset of the CG acceleration and these accelerations were positive throughout the anticipatory phase. The time course of ipsilateral hip acceleration was diphasic during the anticipatory phase: acceleration was initially positive and reached a negative peak at HO. In contrast, the contralateral hip showed no significant acceleration during this phase. Lateral axis The contralateral and ipsilateral hips both accelerated along the lateral axis (OY) before ty (onset of lateral CG acceleration). These accelerations
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V b/s) 2.0
1.6
1.2
i -
0.8
0.4
0
J
0
200
400
600
tx-HO (ms) Fig. 3. Relationship between the duration of global anticipatory movements (&-HO) along the progression axis and the peak of the CG instantaneous velocity (V) in single step (0) and multistep walking (W ).
were initially positive, i.e. directed towards the stance foot, but reached a negative peak at HO, as did the ipsilateral hip along the OX axis. The shoulders (Shi and She) accelerated at the same time as the CG. These accelerations were positive during the anticipatory phase and had the same time course as the CG. Vertical axis
There were no anticipatory movements along the vertical axis. All the acceleration traces show oscillations which were simultaneous to the vertical acceleration of the center of gravity, except for the contralateral hip, which showed no significant acceleration during the anticipatory phase. Relationship between global and local accelerations
There was a highly significant linear correlation (r = 0.92, p < 0.001) between the onset of the ipsilateral shoulder (Shi) acceleration along the progression axis (ShLr) and the onset of global acceleration (tx) (Fig. 5) The linear regression equation (Shti = 1.02 tx - 0.003) suggested that the two occurred at the same time.
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(1 150
205
m.s”
Ins
Fig. 4. Local accelerations. Accelerations are displayed according to the different segments and joints: shank (S), hips (H), shoulders (Sh) and thorax (Th), ipsilateral 6) and contralateral (c) side along the three absolute axes: OX, anteroposterior axis; OY, lateral axis, 02, vertical axis; CG: acceleration of the center of gravity, calculated from the force plate; tx and fy are the onset of dynamic CG phenomena.
4. Discussion
The step initiation phase can be considered as a transition phase between upright posture and level walking. A major problem during multi-step walking is the maintenance of dynamic equilibrium. Stereotypic movement patterns of body segments (trunk, shoulders, arms and pelvis) have been observed (Elftmann, 1939; Murray et al., 1964; Cappozzo, 1981; Thorstensson et al., 1982; 1984; Stokes et al., 1989) which seem to be organized to facilitate gait and minimize force and energy expenditure (Townsend, 1981; Cappozzo, 1983). The present data described a locomotor process which appears during the anticipatory period. It suggests that there are two parts to these anticipatory movements.
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Table 4 Duration
of local anticipatory
Subject
t Six(s)
t Scn(s)
t Hi.&)
t Shix(s)
t Shcx(s)
t Thx(s)
Mean f sd
Mean f sd
Mean f sd
Mean f sd
Mean f sd
Mean + sd
0.480 0.480 0.362 0.403 0.320
0.334 0.406 0.286 0.342 0.298
0.416 0.358 0.281 0.345 0.260
0.415 f 0.360 f 0.252+ 0.354 + 0.220 +
0.436 0.358 0.331 0.374 0.300
movement
along the antero-posterior
axis in single-step
walking
Condition S Sl s2 s3 s4 s5
0.383 0.456 0.351 0.380 0.308
Group
0.375 f 0.063
f + f + f
0.012 0.025 0.019 0.020 0.022
f f + f +
0.010 0.017 0.021 0.031 0.020
f f k f f
0.009 0.030 0.022 0.037 0.017
0.409 f 0.050
0.333 * 0.044
0.388 0.447 0.354 0.417 0.376
* f f + +
0.022 0.015 0.005 0.006 0.012
0.050 0.040 0.010 0.033 0.030
f 0.021 + 0.012 kO.033 f 0.017 + 0.009
0.333 f 0.045
0.323 ?r:0.080
0.360 f 0.046
0.460 0.430 0.446 0.440 0.355
0.390 f 0.415 f 0.340* 0.455 + 0.306 f
0.428 0.403 0.376 0.430 0.370
Condition N f f f + f
0.060 0.006 0.010 0.025 0.019
0.037 0.032 0.031 0.015 0.036
+ f + f f
0.055 0.020 0.015 0.012 0.017
Sl s2 s3 s4 s5
0.402 f 0.022 0.474 f 0.025 0.381 f 0.033 0.392 f 0.029 0.319f0.030
0.465 f 0.501* 0.395 + 0.422 + 0.335 f
Group
0.394 f 0.045
0.424 + 0.052
0.396 f 0.036
0.406 f 0.040
0.380 f 0.050
0.401 f 0.025
0.473 0.485 0.395 0.440 0.340
0.487 0.470 0.393 0.460 0.365
0.501 0.437 0.353 0.439 0.350
0.037 0.022 0.041 0.028 0.030
f f f f +
0.040 0.059 0.006 0.014 0.007
Condition F Sl s2 s3 s4 s5
0.522 f 0.509 f 0.430* 0.512 f 0.363 f
0.033 0.036 0.041 0.033 0.040
0.535 f 0.031 0.514 + 0.022 0.430 f 0.037 0.517*0.041 0.420 f 0.022
0.446 + 0.504 f 0.410* 0.440 f 0.403 f
Group
0.468 + 0.070
0.483 + 0.061
0.440 f 0.040
Six:ipsilateral contralateral
shank; Sex: contralateral shoulder.
shank,
0.014 0.013 0.033 0.036 0.016
Hix:
* + f + +
0.010 0.016 0.015 0.030 0.030
0.429 f 0.050 ipsilateral
f 0.012 f 0.044 +_0.022 f 0.040 + 0.025
0.435 f 0.053
hip, Shix:
ipsilateral
f f f f f
0.011 0.032 0.027 0.033 0.021
0.416 f 0.057 shoulder,
Shcx:
(a) The first one is associated with a change in the initial position of the subject before the onset of dynamic phenomena. Indeed, the antero-posterior abscissa of the center of foot pressure in the initial position (XPO), shifts forward as a function of the peak velocity of the forthcoming movement. Given that (i) the subject started all the movements from the same mark, and (ii) the subject was static (there is no noticeable acceleration in the initial position) the ground projection of the center of gravity and the center of pressure are superimposed (Thomas and Whitney, 1959; Gurfinkel, 1973; Gurfinkel et al., 1976; Breniere et al., 1981). Thus the forward initial shift corresponds to a forward displacement of the center of gravity and initial velocity is going to increase in proportion to the subject’s
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t&ix
b-4 600
I
2004 200
300
400 tx
500
600
700
b)
Fig. 5. Relationship between the onset of the acceleration onset of CG acceleration (tx) along the progression axis.
of the ipsilateral
shoulder
(Shti)
and the
inclination. Consequently, this preparation adjustment can be considered as the early part of the anticipatory process. The duration of global anticipatory movements (&-HO) is correlated with the velocity of the center of gravity along the progression axis (Fig. 3) in both single-step and multi-step walking. The peak progression velocity in multi-step walking is higher than in single step. Nevertheless, the relationship between the peak velocity and the anticipation duration seems to be similar. These results may be considered as a consequence of the biomechanical constraints which are the same in both situations. Indeed, anticipatory movements reflect the need to generate the propulsive forces required to initiate center of gravity displacement (Breniere et al., 19871, and to simultaneously transfer the body weight to the forthcoming stance foot (Roberts, 1978). The dynamic anticipatory phenomena are also represented by the local accelerations before heel-off. The acceleration data shows anticipatory phenomena occur along the antero-posterior and lateral axis before heel-off in all segment levels, except in the contralateral hip. The shanks accelerate first along the antero-posterior axis, before the onset of CG acceleration. The onset and direction of shank acceleration are consistent with the electromyographic phenomena recorded in gait initiation (Carlsiiij, 1966; Herman et al., 1973; Breniere et al., 1981): inhibition of the Soleus and
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activation of the Tibialis Anterior. However, these shank accelerations precede the CG acceleration, i.e. they showed no effect on the center of gravity which is a resultant phenomenon. The shank mass being small compared to the body mass, its shank acceleration could have no dynamic consequence, in terms of force, on the center of gravity. There may also be other local accelerations which compensate for the shank acceleration. But this is improbable, because no local acceleration before or simultaneous to the onset of the shank acceleration was recorded in this study and no limb movement has been recorded in other studies (Herman et al., 1973). The onset of local acceleration of the ipsilateral shoulder along the progression axis occurred at the same time as the onset of CG acceleration (Fig. 5). This simultaneous onset of local and global accelerations suggests that the accelerations produced at the shoulder level contribute a great deal to generating the propulsive forces needed to execute voluntary movement. The acceleration of the center of gravity along the progression axis appears to be mainly due to shoulder acceleration. As in multi-step walking, local movements seem to be specific to the forthcoming movement and can be interpreted as local organization of these movements which control the execution of the step. (b) The second component of anticipatory movement is the postural adjustment to the disruption induced by heel-off. The time courses of local and global accelerations at heel-off can be analyzed in terms of postural adjustments and disruptions resulting from heel-off (Breniere and Dietrich, 1992). The three components of the acceleration of the ipsilateral hip are large (negative peak) at heel-off. The hip accelerates backwards, downwards and towards the stepping foot in a falling movement corresponding to the lifting of the heel. These changes in local acceleration can be interpreted as a consequence of the disruption induced by the vertical fall of the center of gravity occurring at HO. The contralateral hip does not accelerate along the progression and lateral axes, and is a fixed point for the movement in these directions. In contrast, the upper trunk (shoulders and thorax) show no significant change in local acceleration at heel-off. Thus, these segments are not affected by the disruption created by heel-off. This lack of local disruption of the upper trunk indicates that local accelerations balance the effect of the negative hip acceleration. This implies that the upper part of the trunk is dissociated from the pelvic segment during the anticipatory movement. The local anticipatory movements of the upper trunk appear to be orga-
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nized to minimize the disruption provoked by heel-off. This disruption has only a local pelvic effect, mainly at ipsilateral hip. All the initial accelerations are positive along the antero-posterior and lateral axes, i.e. they oppose the disruption (negative acceleration). Thus, the anticipatory movements anticipate not only the aim of gait initiation (i.e. displacement, velocity), but also the disruption associated with heel-off. These anticipatory movements include ‘postural adjustments’, according to the hypothesis that postural adjustments create the initial dynamic conditions required to balance the effect of the voluntary movement which tend to disturb the postural equilibrium (Bouisset and Zattara, 1987). In conclusion, the initiation of single step seems to include postural functions related to the disruption associated with heel-off, the transfer of body weight to the forthcoming foot, and dynamic changes required to create the propulsive forces necessary to initiate displacement of the body CG (Breniere et al., 1987). These different components of the initiation process are reflected in local acceleration of body segments.
References Belenkii, Y.Y., V. Gurfinkel and Y.I. Paltsev, 1967. Element of control of voluntary movements. Biofizika 12, 135-141. Bouisset, S. and M. Zattara, 1981. A sequence of postural movements precedes voluntary movement. Neuroscience Letters 22, 263-270. Bouisset, S. and M. Zattara, 1987. Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement. Journal of Biomechanics 20, 735-742. Breniere, Y. and G. Dietrich, 1992. Heel-off perturbation during gait initiation: Biomechanical analysis using triaxal accelerometry and a force plate. Journal of Biomechanics 25, 121-127. Breniere, Y., G. Dietrich and M.C. Do, 1988. ‘Analytical expression of anticipatory movements in gait initiation’. In: G. de Groot, A.P. Hollander, P.A. Huijing and G.J. van IngenSchenau. (Eds.), Int. series on biomechanics, Vol. 7-A, Biomechanics XI-A (pp. 371-376). Amsterdam: Free University Press. Breniere, Y. and M.C. Do, 1986. When and how does steady state gait movement induced from upright posture begin? Journal of Biomechanics 19, 1035-1040. Breniere, Y. and M.C. Do, 1987. Mouvements et ajustements posturaux anticipateurs de la marche. Journal de Biophysique et Biomecanique 10, 39-40. Breniere, Y., M.C. Do and S. Bouisset, 1987. Are dynamic phenomena prior to stepping essential to walking? Journal of Motor Behavior 19, 62-76. Breniere, Y., M.C. Do and J. Sanchez, 1981. A biomechanical study of the gait initiation process. Journal de Biophysique et Medecine NuclCaire 5, 197-205. Cappozzo, A., 1981. Analysis of the linear displacement of the head and trunk during walking at different speeds. Journal of Biomechanics 14, 411-425.
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Cappozzo, A., 1983. The forces and couples in the human trunk during level walking. Journal of Biomechanics 16, 265-277. Carl&, S., 1966. The initiation of walking. Acta Anatomica 65, l-9. Cook, T. and B. Cozzens, 1976. ‘The initiation of gait’. In: R.M. Herman, S. Grillner, R.S. Stein and P.S.G. Stuart (Eds.), Neural control of locomotion (pp. 65-76). New York Plenum Press. Cordo, P.J. and L.M. Nashner, 1982. Properties of postural movements related to a voluntary movement. Journal of Neurophysiology 47, 287-303. Elftmann, H., 1939. The rotation of the body in walking. Arbeitsphysiologie, 477-484. Gurfinkel, E.V., 1973. Physical foundations of stabilography Agressologie. 14, 9-14. Gurfinkel, E.V., MI. Lipshits, S. Mori and K.E. Popov, 1976. Postural reactions of the controlled sinusoidal displacement of the supporting platform. Agressologie 17, 71-76. Herman, R., T. Cook, B. Cozzens and W. Freedman, 1973. ‘Control of postural reactions in man: The initiation of gait’. In: R.S. Stein, K.G. Pearson, R.S. Smith and J.B. Redford, Control of posture and locomotion (pp. 363-388). New York Plenum Press. Lee, W.A., 1980. Anticipatory control of postural and task muscles during rapid arm flexion. Journal of Motor Behavior 12, 185-196. Lipshits, MI., K. Mauritz and K.E. Popov, 1981. Quantitative analysis of anticipatory postural components of a complex voluntary movement. Translated from Fiziologiya Cheloveka 7, 411-419. Mann, R.A., J.L. Hagy, V. White and D. Liddell, 1979. The initiation of gait. Journal of Bone and Joint Surgery 61-A, 232-239. Murray, M.P., A.B. Dought and R.C. Kory, 1964. Walking pattern of normal men. Journal of Bone and Joint Surgery 46, 335. Nashner, L.M., 1977. Fixed patterns of rapid postural responses among leg muscles during stance. Experimental Brain Research 30, 13-24. Roberts, T.D.M., 1978. Neurophysiology of postural mechanisms. London: Butterworth. Stokes, V.P., C. Andersson and H. Forssberg, 1989. Rotational and translational movement features of pelvis and thorax during adult human locomotion. Journal of Biomechanics 22, 43-50. Thomas, D.P. and R.J. Whitney, 1959. Postural movements during normal standing in man. Journal of Anatomy 93, 525-539. Thorstensson, A., H. Carlson, M.R. Zomlefer and J. Nilsson, 1982 Lumbar back muscle activity in relation to trunk movements during locomotion in man. Acta Physiologica Scandinavica 116, 13-20. Thorstensson, A., J. Nilsson, H. Carlson, and M.R. Zomlefer, 1984. Trunk movements in human locomotion. Acta Physiologica Scandinavica 121, 9-22. Townsend, M.A., 1981. Dynamics and coordination of torso motions in human locomotion. Journal of Biomechanics 14, 727-738. Zattara, M. and S. Bouisset, 1986. Chronometric analysis of posturo-kinetic programming of voluntary movement. Journal of Motor Behavior 18, 215-223.
Human
Organization
Movement
Science
13 (1994) 195-210
of local anticipatory movements in single step initiation
Gilles Dietrich *, Yvon Breniere, Manh Cuong Do Laboratoire de Physiologic du Mouvement, U&l.
CNRS 631,
Universite' Paris-&d, 91405 France
Abstract The study deals with the segmental organization of single- and multi-step walking. Local (segmental) and global (center of gravity) anticipatory movements associated with the initiation of the stepping movement were investigated in 5 normal subjects. Each subject performed a single-step walking task under three conditions of step velocity. Global anticipatory phenomena were indicated by the acceleration of the center of gravity and displacement of the center of foot pressure. Local anticipatory movements, i.e. the accelerations of trunk, shoulders, hips and shanks were recorded simultaneously. The biomechanical data suggest that there are two distinct parts in the anticipatory phase. The earlier correspond to the static postural changes, which are correlated to the velocity of the forthcoming movements. As in multi-step walking, global and local accelerations are specific to the forthcoming movement. The latter part aims to counterbalance the disruption induced by heel-off and is related to postural necessities. These results suggest that the same parameters are programmed in the execution of the single- or multi-step walking process.
1. Introduction
Human voluntary nomena (Belenkii et limb movements was Nashner (1982). The
* Corresponding
movements are preceded by dynamic postural pheal., 1967). The posturo-kinetic organization of upper studied by Bouisset and Zattara (1981) and Cordo and anticipatory movements depend on the initial condi-
author.
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B.V. All rights reserved
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tions (Lipshits et al., 1981; Cordo and Nashner, 1982) and are specific to their parameters (Lee, 1980; Bouisset and Zattara, 1981; Zattara and Bouisset, 1986). Anticipatory movements similar to those which precede mono-articular movement also accompany complex movements, such as gait (Carlsoii, 1966; Herman et al., 1973; Mann et al., 1979; Breniere et al., 1981). Biomechanical studies of the gait initiation process (Breniere et al., 1987) have shown that the anticipatory movements along the progression axis are correlated with the parameters of gait, principally its velocity. However, the duration of the anticipatory movements along the lateral axis is independent of velocity. The aim of this work was to study the anticipatory phenomena in single-step execution and to compare these changes with the initiation of multi-step walking. If the initiation process for a single step is similar to the initiation of multi-step walking it could constitute a simple paradigm for the studies of gait initiation in disabled people. The anticipatory movements were studied with a force plate and complementary kinematic data were obtained from accelerometers. The force plate data indicated changes in the center of gravity during the initiation of single-step and multi-step walking, while the accelerometer data allowed a description the chronology of the anticipatory phenomena. This technique was used by Bouisset and Zattara (1981) to study local anticipatory movements.
2. Methods To relate anticipatory phenomena to movement execution, a relevant parameter must be considered. The instantaneous velocity of the center of gravity has been used to study multi-step walking (Breniere et al., 1987; 1988). The same parameter was used to compare single-step and multi-step walking. To initiate walking, the subject was instructed to perform the movement at three different speeds: slow (S), normal (N) and fast (F). 2.1. Experimental
device
Accelerations of the center of gravity were recorded from a force plate and the kinematics of body segments during the anticipatory phase with accelerometers. The force platform (fully described by Breniere et al., 1981) is an equilateral triangle (2 meters on each side) suspended by its apexes and immobilised by cables linked to a system of nine unidirectional
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extensiometry gauges (SEDEME, DC O-1000 N type). Three of these force transducers were vertical and six were horizontal. This type of suspension provides the mechanical system with six degrees of freedom. The mechanical characteristics of its constitutive parts give the loaded platform a natural frequency of 50 Hz. The frequency range of movements was l10 Hz. Six mono-axial accelerometers (ENTRAN, ECG-240-5D, +5 g> were mounted on supports so as to constitute two triaxial accelerometers and attached to different body segments on appropriately shaped splints. The active axis of each set of accelerometers and the axis of the force plate were initially parallel. Thus, measurements of local acceleration during this phase were not distorted by gravitational acceleration. The locations of the accelerometers were chosen after preliminary experiments, so as to record the accelerations of the shank, thigh, forearm, trunk, wrist and head accurately. The displacement of the axes of the accelerometer was checked using an opto-electronic system (SELSPOT II). This experiment showed that there was no displacement of the accelerometers axes during the anticipation phase. The accelerometers were finally placed at the shoulders (Sh), hips (H), thorax (Th) and shank (S) (Fig. 11, corresponding to five groupings (five sessions): Session 1 (ipsilateral Hip (Hi), contralateral Hip (Hc)), Session 2 (Hi, ipsilateral Shoulder (Shi)), Session 3 (Hi, contralateral Shoulder (She)), Session 4 (Hi, Thorax (Th)) and Session 5 (She, Shi). 2.2. Data acquisition Analog data (sampling interval, 5 ms) from the force plate and the accelerometers was acquired, processed and stored on a PDP 11/34. Data collection was triggered at the same time as stepping foot heel-off (HO). This was sensed by force transducers placed under both feet. The system made it possible to record data over a three-second interval: one second before heel-off, and two seconds after. The center of gravity (CG) acceleration (?G) can be calculated, from Newton’s first law, as:
where m is the subject’s mass, l? the subject’s weight and R’ the ground reaction force, which is calculated from the force plate (Fig. 1). The three components of the acceleration of the CG (X’G, Y”G and Z”G) were
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0
Fig. 1. Experimental procedure. Location of the accelerometers 6, Shanks; H, hips; Th, trunk; and Sh, shoulders); R’ is the resultant of the ground reaction on the force plate; V? is the subject’s weight and G is the location of the center of gravity (03).
calculated from this first equation. The antero-posterior component of the velocity of the CG (X’G) was calculated from the acceleration of the CG. The position of the center of foot pressure (XP and W) was calculated as the barycenter of the vertical forces. The force was determined with a precision of 0.5 Newton. The accuracy of the position of the center of foot pressure was 2 millimeters and acceleration was measured with an accuracy of 5 mm.sK2. 2.3. Experimental procedure Each of the five subjects took part in five experimental sessions, corresponding to five different triaxial accelerometer groupings. The subjects stood upright, barefoot and motionless on the force plate and performed multi-step and single-step walking at three speeds (N, F, S). The signal to
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start walking was given by turning off a light. The subject always started from the same marks on the force plate. He/she performed the movement (multi-step and single-step walking) in each series seven times, corresponding to the three experimental conditions. Data was analyzed by analysis of variance (ANOVA) plus Newman-Keuls test.
3. Results 3.1. Global anticipatory phenomena Anticipatory
movements
Anticipatory changes in the center of gravity (CG) along the progression axis were analyzed from the onset (tx) of CG acceleration (X”G) to heel-off time (HO). The antero-posterior CG acceleration (X”G) was positive during this anticipatory phase (Fig. 21, i.e. directed forward, reversed during the execution phase, after HO, and was zero at time tv (time to peak velocity). The CG acceleration along the lateral axis (Y”G) started at time ty before HO. This acceleration was positive and directed to the forthcoming stance foot during the anticipatory phase, i.e., from ty to HO (@-HO). The duration of global anticipatory movement (HO-& and HO-@) was measured along the progression axis and lateral axis between the first global dynamic events (tx and ty> and HO in both single-step and multi-step walking. In single-step walking, the mean duration of the anticipatory phase, along the progression axis (HO-tx) increased from 0.335 + 0.058 s (mean + sd) to 0.439 + 0.048 s (Table 1) and the influence of the experimental condition was highly significant (F(2,8) = 84.5, p < 0.001). In multi-step walking, the duration of the anticipatory phase ranged from 0.492 s f 0.058 to 0.600 s f 0.048 s and the differences between experimental conditions were highly significant as well (F(2,8) = 112.5, p < 0.001). Along the lateral axis, the duration of anticipatory phenomena (HO-ty) in single step ranged from 0.334 + 0.051 s to 0.397 + 0.033 s. The F value, (F(2,8) = 8.5), indicated no significant differences. The initial coordinates of foot pressure (XPO, WO) before the start signal indicated the initial posture of the subject. The results (Table 2) show that the antero-posterior coordinate of the initial position depended on the experimental conditions in a significant way, between conditions S
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I I
0.2
m/s
0.2
m
1 IILs-z I
tx
I HO
500
ms
Fig. 2. Force plate records (‘N’ condition trial). X”G: CG acceleration, forward (f) and backward (b), along the antero-posterior axis (OX); tx: onset of change in acceleration along OX; Y”G: CC acceleration left (I) and right (r), along the lateral axis COY); fy: onset of acceleration change along the OY axis; Z”G: acceleration of G up (u) and down cd), along the vertical axis (OZ); XP: displacement of the foot pressure along the OX axis; YP: displacement of center of foot pressure along the OY axis; X’G: instantaneous velocity of the center of gravity along the OX axis; VI instantaneous peak velocity; HO: heel-off time.
(t = 3.16, p < 0.01) and between conditions F and N (t = 1.98, p < 0.05). The displacement of the center of foot pressure (XP and YP) started at time tx. The center of foot pressure moved backwards and towards the forthcoming stepping foot during the anticipatory phase.
Velocity of the center of gravity
The velocity of the CG (X’G) along the progression axis was calculated from the acceleration by Euler’s algorithm of integration. This progression velocity reached a maximum value ‘V’ at time ‘tv’. The progression velocity has an initial value ‘~0’ at HO. The mean values of the peak progression velocity (V) in single step ranged from 0.617 f 0.101 m/s to 1.448 L-0.110 m/s depending on experimental conditions (Table 3). It varied significantly (F(2,8) = 159.0,
G. Dietrich et al. /Human Table 1 Duration of ‘global’ anticipatory experimental conditions Subject
movements.
Single-Step
Movement
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Mean values ( f sd) are given for 35 trials in each of the 3
walking
&-HO(s) Mean + sd Condition
Multi-step
walking
tr-HO(s) Mean + sd S
Sl s2 s3 s4 s5
0.401 0.397 0.290 0.332 0.253
f 0.035 * 0.022 f 0.018 + 0.03 1 f 0.037
Group
0.335 f 0.058 Condition
201
0.502 0.455 0.525 0.490 0.489
k * k + f
0.035 0.022 0.018 0.03 1 0.037
0.492 f 0.058
N
Sl s2 s3 s4 S5
0.429 0.436 0.394 0.394 0.328
Group
0.389 f 0.041
0.619 0.523 0.455 0.523 0.483
+ 0.040 f 0.026 f 0.035 f 0.020 f 0.020
+ f + + f
0.039 0.058 0.025 0.041 0.025
0.521+ 0.061
Condition F Sl s2 s3 s4 s5
0.478 0.502 0.407 0.447 0.368
Group
0.439 * 0.048
tx-HO:
duration
of global
0.620 0.669 0.594 0.550 0.569
+ 0.023 f 0.034 f 0.030 + 0.037 k 0.021
anticipatory
+_0.026 + 0.030 f 0.043 f 0.070 f 0.011
0.600 k 0.048 movements.
p < 0.001). The mean I/ values in multi-step
walking varied from 1.070 + 0.070 m/s to 1.730 + 0.110 m/s (Table 3) and the mean values were also significantly different (p < 0.001) according to the experimental conditions.
Relationship between CG progression velocity and the duration of anticipatory movements
The duration of global anticipatory movements (time tx to HO) in single-step and multi-step walking was linearly related to the progression velocity of the CG (Fig. 3). The correlation coefficients were respectively r = 0.75 - (p < 0.01) and r = 0.79 - ( p < 0.01) and the two slope values of the linear regression were close, respectively 0.138 m/s and 0.140 m/s.
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Table 2 Initial location of the center of of pressure (XPO YPO) Subject
XPO (m) Mean + sd Condition
Sl s2 s3 s4 s5
ITO (ml Mean + sd S
0.428 + 0.002 0.396 f 0.005 0.432+ 0.010 0.411+ 0.008 0.414 & 0.009
1.026 + 1.006 + 1.017 + 1.009 f 1.016 f
0.035 0.035 0.035 0.035 0.025
1.026 f 1.008 f 1.017 f 1.009 f 1.014 f
0.030 0.020 0.030 0.025 0.025
1.027 f 1.007 f 1.017 f 1.011+ 1.013 f
0.035 0.030 0.035 0.035 0.020
Condition N Sl s2 s3 S4 s5
0.429 f 0.005 0.397 f 0.007 0.440 + 0.007 0.410 f 0.008 0.414 + 0.009 Condition F
Sl s2 s3 s4 s5
0.430 +_0.005 0.398 + 0.006 0.451 f 0.010 0.417 + 0.008 0.418 + 0.009
3.2. Local anticipatory phenomena For a given experimental condition, the amplitude and the time of each acceleration were reproducible for both individuals and between individuals (Fig. 4). Progression axis
The local accelerations along the antero-posterior axis (OX) started before HO. The mean duration of local anticipatory accelerations ranged from 0.198 + 0.044 s to 0.517 &-0.050 s depending on the experimental conditions (Table 4). The shanks accelerated along the antero-posterior axis before tx (3090 ms for the contralateral shank SC). The initial acceleration of all the body segments and the center of gravity was positive, i.e. directed forward. The onsets of shoulder acceleration (Shix and Shcx) coincided with the
G. Dietrich et al. /Human Table 3 Peak of velocity. The velocity was calculated values (* sd) are given for 35 trials Subject
Movement
by integrating
Science 13 (1994) 195-210
the antero-posterior
Single step
Multi-step walking
Peak velocity
Peak velocity (m/s) Mean + sd
(m/s) Mean + sd
CG acceleration.
203
Mean
Condition S Sl s2 s3 s4 SS
0.624+0.116 0.794 f 0.017 0.615 f 0.032 0.489 f 0.020 0.564 f 0.018
1.120*0.050 0.880 f 0.035 1.140+0.060 1.100~0.080 1.020 f 0.060
Group
0.617+0.101
1.070~0.070
Condition N Sl s2 s3 s4 s5
0.917 1.046 0.892 0.962 0.923
f 0.033 & 0.027 f 0.030 f 0.008 k 0.023
Group
0.948 f 0.054
1.460 f 0.040 1.210+0.050 1.380+0.010 1.440 f 0.040 1.260 + 0.020 1.350 + 0.06
Condition F Sl s2 s3 s4 s5
1.467+0.038 1.586 f 0.021 1.274+0.100 1.531~0.031 1.382~0.008
1.540+ 0.060 1.900 f 0.070 1.800f0.120 1.700 + 0.080 1.720+0.140
Group
1.448+0.110
1.730f0.110
onset of the CG acceleration and these accelerations were positive throughout the anticipatory phase. The time course of ipsilateral hip acceleration was diphasic during the anticipatory phase: acceleration was initially positive and reached a negative peak at HO. In contrast, the contralateral hip showed no significant acceleration during this phase. Lateral axis The contralateral and ipsilateral hips both accelerated along the lateral axis (OY) before ty (onset of lateral CG acceleration). These accelerations
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V b/s) 2.0
1.6
1.2
i -
0.8
0.4
0
J
0
200
400
600
tx-HO (ms) Fig. 3. Relationship between the duration of global anticipatory movements (&-HO) along the progression axis and the peak of the CG instantaneous velocity (V) in single step (0) and multistep walking (W ).
were initially positive, i.e. directed towards the stance foot, but reached a negative peak at HO, as did the ipsilateral hip along the OX axis. The shoulders (Shi and She) accelerated at the same time as the CG. These accelerations were positive during the anticipatory phase and had the same time course as the CG. Vertical axis
There were no anticipatory movements along the vertical axis. All the acceleration traces show oscillations which were simultaneous to the vertical acceleration of the center of gravity, except for the contralateral hip, which showed no significant acceleration during the anticipatory phase. Relationship between global and local accelerations
There was a highly significant linear correlation (r = 0.92, p < 0.001) between the onset of the ipsilateral shoulder (Shi) acceleration along the progression axis (ShLr) and the onset of global acceleration (tx) (Fig. 5) The linear regression equation (Shti = 1.02 tx - 0.003) suggested that the two occurred at the same time.
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Movement Science 13 (1994) 195-210
(1 150
205
m.s”
Ins
Fig. 4. Local accelerations. Accelerations are displayed according to the different segments and joints: shank (S), hips (H), shoulders (Sh) and thorax (Th), ipsilateral 6) and contralateral (c) side along the three absolute axes: OX, anteroposterior axis; OY, lateral axis, 02, vertical axis; CG: acceleration of the center of gravity, calculated from the force plate; tx and fy are the onset of dynamic CG phenomena.
4. Discussion
The step initiation phase can be considered as a transition phase between upright posture and level walking. A major problem during multi-step walking is the maintenance of dynamic equilibrium. Stereotypic movement patterns of body segments (trunk, shoulders, arms and pelvis) have been observed (Elftmann, 1939; Murray et al., 1964; Cappozzo, 1981; Thorstensson et al., 1982; 1984; Stokes et al., 1989) which seem to be organized to facilitate gait and minimize force and energy expenditure (Townsend, 1981; Cappozzo, 1983). The present data described a locomotor process which appears during the anticipatory period. It suggests that there are two parts to these anticipatory movements.
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Table 4 Duration
of local anticipatory
Subject
t Six(s)
t Scn(s)
t Hi.&)
t Shix(s)
t Shcx(s)
t Thx(s)
Mean f sd
Mean f sd
Mean f sd
Mean f sd
Mean f sd
Mean + sd
0.480 0.480 0.362 0.403 0.320
0.334 0.406 0.286 0.342 0.298
0.416 0.358 0.281 0.345 0.260
0.415 f 0.360 f 0.252+ 0.354 + 0.220 +
0.436 0.358 0.331 0.374 0.300
movement
along the antero-posterior
axis in single-step
walking
Condition S Sl s2 s3 s4 s5
0.383 0.456 0.351 0.380 0.308
Group
0.375 f 0.063
f + f + f
0.012 0.025 0.019 0.020 0.022
f f + f +
0.010 0.017 0.021 0.031 0.020
f f k f f
0.009 0.030 0.022 0.037 0.017
0.409 f 0.050
0.333 * 0.044
0.388 0.447 0.354 0.417 0.376
* f f + +
0.022 0.015 0.005 0.006 0.012
0.050 0.040 0.010 0.033 0.030
f 0.021 + 0.012 kO.033 f 0.017 + 0.009
0.333 f 0.045
0.323 ?r:0.080
0.360 f 0.046
0.460 0.430 0.446 0.440 0.355
0.390 f 0.415 f 0.340* 0.455 + 0.306 f
0.428 0.403 0.376 0.430 0.370
Condition N f f f + f
0.060 0.006 0.010 0.025 0.019
0.037 0.032 0.031 0.015 0.036
+ f + f f
0.055 0.020 0.015 0.012 0.017
Sl s2 s3 s4 s5
0.402 f 0.022 0.474 f 0.025 0.381 f 0.033 0.392 f 0.029 0.319f0.030
0.465 f 0.501* 0.395 + 0.422 + 0.335 f
Group
0.394 f 0.045
0.424 + 0.052
0.396 f 0.036
0.406 f 0.040
0.380 f 0.050
0.401 f 0.025
0.473 0.485 0.395 0.440 0.340
0.487 0.470 0.393 0.460 0.365
0.501 0.437 0.353 0.439 0.350
0.037 0.022 0.041 0.028 0.030
f f f f +
0.040 0.059 0.006 0.014 0.007
Condition F Sl s2 s3 s4 s5
0.522 f 0.509 f 0.430* 0.512 f 0.363 f
0.033 0.036 0.041 0.033 0.040
0.535 f 0.031 0.514 + 0.022 0.430 f 0.037 0.517*0.041 0.420 f 0.022
0.446 + 0.504 f 0.410* 0.440 f 0.403 f
Group
0.468 + 0.070
0.483 + 0.061
0.440 f 0.040
Six:ipsilateral contralateral
shank; Sex: contralateral shoulder.
shank,
0.014 0.013 0.033 0.036 0.016
Hix:
* + f + +
0.010 0.016 0.015 0.030 0.030
0.429 f 0.050 ipsilateral
f 0.012 f 0.044 +_0.022 f 0.040 + 0.025
0.435 f 0.053
hip, Shix:
ipsilateral
f f f f f
0.011 0.032 0.027 0.033 0.021
0.416 f 0.057 shoulder,
Shcx:
(a) The first one is associated with a change in the initial position of the subject before the onset of dynamic phenomena. Indeed, the antero-posterior abscissa of the center of foot pressure in the initial position (XPO), shifts forward as a function of the peak velocity of the forthcoming movement. Given that (i) the subject started all the movements from the same mark, and (ii) the subject was static (there is no noticeable acceleration in the initial position) the ground projection of the center of gravity and the center of pressure are superimposed (Thomas and Whitney, 1959; Gurfinkel, 1973; Gurfinkel et al., 1976; Breniere et al., 1981). Thus the forward initial shift corresponds to a forward displacement of the center of gravity and initial velocity is going to increase in proportion to the subject’s
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t&ix
b-4 600
I
2004 200
300
400 tx
500
600
700
b)
Fig. 5. Relationship between the onset of the acceleration onset of CG acceleration (tx) along the progression axis.
of the ipsilateral
shoulder
(Shti)
and the
inclination. Consequently, this preparation adjustment can be considered as the early part of the anticipatory process. The duration of global anticipatory movements (&-HO) is correlated with the velocity of the center of gravity along the progression axis (Fig. 3) in both single-step and multi-step walking. The peak progression velocity in multi-step walking is higher than in single step. Nevertheless, the relationship between the peak velocity and the anticipation duration seems to be similar. These results may be considered as a consequence of the biomechanical constraints which are the same in both situations. Indeed, anticipatory movements reflect the need to generate the propulsive forces required to initiate center of gravity displacement (Breniere et al., 19871, and to simultaneously transfer the body weight to the forthcoming stance foot (Roberts, 1978). The dynamic anticipatory phenomena are also represented by the local accelerations before heel-off. The acceleration data shows anticipatory phenomena occur along the antero-posterior and lateral axis before heel-off in all segment levels, except in the contralateral hip. The shanks accelerate first along the antero-posterior axis, before the onset of CG acceleration. The onset and direction of shank acceleration are consistent with the electromyographic phenomena recorded in gait initiation (Carlsiiij, 1966; Herman et al., 1973; Breniere et al., 1981): inhibition of the Soleus and
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activation of the Tibialis Anterior. However, these shank accelerations precede the CG acceleration, i.e. they showed no effect on the center of gravity which is a resultant phenomenon. The shank mass being small compared to the body mass, its shank acceleration could have no dynamic consequence, in terms of force, on the center of gravity. There may also be other local accelerations which compensate for the shank acceleration. But this is improbable, because no local acceleration before or simultaneous to the onset of the shank acceleration was recorded in this study and no limb movement has been recorded in other studies (Herman et al., 1973). The onset of local acceleration of the ipsilateral shoulder along the progression axis occurred at the same time as the onset of CG acceleration (Fig. 5). This simultaneous onset of local and global accelerations suggests that the accelerations produced at the shoulder level contribute a great deal to generating the propulsive forces needed to execute voluntary movement. The acceleration of the center of gravity along the progression axis appears to be mainly due to shoulder acceleration. As in multi-step walking, local movements seem to be specific to the forthcoming movement and can be interpreted as local organization of these movements which control the execution of the step. (b) The second component of anticipatory movement is the postural adjustment to the disruption induced by heel-off. The time courses of local and global accelerations at heel-off can be analyzed in terms of postural adjustments and disruptions resulting from heel-off (Breniere and Dietrich, 1992). The three components of the acceleration of the ipsilateral hip are large (negative peak) at heel-off. The hip accelerates backwards, downwards and towards the stepping foot in a falling movement corresponding to the lifting of the heel. These changes in local acceleration can be interpreted as a consequence of the disruption induced by the vertical fall of the center of gravity occurring at HO. The contralateral hip does not accelerate along the progression and lateral axes, and is a fixed point for the movement in these directions. In contrast, the upper trunk (shoulders and thorax) show no significant change in local acceleration at heel-off. Thus, these segments are not affected by the disruption created by heel-off. This lack of local disruption of the upper trunk indicates that local accelerations balance the effect of the negative hip acceleration. This implies that the upper part of the trunk is dissociated from the pelvic segment during the anticipatory movement. The local anticipatory movements of the upper trunk appear to be orga-
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nized to minimize the disruption provoked by heel-off. This disruption has only a local pelvic effect, mainly at ipsilateral hip. All the initial accelerations are positive along the antero-posterior and lateral axes, i.e. they oppose the disruption (negative acceleration). Thus, the anticipatory movements anticipate not only the aim of gait initiation (i.e. displacement, velocity), but also the disruption associated with heel-off. These anticipatory movements include ‘postural adjustments’, according to the hypothesis that postural adjustments create the initial dynamic conditions required to balance the effect of the voluntary movement which tend to disturb the postural equilibrium (Bouisset and Zattara, 1987). In conclusion, the initiation of single step seems to include postural functions related to the disruption associated with heel-off, the transfer of body weight to the forthcoming foot, and dynamic changes required to create the propulsive forces necessary to initiate displacement of the body CG (Breniere et al., 1987). These different components of the initiation process are reflected in local acceleration of body segments.
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