Movement Variability and Muscle Activity Relative ... - Semantic Scholar

ORIGINAL RESEARCH Motor Control, 2008, 12, 283-295 © 2008 Human Kinetics, Inc.

Movement Variability and Muscle Activity Relative to Center of Pressure During Unipedal Stance on Solid and Compliant Surfaces James L. Croft, Vinzenz von Tscharner, and Ronald F. Zernicke Compliant surfaces are used to challenge postural stability, but assessments are frequently limited to summary measures of center of pressure that do not provide insights into the temporal dynamics of motor coordination. Here, we measured center-of-pressure changes on three surfaces (solid, foam, and air-filled disc) and quantified the relative timing of changes in joint angles and muscle activity with respect to center-of-pressure changes. Nine active male subjects (20–30 years old) performed ten 30-s trials of unipedal stance on each of the three surfaces. Sway range, mean sway, mean sway velocity, path length, and fitted ellipse area increased, monotonically, from solid surface to foam to air-filled disc. The number of significant cross-correlations was greater for the compliant surfaces compared with the solid surface. Muscle activity preceded changes in center-of-pressure displacement, with the type of surface affecting the magnitude of the lead in the mediolateral direction. Center of pressure was more constrained on less stable surfaces and in the mediolateral direction. Keywords: posture, balance, sway, foam, compliant surface, COP, center of pressure

The control of posture during upright stance is influenced by the compliance of the supporting surface. Numerous studies have reported that postural sway was greater on compliant surfaces (Allum et al., 2001; Birmingham, 2000; Riemann, Myers, & Lephart, 2003; Teasdale, Stelmach, & Breunig, 1991; Vuillerme et al., 2001), but much of the available motor control data do not quantify the interrelations among joint angle kinematics, muscle activity, and center-of-pressure variations. Few studies have investigated movement strategies during stance on a compliant surface. Allum et al. (2001) measured trunk sway of normal and vestibular-loss patients under a variety of conditions including bipedal and unipedal stance on solid Croft is with the Institute of Sport and Recreation Research, New Zealand School of Sport and Recreation, Faculty of Health & Environmental Sciences, AUT University, Auckland 1142, New Zealand. von Tscharner is with the Faculty of Kinesiology, University of Calgary, Calgary, AB Canada. Zernicke is with the Dept. of Orthopaedic Surgery and the Division of Kinesiology, Dept. of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48105. 283

284 Croft, von Tscharner, and Zernicke

and foam surfaces. Their sway values, however, were only compared between groups and not between conditions. Trunk sway of normal subjects was also assessed on a foam support and an ankle-sway-referenced surface, but comparison to a solid surface was not included (Allum, Zamani, Adkin, & Ernst, 2002). Riemann et al. (2003) measured displacement and orientation of the lower leg, thigh, and trunk (sacrum and C7 vertebra) and found that during single-limb stance on a compliant surface, angular changes were significantly greater at the ankle joint than at the hip joint, knee joint, or trunk. They suggested that the higher mass of the trunk might have made it less suitable for the quick adjustments required when standing on unstable surfaces. Compliant surfaces potentially disturb upright stance by reducing sensory input and decreasing the ability to effectively use corrective ankle torque. Foam increases contact area between the feet and ground, making plantar pressures more evenly distributed across the soles (Wu & Chiang, 1996), which reduces the accuracy of cutaneous input. Meyer, Oddsson, and De Luca (2004) showed that anesthesia of the plantar surface of the foot affected postural sway, but only in more difficult conditions (with eyes closed or unipedal stance). Using a diffusion analysis (Collins & De Luca, 1993), Meyer et al. (2004) showed that the long-term regions (which are thought to indicate closed-loop feedback) were unaffected by removal of cutaneous input, suggesting that its primary function during unperturbed stance was not for feedback control. There is growing evidence that a feedforward model is more likely to control quiet stance than feedback control to compensate for neural transmission delays (Morasso & Schieppati, 1999; Loram, Maganaris, & Lakie, 2005). During quiet stance, anterior-posterior motion of center of pressure and center of mass were correlated with lateral gastrocnemius activity with a phase lead of the plantar flexors (Gatev, Thomas, Kepple, & Hallett, 1999; Masani, Popovic, Nakazawa, Kouzaki, & Nozaki, 2003). In addition, muscle length changes measured with ultrasound images have shown that plantar flexor activity makes small ballistic adjustments that precede center-of-mass position (Loram et al., 2005). Compliant surfaces decrease the ability to use torque generated at the ankle to maintain upright posture. For a specific ankle torque, subjects had greater sway on a compliant surface compared with a solid surface (Fitzpatrick, Burke, & Gandevia, 1996). In addition, sway had a greater phase lag relative to ankle torque, possibly owing to increased muscle shortening required to develop force. Such phase lag might be affected by the compliance of the surface. Building on those previous results, in the current study we investigated two compliant surfaces that are commonly used in rehabilitation (foam and air-filled disc) and are assumed to create different levels of instability. We sought to investigate control strategies on each surface and tested the following hypotheses: (1) summary center-of-pressure measures would be greatest on the air-filled disc and least on the solid surface, (2) muscle activity and center-of-pressure changes would be more highly correlated in the mediolateral versus the anterior-posterior direction, and (3) muscle activity in the medial gastrocnemius, soleus, and peroneus (fibularis) would occur significantly earlier relative to center-of-pressure movement on compliant surfaces compared with solid surfaces.

Movement Variability on Compliant Surfaces 285

Methods Subjects Nine healthy male volunteers between the ages of 20 and 30 (25.1 ± 2.0) with no history of ankle sprains were recruited as subjects. All subjects were naive to the specific purpose of the experiment. A single gender was chosen because previous studies reported differences in mediolateral movement for women and men during quiet stance (Panzer, Bandinelli, & Hallett, 1995).

Protocol Before participation, all subjects gave informed consent approved by the Conjoint Health Research Ethics Board of the University of Calgary. Each subject wore shorts and was unshod. A “neutral” trial was collected while each subject stood with both feet on the force platform, with his arms crossed and hands placed on opposite shoulders and the lower limbs aligned with the laboratory coordinate system. Changes in joint angles were calculated relative to this neutral position. Participants remained “as stationary as possible” while standing on their right foot on each of three surfaces (solid, foam, and air-filled disc) for 30 s with their arms crossed and hands placed on opposite shoulders. The position of the left leg was not standardized. The foam was an Airex Balance Pad, which was a highdensity (50 kg/m3), closed-cell foam pad (47 × 39 × 7 cm, 0.7 kg). The air-filled disc was an “active sitting disc” (Fitter, AB, Canada) with a diameter of 28.5 cm and was inflated to a height of 10 cm. Deformation of the foam during single-leg stance ranged from 3.4 to 4.3 cm with a mean of 3.9 ± 0.3 cm. Deformation of the disc ranged from 4.2 to 6.2 cm with a mean of 5.2 ± 0.7 cm. These materials and shapes were chosen to replicate those used in rehabilitation and provide a range of instability. Each condition was presented in sequential order with each of the surfaces occurring 10 times, for a total of 30 trials. A sequential order was chosen over a random design to replicate the clinical situation. If the arms were removed from the shoulders or the left foot touched the floor, the trial was terminated and redone up to a maximum of three extra trials per surface. If more than three trials were required, the subject was excluded from analysis. Subjects took 1 min of rest after each trial and 5 min after each surface condition.

Force-Plate Data Force data were collected (4000 Hz per channel) from a force plate (Kistler Instruments, Z4852C, Winterthur, Switzerland). Anterior-posterior and mediolateral components of the center-of-pressure trace were calculated using the standard formulas provided by the force-plate manufacturer (see also Gatev et al., 1999). Path length was calculated for each trial as the sum of changes in center of pressure over time. Anterior-posterior, mediolateral, and planar center-of-pressure trajectories were summarized by root mean square sway, mean sway velocity, and ellipse area (calculated using principal components to define ellipse axes; Oliveira, Simpson, & Nadal, 1996). These summary measures were compared across surfaces using


a repeated-measures ANOVA (SPSS, SPSS Inc.) with significance levels for post hoc comparisons calculated using a Bonferroni correction to avoid cumulative Type I errors (p / number of planned comparisons).

Electromyography (EMG) EMG data were collected from the midmuscle of the tibialis anterior, medial gastrocnemius, soleus, peroneus (fibularis), gluteus medius, adductor musculature, biceps femoris, and vastus medialis muscles unilaterally on the right side using bipolar surface electrodes (Noraxon USA Ltd., Scottsdale, AZ). The specific adductor muscle location was standardized as 60% of femur length from the medial femoral epicondyle. The ground electrode was placed on the fibular head. The skin was shaved and cleaned using a mild rubbing alcohol swab before electrode application on the skin, and electrode leads were secured to the lower limb with adhesive tape. Myoelectric signals were preamplified at source (bandwidth 10–500 Hz, 3 dB; Biovision, Wehrheim, Germany) and recorded at 4000 Hz.

Kinematic Data Three-dimensional kinematic data were collected (1000 Hz) using an infrared Motion Analysis system (Santa Rosa, CA) on the same computer as the EMG and force platform analog data. Using marker sets based on the Grood and Suntay (1983) convention, retro-reflective markers were affixed bilaterally to the calcaneus, first and fifth metatarsal heads, anterior superior iliac spine, and to the first sacrolumbar vertebra. Marker sets for the lower leg and thigh were fixed to a rigid plastic shell, which was attached using Velcro to a neoprene wrap around the respective segment. For the neutral trial, additional markers were placed bilaterally on the lateral malleolus, medial malleolus, lateral femoral epicondyle, medial femoral epicondyle, and greater trochanter to define joint centers. Local coordinate systems were created for each segment. The position and orientation of these coordinate systems for each frame of data were compared with the neutral trial to allow joint angles to be calculated around each axis defined in the neutral trial. Care was taken to align the subjects’ feet, legs, pelvis, and trunk with the global coordinate system before the neutral trial. Joint angles for ankle (ankle and subtalar joint complex), knee, hip, and trunk were used in the subsequent analysis.

Signal Analysis EMG signals were resolved into time-frequency space using wavelet analysis (von Tscharner, 2000). Wavelet-transformed signals consisted of both intensity and phase. The intensity is a measure of the power of the signal within a frequency band. The mean intensities of the wavelets (6.9, 19.3, 37.7, 62.1, 92.4, 128.5, 170.4, 218.1, 271.5, 330.6, 395.4, 465.9, 542.1 Hz) were used as input for the cross-correlation analyses. Joint angles and center-of-mass position were smoothed using a wavelet filter (cutoff frequency = 60 Hz, mode = 3). Center of pressure was smoothed using a wavelet filter (cutoff frequency = 60 Hz, mode = 3) and resampled to 1000 Hz. Cross-correlation was determined for each of the pairs of time series between anterior-posterior center of pressure and ankle dorsi/plantar flexion angles, knee


flexion/extension angles, hip flexion/extension angles, and EMG of tibialis anterior, medial gastrocnemius, soleus, rectus femoris, and biceps femoris; anterior-posterior mediolateral center of pressure and ankle inversion/eversion angles, knee abduction/adduction angles, hip abduction/adduction angles, and EMG of peroneus, adductors, and gluteus magnus. For a pair of signals, x and y, each 30-s trial of x was divided into 29 equal windows of 2000 points (i.e., 2 s), thus overlapping each window with the next window by half. For each window, a corresponding window of the same size was selected in signal y. The offset (t) of window y from window x ranged from –500 to +500 points (i.e., maximal offset of y compared with x was 500 ms leading or lagging). The Pearson correlation was calculated for each pair of windows for each value of t, giving a cross-correlation curve for each window in x. The average of all 29 windows was calculated for each trial. The mean of each trial cross-correlation function was calculated for each surface for each subject. The correlation coefficients of the maxima were transformed into Z scores for purposes of hypothesis testing (Hardle & Simar, 2003) as in previous studies (Gatev et al. 1999; Masani et al. 2003). Following significance testing, the Z scores were back converted to correlations for ease of understanding. The significance level for cross-correlations was p < .001 or Zr > .31. Since 14 cross-correlations were planned for each of the three surfaces, significance levels for post hoc comparisons were calculated using a Bonferroni correction to avoid cumulative Type I errors (.05 / 42). Although the authors appreciate that applying the Bonferroni correction in this manner might be overly conservative, it affords focus on a smaller subset of meaningful data. Only those variables with significant cross-correlational maxima were analyzed for time shifts. The time shift was calculated from the mean cross-correlational curve for each surface for each subject. The difference in time shift between surfaces was compared using a repeated-measures ANOVA (SPSS, SPSS Inc.) with the significance level set at p = .05. In calculating cross-correlational functions between joint angles and center of pressure, anatomical angles were assumed to be aligned with the force-plate coordinate system. Because cross-correlation compared the form of the signals and not the absolute amplitude, if there were a systematic offset, there would be no effect on the cross-correlational maxima.

Results Center-of-Pressure Measures Values on the foam differed significantly from the solid surface on anterior-posterior range, center of pressure path length, and the mean sway velocity measures (Table 1). All values for the disc were significantly different from those on the solid and foam surfaces (Table 1).

Cross-Correlations The mean cross-correlational curve for each subject and surface was calculated, and the maximum of each curve was converted to a Z score for significance testing. The raw R values (Figure 1) are reported instead of the converted Z scores for ease

Table 1 Summary of Center-of-Pressure Measures (Mean ± SD) Measure

Solid

Foam

Disc

Anterior-posterior range (mm) 44.0 ± 4.3 60.0 ± 6.9s 69.9 ± 11.5sf Mediolateral range (mm) 31.3 ± 2.8 33.8 ± 3.7 49.0 ± 6.4sf Mean anterior-posterior sway (mm) 9.1 ± 1.2 10.5 ± 1.5 13.2 ± 2.3sf Mean mediolateral sway (mm) 6.2 ± 0.7 6.7 ± 0.6 8.7 ± 0.7sf Mean anterior-posterior velocity (mm/s) 22.8 ± 5.2 33.9 ± 4.1s 42.7 ± 7.5sf Mean mediolateral velocity (mm/s) 28.1 ± 6.2 36.0 ± 4.8s 51.9 ± 7.1sf Ellipse area (mm) 686.6 ± 160.9 863.2 ± 172.8 1397.1 ± 340.0sf Path length (mm/trial) 1107.8 ± 185.1 1629.0 ± 197.2s 2270.3 ± 359.1sf s f

Significant difference on this surface compared with the solid surface. Significant difference on this surface compared with the foam (p < .006).

Figure 1 — Means (SD) of maxima of cross-correlations with center of pressure. The horizontal line represents the level of significance. Upper: maxima of cross-correlations with anterior-posterior component of center of pressure. Ank = ankle dorsiflexion angle; Hip = hip flexion angle; Trunk = trunk extension angle; MG = medial gastrocnemius activity; COM = anterior-posterior location of center of mass. Lower: maxima of cross-correlations with mediolateral component of center of pressure. Ank = ankle eversion angle; Knee = knee abduction angle; Hip = hip abduction angle; Trunk = trunk L side flexion angle; HipC = contralateral hip adduction angle; PER = peroneus longus (fibularis longus) activity; ADD = adductor activity; GM = gluteus medius activity; COM = mediolateral position of center of mass. 288


of interpretation and comparison with previous research, and the critical level for significance is indicated by the horizontal line. In the anterior-posterior direction on the solid and foam surfaces, center of mass and medial gastrocnemius activity were significantly correlated with center of pressure; on the disc, center of mass, medial gastrocnemius activity, and ankle, hip, and trunk angles were correlated with center of pressure. In the mediolateral direction on the solid surface, center of mass, peroneus activity, and ankle angle were significantly correlated to center of pressure; on the foam and disc, center of mass, peroneus activity, and ankle, hip, trunk, and contralateral hip angles were significantly correlated with center of pressure. Mean time shifts were plotted only for those variables with a significant correlational maxima (Figures 2 and 3). The width of each bar represents the standard error.

Figure 2 — Time shifts between anterior-posterior center of pressure (COP) and other signals. Time shifts are only plotted for variables that had a significant cross-correlational maxima. The center of each bar represents the mean time shift of a measure relative to the anterior-posterior component of center of pressure. The length of the bar indicates the standard error of the time shift across subjects. The central line indicates a time shift of zero (i.e., the two signals were synchronized). Bars to the left of the central line indicate that measure preceded the anterior-posterior center-of-pressure signal, and bars to the right indicate that measure occurred after anterior-posterior center of pressure. Ank = ankle dorsiflexion angle; Hip = hip flexion angle; Trunk = trunk extension angle; MG = medial gastrocnemius activity; COM = anterior-posterior location of center of mass.


Figure 3 — Time shifts between mediolateral center of pressure and other signals. Time shifts are plotted for variables that had a significant cross-correlational maxima. The center of each bar represents the mean time shift of a certain measure relative to the mediolateral component of center of pressure (ML-COP). The length of the bar indicates the standard error of the time shift across subjects. The central line indicates a time shift of zero (i.e., the two signals were synchronized). Bars to the left of the central line indicate that a measure preceded the mediolateral center-of-pressure signal, and bars to the right indicate that a measure occurred after mediolateral center of pressure. Ank = ankle eversion angle; Knee = knee abduction angle; Hip = hip abduction angle; Trunk = trunk L side flexion angle; cHip = contralateral hip adduction angle; PER = peroneus longus (fibularis longus) activity; COM = mediolateral position of center of mass.

Relative Timing on the Solid Surface In the anterior-posterior direction, the mean time shift between center of mass and center of pressure was small, with center of mass leading center of pressure by ~17 ms (Figure 2). The only significant correlation of the plantar flexors was medial gastrocnemius, which preceded center-of-pressure changes by ~227 ms. In the mediolateral direction, shift of center of mass relative to center of pressure had low variability, but in contrast to the anterior-posterior direction, center of mass led center of pressure by ~90 ms. Peroneus activity led center of pressure by ~140 ms (Figure 3). The ankle inversion/eversion angle also had a consistent time shift and lagged center of pressure by ~13 ms.


Relative Timing on the Compliant Surfaces The time shifts were analyzed for differences between surfaces with a repeatedmeasures general linear model with an adjusted Bonferroni for post hoc comparisons. In the anterior-posterior direction, time shifts between center of mass and center of pressure were not significantly different between surfaces, but center of mass always led center of pressure by a small amount. The time shift between ankle angle and center of pressure was not significant on the compliant surfaces and lagged center of pressure by ~29 ms on the foam and ~13 ms on the disc. The changes in hip and trunk angles relative to center-of-pressure changes were also significant on the disc, with both occurring after center of pressure. The timing of medial gastrocnemius activity relative to the center of pressure was not significantly different for solid and compliant surfaces. In the mediolateral direction, the center of mass led by a significantly greater amount on the compliant surfaces (~174 ms and ~155 ms) than on the solid surface (~90 ms). The time shift of ankle-angle change relative to center-of-pressure change was small and significantly different on the disc than on either the solid or the foam surface. The time shift of hip, trunk, and contralateral hip angles relative to center of pressure was significant on the compliant surfaces, and all lagged center of pressure. Peroneus activity occurred significantly earlier on the disc (~170 ms) than on the other two surfaces (~140 ms).

Discussion Compliant surfaces are commonly used in rehabilitation and training because of the reduction in sensory feedback and the reduced effectiveness of corrective ankle torque. Typical balance programs start on a solid surface and progress to foam and then to an air-filled disc. This progression was supported by the center-of-pressure measures, which were significantly greater on the disc than on the solid surface, most of which were significantly greater than on the foam surface. However, the temporal dynamics of coordination, identified by cross-correlations of joint angular changes relative to center-of-pressure displacement, were different for each surface, suggesting that different movement solutions were used. On solid surfaces, frontal-plane motions of unipedal stance are predominately controlled at the ankle joint (Tropp & Odenrick, 1988; Hoogvliet, van Duyl, Bakker, Mulder, & Stam, 1997; King & Zatsiorsky, 2002). Two movement patterns have been suggested. A foot-tilt strategy resulting from movements of the subtalar joint and a hip strategy in which rotation of the upper body about the hip joint causes horizontal shear forces (Hoogvliet et al., 1997). In the current study, ankle angle in the frontal plane was highly correlated with center of pressure on all surfaces. These findings confirm those of Riemann, Myers, and Lephart (2003), who reported that angular displacement of the shank was significantly greater than at the knee, hip, or trunk during stance on foam. However, they only compared the amount of angular displacement at each joint and, consequently, were not able to identify which joints were related to postural control. The current results show that on compliant surfaces, hip angle was also correlated with center of pressure, possibly because sufficient ankle torque could not be applied against the compliant surface.


From a practitioner’s perspective, although compliant surfaces might be training ankle proprioceptors, different movement synergies involving hip mechanics may emerge. Whether these synergies transfer to game play is unknown. On all surfaces, muscle activity occurred earlier in the medial gastrocnemius than in the peroneus. One reason could be differences in stiffness of the series elastic components of the tendon of the Achilles for the gastrocnemius compared with the tendon of the peroneus. The Achilles tendon and foot are known to have low stiffness during stance (Loram, Maganaris, & Lackie, 2005), so muscle contraction does not immediately create ankle torque. The stiffness of the peroneal tendon during stance has not been reported, so the stiffness explanation cannot be verified yet. Another reason might be the biomechanical arrangement of these muscles. Plantar flexion occurs mainly at the talocrural joint and eversion at the subtalar joint. The moment arm of the triceps surae (gastrocnemius and soleus) at the talocrural joint is approximately twice the moment arm of peroneus longus at the subtalar joint (Klein, Mattys, & Rooze, 1996). However, the resistive moment arm is much larger in the anterior direction than the medial direction with the foot being much longer than it is wide. This might allow the mediolateral foot-tilt strategy to act more rapidly than the anterior-posterior ankle strategy. We hypothesized that muscle activity would occur significantly earlier relative to center-of-pressure movement on compliant surfaces compared with solid surfaces. In the mediolateral direction, the hypothesis was partly confirmed because peroneus activity occurred significantly earlier on the disc. Although the disc is often used as a progression following a foam surface, the compliance characteristics are different. When a vertical force is applied to the foam, it compresses under the load with the deformation staying local to the applied force. In contrast, when an external force is applied to the disc, the pressure inside the disc increases and is distributed against the inner surface of the disc. From a practical perspective, the different characteristics of the foam and air-filled disc might not afford a linear progression and might require a different type of control. This was evident in the earlier onset of peroneal activity on the disc compared with the foam and involvement of the hip and trunk in the sagittal plane. If the goal is to train the same movement pattern, it might be better to change the compliance of the foam than to use an air-filled disc. In the anterior-posterior direction, surface had no effect on medial gastrocnemius latency, with similar values as reported previously (Gatev et al., 1999; Masani et al., 2003). The moment arm length of resistive force is greater in the anteriorposterior direction compared with the mediolateral direction with the foot being longer than it is wide. Thus, for each unit change in surface deformation at the point of force application, the angular change at the ankle was less in the anterior-posterior direction than in the mediolateral direction. It is possible that the angular changes on these surfaces were not sufficient to require earlier gastrocnemius activity. None of the other muscles that were measured were significantly correlated to center of pressure. Potentially, the soleus, composed of mainly slow-twitch fibers, might be unsuitable for rapid postural adjustments. The proximal muscles (vastus medialis, biceps femoris, adductors, gluteus medius) all had low correlations with


center of pressure, suggesting that control of center of pressure at the hip or knee was less prevalent than at the ankle, possibly being used in more extreme positions. Winter, Prince, Frank, Powell, and Zabjek (1996) discussed how the center of mass is controlled by the center of pressure during quiet standing, but until now, the phase shift between these two signals during quiet stance had not been reported. Time shifts might have been missed in previous studies because of low sampling frequencies (Gatev et al., 1999). With the current results, for both the mediolateral and anterior-posterior directions on the solid surface, center of mass and center of pressure were tightly related, with center of mass changes occurring ~15 ms before center of pressure in the anterior-posterior direction and ~90 ms before center of pressure in the mediolateral direction. Winter et al. (1996) suggested that the displacement difference (in the anteriorposterior direction) between center of mass and center of pressure acted as the error signal in the balance-control system. Cross-correlations between this difference signal and muscle activity also showed significant relations with EMG activity but always lagged muscle activity, suggesting that it too was unlikely to act as a stimulus signal. Without a suitable feedback signal, a feedforward model might exist. Loram et al. (2005) suggested that changes in muscle length and an internal model of the subsequent effect of those changes on center-of-mass dynamics were used to initiate appropriate postural corrections. Then, as ballistic impulses were provided by the muscles, the velocity of the center of mass was used to refine and update the internal model. On a solid surface, changes in muscle length had a predictable effect on center-of-mass dynamics. On a compliant surface, however, changes in length of muscles around the ankle might have been the result of surface deformation and, therefore, might not have given accurate center-of-mass dynamics. For the muscle length to predict accurately the changes in center of mass, an internal model would need to be updated. Such self-supervised learning algorithms were proposed by other researchers and are supported by other motor control tasks (Johansson & Westling, 1984; Shadmehr & Mussa-Ivaldi, 1994; Lackner & DiZio, 1998).

Summary Cross-correlations identified time shifts of muscle activity and joint angles relative to center-of-pressure displacement and confirmed that muscle-activity timing was affected by surface, but only in the mediolateral direction. On the disc, peroneus EMG activity led center of pressure by a greater amount than on the other surfaces, potentially to account for the delay in torque generation resulting from surface compliance. A similar shift was not observed in the anterior-posterior direction, presumably because torque generation was facilitated by a longer resistive moment arm and larger muscles than in the mediolateral direction. The medial gastrocnemius led center of pressure by a greater amount than the peroneus (fibularis), which might have been the result of the difference in ability to generate torque. In the aggregate, these data suggest that the postural-control system might be constrained most in the least stable conditions (surface and direction).


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