THE CONTROL OF SPATIAL BODY ORIENTATION IN HUMAN UPRIGHT STANCE

G. Wu and W. Zhao
Department of Exercise and Sport Science
Department of Mechanical Engineering and Center for Locomotion Studies
The Pennsylvania State University,
University Park, PA 16802

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION AND BACKGROUND

The maintenance of upright balance represents a major goal of human postural control. Although it has been well accepted that the upright balance is controlled by the central nervous system which integrates the information provided by the peripheral sensory systems, the specific nature of the controlled variable(s) is not clear yet.

Researches on pattern generators in control of locomotion have suggested that there exists an elaborate neural circuitry in the spinal cord (at least in animals) that is responsible for automatic execution of basic locomotion patterns (Marsden, 1982). Similarly in the control of human upright balance, a set of automatic postural reflexes has been observed in leg muscles when a person stands on a platform that suddenly moves (Woolacott et al., 1986). Although these reflexes are found to either decrease its magnitude with consecutive movements of the supporting surface when it destabilized the posture, or increase its magnitude when it stabilized the posture (Nashner, 1976), their timings are relatively invariable.

In this study, the peak magnitude and the peak time of body orientations were measured when subjects' postural balance was unexpectedly disturbed. The variations of these two variables with respect to the number of trials were examined. We hypothesized that the human upright balance is controlled by a pattern generator which regulates the timing of body movement.

METHODOLOGY

Postural perturbation was applied to the subject through an unexpected platform movement in the anterior-posterior direction of the subject. The maximum acceleration was 8m/s2, speed 40cm/s and total displacement 8cm. The spatial orientation (or angular displacement) of the head, trunk and arms, thigh, and shank in the sagittal plane was determined based on the measurements of multiple marker positions by a video-based motion analysis system (Peak Performance) with two cameras. An accelerometer was attached to the movable platform to measure its acceleration in the moving direction. The output from the accelerometer was synchronized with the video data and collected at 120Hz.

A total of 10 healthy subjects (ages ranged from 19 to 28 years, with a mean of 23.8 years) participated in this experiment. During the experiment, each subject was tested for a total of 12 conditions, 4 times each. They included: vision/no vision, moving forward/backward, and standing on hard/soft/reduced surfaces. All these conditions were provided in a random, but blocked order.

Before each trial, the subject was instructed to stand quietly on the movable platform, with feet comfortably separated in the lateral direction, arms folded across the chest, and eyes looking at a target 25m in front of the subject. The subject was given a random number between 50 and 100, and was asked to continuously subtract by 3 as quickly and accurately as possible until the platform movement started. In response to the platform movement, the subject was instructed to try to maintain the upright balance by moving any part of the body except for the arms.

The perturbation onset time was first determined based on the first point at which the platform acceleration was above the mean plus three times the standard deviation of the baseline level. Two parameters were calculated: first maximum peak magnitude (Pmax); first peak time (Tp) defined as the time at which the magnitude reached to a maximum for the first time. Data were analyzed using Analysis of Variance (ANOVA) to identify differences among the four trials of all the conditions. If the test proved to be significant, Least Squares Means analysis was made to detect which differences among the trials were significant (p < 0.05).

RESULTS

The means of all four trials of both peak magnitude and the peak time of the orientation angles are shown in Fig. 1. Those trials that are statistically significant are summarized in Table 1.

First maximum peak amplitude (Pmax): The results (Table 1) showed that the difference in Pmax between the first and the later trials was significant for the shank, thigh and trunk segments. In general, the significant difference occurred at about the second trial. In all cases, no significant difference was found between the third and the fourth trial. In addition, these differences were independent of surface, vision, and movement direction conditions.

First peak time (Tp): For most of the cases the ANOVA test determined that there was not enough evidence to suggest a difference among the mean of the trials except for the thigh angle between the first and the last trials. The results of all the other variables were not affected by factors such as surface, visual condition, and movement direction.

Fig. 1. Means+stds. of maximum peak orientation angles and the first peak time of four body segments.

Table 1: Statistically significant difference between trials for the maximum peak amplitude and peak time. All the results are not affected by surface, vision and movement direction. (NS: not significant among trials.)

DISCUSSION AND CONCLUSIONS

It is observed in this study that both the first maximum peak magnitude and the first peak time of the head orientation angle in space do not show any sign of adaptation. This suggests that the control of the stability of the head has a stable pattern which regulates both the magnitude and the timing. The fact that these characteristics of the head movement are not effected by vision and surface conditions further suggests that the pattern generator for head stabilization is probably controlled by other mechanisms than visual and somatosensory systems in the foot. Early studies by others (Roberts, 1973) have suggested that the orientation of the head in space is stabilized by means of vestibular and neck reflexes.

Differently from the head, it is interesting to find in this study that the magnitude of the spatial orientations of the trunk and legs are adapted when multiple trials with similar conditions are experienced by the subject, although the timing at which the first peak value occurs does not seem to be adaptive. This finding establishes the fact that the timing and the amplitude of body movement are independently controlled. In fact, this is consistent with the results in an indirectly related study by Radin et al. (1991) on the effect of knee pain on general gait patterns: that the timing of the peak ground reaction force was significantly altered in the knee pain group compared to the normal subjects while the magnitude was kept the same.

REFERENCES

Marsden, C.D. The mysterious motor function of the basal ganglia. Neurology. 32:514-39, 1982

Nashner, L.M. Adapting reflexes controlling the human posture. Exp Brain Res. 26: 59-72,1976 .

Radin, E.L. et al. Relationship between lower dynamics and knee joint pain. J Ortho Res. 9: 398- 405, 1991

Roberts, T.D.M. Reflex balance. Nature, 244:156- 158, 1973

Woolacott, M. et al., Aging and postural control: changes in sensory organization and muscular coordination. Int J Aging Hum Dev. 23:97-114, 1986

ACKNOWLEDGMENT

The authors would like to thank Mary Becker for recruiting subjects. This work was supported by a grant from the Whitaker Foundation and by a National Institute of Health grant No. 1R29AG11602-01A2.