RELATION BETWEEN ANKLE MOVEMENT AND POSTURAL REFLEXES

G. Wu (1) and J. Chiang (2)
(1) Department of Exercise and Sport Science
(2) Bioengineering Program 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 control of human upright posture involves the integration of sensory information from the somatosensory, vestibular, and visual systems. In the past, the contribution of these sensory systems to postural control has been studied extensively except for the specific role of each of the somatosensory receptors. The reason is not only due to the complexity of the somatosensory system but also due to the difficulty in separating each of these receptors.

In particular, the precise role of joint receptors in postural control is still under discussion. While it is generally accepted at present that joint receptors provide position information only at the extremes of joint rotation (Ferell, 1988), it has been shown that joint sensations are critical during low frequency platform movement (Diener et al. 1986).

The purpose of this study was to investigate the relation between the ankle movement and the postural reflexes in the leg muscles during a rapid, toes-up rotation of the supporting base. The ankle joint was either free to move or immobilized to alter the range of the ankle motion. The latencies of the leg muscle responses were recorded and correlated to the ankle movement.

METHODOLOGY

Fifteen male subjects were included in this study with mean weight of 66±3kg, mean age of 25±4 years. During testing, subjects stood barefoot on a rotational platform with eyes closed. The ankle joints were either allowed to move freely or constrained in the sagittal plane by a pair of ankle immobilizers that were firmly placed around both ankle joints (Chiang, 1995). The platform was rotated in the toes-up direction with an amplitude of 8¡, and velocity of 60¡/s. In response to the platform movement, subjects were instructed to keep their body as still as possible and not to move their feet unless they absolutely had to. A total of 10 trials was repeated, 5 for each ankle condition.

The following biomechanical variables were directly measured: ankle rotation in the sagittal plane by a strain gauge goniometer (Penny & Giles Inc.), EMG signals from the lateral gastrocnemius and tibialis anterior muscles of the right leg by a pair of EMG electrodes (Therapeutics Unlimited), pressure distribution under the feet by a pair of pressure detector insoles (Pedar system, Novel), and platform acceleration by a linear accelerometer (Kistler Instrument Ltd.). Pressure data were collected at 50 Hz for 4 seconds, and the signals from the goniometer, the EMG electrodes and the accelerometer were collected at 1000 Hz for 4 seconds.

The peak pressures were determined in forefoot, midfoot and rearfoot regions. The dynamic change and the maximum range of ankle rotation during the platform movement were calculated. In addition, the angular velocity of the ankle joint movement was calculated using numerical differentiation. Both angular displacement and velocity data were then low- pass filtered by a 4th order numerical Butterworth filter with a cut-off frequency of 25 Hz. The muscle latencies were calculated according to the time between the perturbation onset time and the rising burst of muscle response. They were then categorized into short latency (30 to 69 ms post perturbation onset), medium latency (70 to 119 ms post) for the gastrocnemius muscle, and long latency (120 to 200 ms post) for the tibialis anterior muscle.

Analysis of variance (ANOVA) was used to identify differences between two ankle conditions for three muscle latencies, regional peak pressures and maximum range of ankle joint rotation. This analysis was followed by a Least Squares Means test when significant main effect differences were found. These effects were considered significant at p < 0.05.

RESULTS

Ankle joint rotation: During the entire perturbation period the ankle movement increased monotonously with time (Fig. 1). When the ankle joints were free to move, they changed about 1.2¡ at 50ms and up to 8¡ at 300ms. When both ankles were constrained with the immobilizer, they changed very little (about 1¡ maximum) throughout the entire perturbation period.

Fig. 1. Ankle angle after onset of the platform movement for both ankle conditions.

Pressure: The regional peak pressures for both ankle conditions were remarkably similar (Fig. 2). Statistical analysis revealed that the peak pressures between the two ankle conditions were not significantly different (p>0.05).

Fig. 2. Percent differences of plantar pressures between two ankle conditions.

Muscle latencies: The means and standard deviations of latencies of leg muscle responses for both ankle conditions are shown in Fig. 3. Statistical analysis revealed that the short latencies were not significantly different between ankle conditions (p = 0.39). However, when the ankle joints were immobilized, both medium and long latencies were significantly delayed compared to those without having the ankles fixed (p < 0.044).

Fig. 3. Mean+std. of short, medium and long latencies for both ankle conditions. Symbol * indicates significant difference between two conditions (p<0.05).

DISCUSSIONS

In this study, three variable, namely, ankle joint movement, plantar pressure under the feet and postural reflexes in the leg muscles were monitored during a fast, toes-up rotation of a supporting base. Comparing to the condition when the ankles were allowed to move freely, it is found that when both ankle joints were immobilized both medium and long latency responses in the leg muscles were significantly delayed. Furthermore, the ankle movements were found to be greatly limited while the plantar pressures were not altered. Therefore, it can be concluded that the input of joint receptors could affect the medium and long latency muscle responses.

REFERENCES

Ferell, W.R. Discharge characteristics of joint receptors in relation to their proprioceptive role. In: Soukoup, T. et al. (Eds), Mechanoreceptors: dev struc func, Plenum NY, 1988

Diener et al. Role of visual and static vestibular influences on dynamic posture control. Human Neurobiol. 5:105-113, 1986

Chiang, J. The effect of plantar cutaneous mechanoreceptor input on dynamic upright posture. M.S. Thesis, Penn State University, 1995

ACKNOWLEDGMENT

This work was supported by a grant from the Whitaker Foundation and by a National Institute of Health grant No. 1R29AG11602-01A2