Human stance on a sinusoidally translating platform: balance control by feedforward and feedback mechanisms

Abstract

With subjects standing on a treadmill moving sinusoidally backward and forward, recordings of electromyographic (EMG) leg and trunk muscle activity, head and joint movements and platform torque were made with the subjects' eyes open or closed. The sinusoidal frequency was changed, stepwise and randomly, between 0.5, 0.3 and 0.25 Hz. The amplitude of the deflection was constant at +/- 12 cm. During an adapted sinus cycle, the maximum leg muscle EMG activity was recorded in the tibialis anterior around the posterior turning point and in the gastrocnemius around the anterior turning point in the treadmill cycle. This activity was associated with a forward inclination of the body around the posterior point and a straightening of the body at the anterior point. Both the degree of body inclination and the corresponding EMG activity were dependent upon the sinusoidal frequency. The programmed adjustment of the body inclination was such that the result of inertial and gravitational forces acting on the body coincided with the axis of the body at the posterior turning point. At the anterior point, the adjustment was achieved mainly by strong activation of the leg extensors. The latencies of the compensatory muscle responses to a change in treadmill frequency were significantly shorter at the posterior point for the gastrocnemius than for the tibialis anterior, and at the anterior point for the tibialis anterior than for the gastrocnemius. No correlated changes were seen in the corresponding head and joint movements. The difference in latency can best be attributed to the different body postures during the sinusoid. Early activation of the gastrocnemius is required due to the forward-directed impulse to the inclined body at the posterior point, and of the tibialis anterior muscle due to the backward-directed impulse to the erect body at the anterior point. It is suggested that afferent input from extensor load receptors provides information about the position of the body's centre of gravity relative to the support surface and determines the generation of the EMG responses. Adaptation of both the EMG and biomechanical patterns to a new sinusoidal frequency of the treadmill occurred within four cycles after the change. Biomechanically, this was reflected as a change in the body posture. Vision did not significantly affect these changes. In conclusion, standing on a sinusoidally moving platform, the nervous system acts to control the position of the body's centre of gravity relative to the feet. Body posture is adjusted in such a way that the forces acting on the body during the treadmill movements become minimised.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of position and movement is simplified by combined muscle spindle and Golgi tendon organ feedback

“…In the literature, it has been suggested that force feedback from GTOs enhances the stabilization of posture (e.g., Dietz et al 1992Dietz et al , 1993van Soest and Rozendaal 2008). It has been theorized that GTO feedback needs to be positive (for an overview see Prochazka et al 1997a) and indeed seems to be supported experimentally (e.g., Dietz et al 1993;Pratt 1995;Prochazka et al 1997aProchazka et al , 1997b. In the proposed scheme of low-level position control, spindle and GTO afferents are combined to sense muscle-tendon complex length, which is subtracted from a reference MTC length and fed back to the ␣-motoneuron.…”

Postural muscle activity patterns during standing at rest and on an oscillating floor

“…We previously reported that the improvement in steadiness of standing posture reached a plateau for a total of 5 minutes during floor oscillation [11]. Moreover, the anteroposterior position of the center of foot pressure (CFP) in the standing posture during floor oscillation shifts forward as oscillation frequency rises [8,22] and the momentum of the anteroposterior direction around the ankle joint changes according to the distance of CFP position from the heel [24]. As CFP position shifts forward, the activity of triceps surae increases while that of tibialis anterior changes very little [10].…”

“…However, this postulated change in trunk and thigh muscle activity has not been discussed in relation to dominance of anterior or posterior muscle activity. 5 It has been reported that in the lower leg, periodic muscle activity corresponding to floor oscillation curve increases at relatively higher frequencies of oscillation, with continuous muscle activity not related to the oscillation decreasing in the way between directional changing points of oscillation and periodic ballistic activity increasing where oscillation direction changes [8]. However, the periodicity of the trunk and thigh muscles has not been sufficiently investigated.…”

Condición muscular y estabilidad del tronco en judocas de nivel nacional e internacional

“…Diversos autores (Perrin et al, 2002;Yoshitomi et al, 2006) señalan que el entrenamiento reta al sistema nervioso central a controlar la posición del centro de gravedad del cuerpo favoreciendo la organización de patrones posturales en tareas de equilibrio en función de la información sensorial disponible y los condicionantes biomecánicos (Allum et al, 1998;Dietz et al, 1993). Estos resultados, unidos a la ausencia de diferencias entre los judocas al analizar la respuesta involuntaria del tronco ante las cargas súbitas, podrían sugerir que la diferencia en el control del tronco entre judocas de competición se manifiesta en condiciones dinámicas donde el control voluntario es un factor determinante.…”