Directional sensitivity of stretch reflexes and balance corrections for normal subjects in the roll and pitch planes

A large body of evidence has been collected which describes the response pa rameters associated with automatic balance corrections in man to perturbati ons in the pitch plane. However, perturbations to human stance can be expec ted from multiple directions. The purpose of the present study was to descr ibe the directional sensitivities of muscle responses re-establishing distu rbed stance equilibrium in normal subjects. The contributions of stretch re flex and automatic balance-correcting responses to balance control, and con comitant biomechanical reactions, were examined for combinations of pitch a nd roll perturbations of the support surface. More specifically, muscle res ponses, initial head accelerations and trunk velocities were analyzed with the intention of identifying possible origins of directionally specific tri ggering signals and to examine how sensory information is used to modulate triggered balance corrections with respect to direction. Fourteen healthy a dults were required to stand on a dual-axis rotating platform capable of de livering rotational perturbations with constant amplitude (7.5 degrees) and velocity (50 degrees/s) through multiple directions in the pitch and roll planes. Each subject was randomly presented with 44 support surface rotatio ns through 16 different directions separated by 22.5 degrees first under ey es-open, and then, for a second identical set of rotations, under eyes-clos ed conditions. Bilateral muscle activities from tibialis anterior, soleus, lateral quadriceps and paraspinals, were recorded, averaged across directio n, and areas, calculated over intervals with significant bursts of activity . Trunk angular velocity and ankle torque data were averaged over intervals corresponding to significant biomechanical events. Stretch reflex (interva ls of 40-100, 80-120 ms) and automatic balance-correcting responses (120-22 0, 240-340 ms) in the same muscle were sensitive to distinctly different di rections. The directions of the maximum amplitude of balance-correcting act ivity in leg muscles were oriented along the pitch plane, approximately 180 degrees from the maximum amplitude of stretch responses. Ankle torques for almost all perturbation directions were also aligned along the pitch plane . Stretch reflexes in paraspinal muscles were tuned along the 45 degrees pl ane but at 90 degrees to automatic balance corrections and 180 degrees to u nloading responses in the same muscle. Stretch reflex onsets in paraspinal muscles were observed at 60 ms, as early as those of soleus muscles. In con trast, unloading reflexes in released paraspinal muscles were observed at 4 0 ms for perturbations which caused roll of the trunk towards the recorded muscle. Onsets of trunk roll velocities were earlier and more rapid than th ose: observed for pitch velocities. Trunk pitch occurred for pure roll dire ctions but not vice versa. When considered together, early stretch and unlo ading of paraspinals,;md concomitant roll and pitch velocities of the trunk requiring a roll-and-pitch-based hip torque strategy, bring into question previous hypotheses of an ankle-based trigger signal or ankle-based movemen t strategies for postural balance reactions. These findings are compatible with the hypothesis that stretch-, force- and joint-related proprioceptive receptors at the level of the trunk provide a directionally sensitive trigg ering mechanism underlying a minimally two-stage (pitch-based leg and pitch -and-roll-based trunk) balance-correcting strategy. Accelerometer recording s from the head identified large vertical linear accelerations only for pit ch movements and angular roll accelerations only during roll perturbations with latencies as early as 15 ms. Thus, it appears that balance corrections in leg and trunk muscles may rece ive strong, receptor-dependent (otolith or vertical canal) and directionall y sensitive amplitude-modulating input from vestibulospinal signals.