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

Abstract

A large body of evidence has been collected which describes the response parameters associated with automatic balance corrections in man to perturbations in the pitch plane. However, perturbations to human stance can be expected from multiple directions. The purpose of the present study was to describe the directional sensitivities of muscle responses re-establishing disturbed stance equilibrium in normal subjects. The contributions of stretch reflex and automatic balance-correcting responses to balance control, and concomitant biomechanical reactions, were examined for combinations of pitch and roll perturbations of the support surface. More specifically, muscle responses, initial head accelerations and trunk velocities were analyzed with the intention of identifying possible origins of directionally specific triggering signals and to examine how sensory information is used to modulate triggered balance corrections with respect to direction. Fourteen healthy adults were required to stand on a dual-axis rotating platform capable of delivering 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 rotations through 16 different directions separated by 22.5 degrees first under eyes-open, and then, for a second identical set of rotations, under eyes-closed conditions. Bilateral muscle activities from tibialis anterior, soleus, lateral quadriceps and paraspinals were recorded, averaged across direction, 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 (intervals of 40-100, 80-120 ms) and automatic balance-correcting responses (120-220, 240-340 ms) in the same muscle were sensitive to distinctly different directions. The directions of the maximum amplitude of balance-correcting activity 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 plane but at 90 degrees to automatic balance corrections and 180 degrees to unloading responses in the same muscle. Stretch reflex onsets in paraspinal muscles were observed at 60 ms, as early as those of soleus muscles. In contrast, unloading reflexes in released paraspinal muscles were observed at 40 ms for perturbations which caused roll of the trunk towards the recorded muscle. Onsets of trunk roll velocities were earlier and more rapid than those observed for pitch velocities. Trunk pitch occurred for pure roll directions but not vice versa. When considered together, early stretch and unloading of paraspinals, and concomitant roll and pitch velocities of the trunk requiring a roll-and-pitch-based hip torque strategy, bring into qu...

The influence of artificially increased hip and trunk stiffness on balance control in man

“…2, 3, 7). Trunk angular velocity was calculated as the average over the intervals between 90-130 and 180-220 ms for roll and pitch, respectively, that is at the time when these velocities are known to peak, on average, from previous studies (Carpenter et al 1999). All angular velocity traces (two each for the trunk and arm) were integrated off-line using trapezoid integration to yield angular displacement.…”

“…Outcome measures EMG and biomechanical measurements were obtained using previously described techniques (Carpenter et al 1999;. To record EMG activity, pairs of silver-silver chloride electrodes were placed approximately 3 cm apart along the muscle bellies of left tibialis anterior, left soleus, left peroneus longus, left medial gastrocnemius and bilaterally on paraspinals at the L1-L2 level of the spine, and on the left medial deltoid (pars acromialis) muscles.…”

“…Presumably the use of multiple joints would require use of feedback information from all these joints, as well as increased interlink coordination, to maintain stability compared to the requirements to control motion about the ankle joint alone. Furthermore, adequate balance corrections in the pitch and roll planes must be processed-and perhaps generated-sequentially as the trunk moves more rapidly in roll than pitch when compensating for multidirectional falls (Carpenter et al 1999). …”

“…The corsets should also induce marked instability for another reason. Prior studies suggested that the difference in trunk flexibility in the roll and pitch directions is crucial to maintaining balance following multidirectional perturbations (Carpenter et al 1999;Allum et al 2002). Thus eliminating or changing this difference with corsets might lead to difficulties in CNS processing of balance corrections.…”

“…Recently, however, this concept has been challenged for motion occurring during quiet stance (Aramaki et al 2001;Loram and Lakie 2002). Indeed, as body motion increases in amplitude and direction from that of quiet standing (Fitzpatrick et al 1992(Fitzpatrick et al , 1994Winter et al 1996Winter et al , 1998Gatev et al 1999;Accorneo et al 1997), to that induced by a support surface perturbation in the pitch plane alone (Cordo and Nashner 1982;Allum et al 1993;Horak et al 1997), and finally to that induced by combined roll and pitch plane perturbations (Moore et al 1988;Maki et al 1994aMaki et al , 1994bHenry et al 1998b;Carpenter et al 1999;Allum et al 2002), the multi-link nature of human postural corrections becomes increasingly prominent. This multi-link strategy involves hinging at the knees, hips and lower vertebral column, in addition to ankle joint motion .…”