Decoupling of stretch reflex and background muscle activity during anticipatory postural adjustments in humans

The benefit of knowledge: postural response modulation by foreknowledge of equilibrium perturbation in an upper limb task

For whole-body sway patterns, a compound motor response following an external stimulus may comprise reflexes, postural adjustments (anticipatory or compensatory), and voluntary muscular activity. Responses to equilibrium destabilization may depend on both motor set and a subject`s expectation of the disturbing stimulus. To disentangle these influences on lower limb responses, we studied a model in which subjects (n = 14) were suspended in the air, without foot support, and performed a fast unilateral wrist extension (WE) in response to a passive knee flexion (KF) delivered by a robot. To characterize the responses, electromyographic activity of rectus femoris and reactive leg torque was obtained bilaterally in a series of trials, with or without the requirement of WE (motor set), and/or beforehand information about the upcoming velocity of KF (subject`s expectation). Some fast-velocity trials resulted in StartReact responses, which were used to subclassify leg responses. When subjects were uninformed about the upcoming KF, large rectus femoris responses concurred with a postural reaction in conditions without motor task, and with both postural reaction and postural adjustment when WE was required. WE in response to a low-volume acoustic signal elicited no postural adjustments. When subjects were informed about KF velocity and had to perform WE, large rectus femoris responses corresponded to anticipatory postural adjustment rather than postural reaction. In conclusion, when subjects are suspended in the air and have to respond with WE, the prepared motor set includes anticipatory postural adjustments if KF velocity is known, and additional postural reactions if KF velocity is unknown.

Do postural constraints affect eye, head, and arm coordination?

If a whole body reaching task is produced when standing or adopting challenging postures, it is unclear whether changes in attentional demands or the sensorimotor integration necessary for balance control influence the interaction between visuomotor and postural components of the movement. Is gaze control prioritized by the central nervous system (CNS) to produce coordinated eye movements with the head and whole body regardless of movement context? Considering the coupled nature of visuomotor and whole body postural control during action, this study aimed to understand how changing equilibrium constraints (in the form of different postural configurations) influenced the initiation of eye, head, and arm movements. We quantified the eye-head metrics and segmental kinematics as participants executed either isolated gaze shifts or whole body reaching movements to visual targets. In total, four postural configurations were compared: seated, natural stance, with the feet together (narrow stance), or while balancing on a wooden beam. Contrary to our initial predictions, the lack of distinct changes in eye-head metrics; timing of eye, head, and arm movement initiation; and gaze accuracy, in spite of kinematic differences, suggests that the CNS integrates postural constraints into the control necessary to initiate gaze shifts. This may be achieved by adopting a whole body gaze strategy that allows for the successful completion of both gaze and reaching goals.

NEW & NOTEWORTHY Differences in sequence of movement among the eye, head, and arm have been shown across various paradigms during reaching. Here we show that distinct changes in eye characteristics and movement sequence, coupled with stereotyped profiles of head and gaze movement, are not observed when adopting postures requiring changes to balance constraints. This suggests that a whole body gaze strategy is prioritized by the central nervous system with postural control subservient to gaze stability requirements.