Muscle coactivation can prime the nervous system for rapid feedback control

Abstract

Humans often interact with environments where they may encounter disturbances that can vary in direction and amplitude between or within movement. In turn, these disturbances can threaten success of their actions. Recent advances in Optimal Feedback Control highlight the flexible use of sensory feedback when dealing with unpredictable disturbances. In this thesis, we investigated how features of the body or task alter how the healthy nervous system processes sensory feedback. Chapter 2 examined whether right-handed individuals (N=28) express differences in stretch responses when controlling the posture of the dominant and nondominant arms. Across two experiments, we found no differences in stretch responses between the two arms. Instead, behavioral and muscle responses correlated across arms. Chapter 3 tested whether participants (N=70) alter their responsiveness to sensory feedback when reaching in the presence of random, time-varying torque disturbances. Across three experiments, we demonstrated that participants increased and tuned their responses to proprioceptive and visual feedback as well as muscle coactivation to the variability of the random torque disturbances. Chapter 4 investigated whether participants (N=90) modulate their responsiveness to sensory feedback when exposed to unpredictable visuomotor rotations that can vary in amplitude and direction between reaching movements. Across three experiments, we showed that participants increased and tuned their responses to visual and proprioceptive feedback and muscle coactivation to the variability of the unpredictable visuomotor rotations. Moreover, increases in muscle coactivation correlated with the vigor of corrective responses to visual and proprioceptive feedback. Chapter 5 examined the role of muscle coactivation in upregulating responses to sensory feedback when initiating and controlling voluntary reaching movements. Across four experiments (N=80), we found that participants spontaneously increased muscle coactivation and responses to sensory feedback when the task imposed greater temporal urgency. Moreover, instructions to coactivate upregulated responses to visual, auditory, and somatosensory feedback and expedited the initiation and control of voluntary reaching movements. Collectively, the studies included in this thesis highlight the important role of muscle coactivation in upregulating responses to sensory feedback to provide mobility. The findings raise questions about the traditional role of muscle coactivation in leveraging viscoelastic properties of skeletal muscles to provide stability.