Long-latency and Volitional Responses Modulate Upper Extremity Movements During a Novel Visuomotor Tracking Task
Authors: Dubey Olga , Shiao Chishan , Petrie Michael , Shields Richard 
Published: May 2024

Precise upper limb movements are essential for daily tasks and motor function. These movements are governed by feedforward and feedback mechanisms. Feedforward responses enable anticipatory movement planning, while feedback responses utilize sensory information for real-time corrections. Long-latency reflexes (LLRs) represent crucial feedback responses during unexpected perturbations. LLRs are integral in maintaining motor control and can adapt based on various factors. However, the mechanisms governing LLRs, especially in the upper extremity, remain unclear. Methods: Forty healthy participants performed a visuomotor elbow flexion task with one arm while the opposite limb remained static. Muscle activity of the biceps and triceps muscles of both upper extremities was recorded during perturbations. Participants completed a task with varying speeds and resistance levels. Electromyography data was analyzed across pre-perturbation, short-latency reflex (SLR), long-latency reflex (LLR), and voluntary response (VR) phases. Results: In response to unexpected upper extremity perturbations, participants predominantly relied on two core strategies. Inhibitory long-latency reflexes within the biceps played a central role, with participants emphasizing inhibition to maintain movement stability in a window of 50-120 ms after the disturbance. Furthermore, volitional control, primarily executed through the triceps muscles, emerged as another dominant approach, allowing participants to actively regain precision and control after the perturbation in a volitional manner starting from 120 ms. Participants’ response to perturbations reflected the task context and perceived urgency. Over repeated trials, participants refined their response strategies, becoming more precise and effcient. Variations in movement speed and resistance influenced muscle activity. Preliminary evidence suggested the involvement of cross-transcortical communication in response to perturbations, with delayed responses observed in the non-participating upper extremity. Conclusion: This study reveals that participants use both long-latency and volitional responses to counteract perturbations during an upper extremity visuomotor task, adapting their response strategies over time. Biceps inhibitory LLRs and triceps voluntary activation characterize feedback control in this context. These findings shed light on the dynamic nature of upper extremity feedback responses during unpredictable perturbations and suggest the involvement of cross-transcortical communication. Understanding these mechanisms can inform rehabilitation strategies for individuals with neurological conditions affecting motor control. Research Support: This study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development: R01HD084645, R01HD082109. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.