Preparation duration shapes the goal-directed tuning of stretch reflex responses

Stretch reflex responses are important for counteracting perturbations, and the modulation of stretch reflex gains can assist the voluntary movement quality and performance. Recent findings suggest that movement preparation includes goal-directed tuning of muscle spindles, facilitating a concurring modulation of reflex gains. In the context of delayed-reach, goal-directed modulation of stretch reflex gains is present when the homonymous muscle is unloaded (i.e., antagonist load) and the preparatory delay is sufficiently long (>250 ms and <750 ms). This study examined how multiple preparatory delays impact the goal-directed modulation of short- and long-latency stretch reflex (SLR and LLR) gains of the pectoralis, anterior and posterior deltoid muscles in a delayed-reach task. We used electromyographic signals to quantify stretch reflex responses of healthy subjects that experienced haptic perturbations induced by a robotic manipulandum in a delayed-reach task. We found that a preparatory delay of 300 ms is sufficient for goal-directed tuning of SLR responses in the pectoralis and posterior deltoid muscles, and 400 ms allows for even stronger goal-directed tuning of stretch reflex responses. In contrast, there was no goal-directed modulation in the SLR of the anterior deltoid. Tuning of LLR gains was robustly goal-dependent throughout, with negligible modulation across preparatory delays. Our results clarify the minimum preparatory time required for goal-directed tuning of stretch reflex gains and characterise the relationship between reflex gains and preparation duration. This relationship is not strictly linear, likely reflecting the interplay of multiple feedback mechanisms functioning at different time frames.