Reticulospinal involvement in the control of complex multi-joint movements requiring endpoint accuracy

Abstract

Introduction The reticulospinal (RS) system is a key descending motor pathway involved in movement coordination, particularly for multi-muscle activities such as posture and locomotion. The StartReact paradigm, which utilizes a loud acoustic stimulus (LAS) to trigger movement via subcortical pathways, has provided insights into RS system function. While StartReact effects are well-documented in simple single-joint tasks, their role in complex, multi-joint movements requiring endpoint accuracy remains unclear. This study aims to expand the existing knowledge of StartReact effects in complex, functional movements representative of daily activities, requiring endpoint accuracy in both upper and lower extremity tasks. Methods Seven healthy participants (5 female, 24.1 ± 6.6 years) performed a reaching and stepping task. For the reaching task, participants sat in front of three targets, reacting to visual cues. For stepping, targets were projected onto the floor, scaled to leg length. Acoustic stimuli were delivered via headphones. Reaction times were assessed using surface electromyography from key muscles, and movement accuracy and kinematics were captured via motion tracking. Results Reaction times were significantly shorter in LAS compared to MAS trials for both the anterior deltoid and biceps brachii, confirming the StartReact effect. LAS trials also showed reduced accuracy, with greater deviation from the target. Variability of reaction times and accuracy showed inter-individual differences. Sagittal movement trajectories showed greater initial vertical displacement and higher vertical endpoint overshoot for LAS trials. Discussion The findings confirm the presence of the StartReact effect in multi-joint reaching movements, as shown by the reduced reaction times. In addition, reduced endpoint accuracy was shown for LAS trials, aligning with Fitts' law. The observed trajectory differences suggest that reticulospinal inputs predominantly influence early movement phases, while corticospinal contributions in later stages may be insufficient for precise endpoint control. The inter-individual variability shown, indicates differences in neuromuscular control mechanisms. Conclusion This study confirms the StartReact effect in complex, targeted multi-joint reaching movements, with faster reaction times in LAS trials driven by reticulospinal input. However, the trade-off between speed and accuracy suggests corticospinal control may be insufficient for refining movement precision. These findings provide insights for neurorehabilitation, emphasizing the potential of reticulospinal pathways in restoring motor function.