Mapping reticulospinal drive across various muscles of the upper and lower extremities.

The reticulospinal (RS) system is a fundamental descending pathway involved in human movement control. However, the relative strength of RS projections across different muscles and its specific contributions to distinct movements are poorly understood. We systematically mapped the RS drive across a broad range of muscles in the upper and lower extremities. The RS drive was assessed in 14 muscles of 29 healthy participants using the StartReact paradigm, characterized by shortened premotor reaction times when movement initiation is paired with a loud versus moderate acoustic stimulus. Reaction times were assessed by surface EMG. RS drive was compared as follows: (1) across individual muscles; (2) between proximal and distal muscles; and (3) between flexor and extensor muscles. The RS drive was lowest in the finger abductor, with significantly reduced values in comparison to those in the shoulder flexor and extensor, the elbow flexor, hip and knee extensors, and the ankle plantar flexor. A proximal-to-distal gradient in RS drive was observed only in the upper extremities, mainly attributable to the low RS drive to the finger abductor. Additionally, the RS drive was greater to flexors than to extensors in the upper extremities. Conversely, the RS drive was enhanced to extensors versus flexors in the lower extremities. Our findings emphasize the presence of RS drive in all examined muscles, with no distinctive proximal-to-distal gradient in RS motor control. Notably, a reversed flexor-extensor bias in RS control was evident between the upper and lower extremities. These findings advance our understanding of RS motor control and might inform the development of targeted neurorehabilitation strategies.