RETICULOSPINAL CONTRIBUTIONS TO UPPER-LIMB FORCE PRODUCTION DURING UPRIGHT STANDING IN STRENGTH-TRAINED AND UNTRAINED INDIVIDUALS

Coordination between limb and trunk muscles plays an essential role in daily actions, such as lifting objects while standing. Previous studies have primarily investigated the neural mechanisms underlying limb-trunk coordination with a focus on the corticospinal pathway. More recently, the reticulospinal tract has been suggested to contribute to coordinated limb-trunk muscle activation. However, it remains unclear whether reticulospinal excitability during coordinated limb-trunk movements is modulated by exercise experience. This study aimed to examine differences in reticulospinal excitability between strength-trained and untrained individuals. Five trained and five untrained healthy men participated in this study. Participants performed a rapid bilateral arm-curl task while standing upright, using a load corresponding to 35% of their one-repetition maximum (1RM), in response to visual, visual-auditory (80 dB), or visual-startling (116 dB) stimuli. Following task familiarization, participants completed six blocks, each consisting of one practice trial followed by five trials per stimulus condition. Electromyography (EMG) was recorded from the right biceps brachii (BB) and erector spinae (ES) muscles during the task. The trained group exhibited significantly greater 1RM than the untrained group (p < 0.001), confirming differences in exercise experience. In the trained group, EMG onsets of BB significantly shortened (p < 0.001) and showed a trend toward shortening in ES (p = 0.074) during visual-startling stimuli, suggestive of increased excitability of reticulospinal pathways, whereas no such changes were observed in the untrained group (p ≧ 0.204). These findings suggest that strength training induces adaptive changes in reticulospinal excitability during coordinated limb-trunk movements under upright postural control.