Motor unit rate coding in intrinsic hand muscles during isolated finger contractions and pinch task

PurposeMuscle force output is modulated via motor unit recruitment and rate coding, yet how rate coding in intrinsic hand muscles differs between isolated and synergistic hand tasks remains unclear. This study examined motor unit discharge behaviour in the first dorsal interosseous (FDI) and thenar during isolated index finger flexion, isolated thumb flexion, and a tip pinch task. MethodsSeventeen participants performed each task at 10%, 20%, and 30% of their maximal voluntary contraction (MVC) while high-density surface electromyograms (HDsEMG) were recorded from both muscles. Motor unit spike trains were completely decomposed from the HDsEMG recordings, tracked across force levels, and their mean discharge rates and recruitment thresholds were calculated. ResultsFor both FDI and thenar muscles, the mean discharge rate increased with force, but the FDI exhibited steeper slopes than the thenar and at 10-20% MVC than 20%-30% MVC. In addition, lower recruitment thresholds and higher mean discharge rates were observed in the FDI compared to the thenar. Task-dependent differences were also observed in the FDI, with the pinch task yielding higher discharge rates than isolated contractions. In the thenar, differences between tasks were limited to higher forces. ConclusionThese findings demonstrate muscle- and task-specific motor unit modulation across forces in the intrinsic hand muscles, where the FDI relies more on rate coding, while the thenar likely prioritizes recruitment to cope with increased force demands.