Short latency afferent inhibition differs with load type during isometric finger abduction

Background Static muscle contraction involves two distinct load types. One type, called a position task, entails holding the limb in a fixed position while counteracting an inertial load, while the other type, known as a force task, involves exerting a consistent force against a solid constraint. While proprioceptive information has been shown to be required more during the position task, it has remained to be elucidated how sensorimotor integration differs between these two tasks. Methods This study investigated differences in short latency afferent inhibition (SAI) and heteronymous reflex responses between the force and position conditions. Sixteen participants performed static contractions of the first dorsal interosseous (FDI) muscle. In the force task, they exerted a constant force corresponding to 10% maximum voluntary contraction (MVC) against a rigid restraint. In the position task, they sustained a target abduction angle of 20° while holding a load equivalent to 10% MVC. SAI was induced by the paired application of electrical stimulation to the right median nerve and transcranial magnetic stimulation over the left motor cortex at an N20 + 2 msec interval. Motor evoked potentials (MEPs) were recorded from the FDI muscle to quantify the magnitude of SAI. Heteronymous short and long latency reflexes (SLR and LLR) were also examined, and their amplitudes were compared between the force and position tasks. Results SAI was significantly attenuated in the position task ( p  < 0.05). Additionally, SLR and LLR amplitudes were significantly greater during position task ( p  < 0.05). Conclusions These findings suggest distinct sensorimotor processing strategies depending on the load type.