Activity of Motor Units in Human Elbow Flexor and Extensor Muscles during Task-Dependent Unloading

Surface EMGs and activity of 73 single motor units (MUs) of the m. triceps brachii caput laterale (TBcl) and 25 units of the m. biceps brachii caput longum (BBcl) were recorded in five adults during unloading within the ramp phase of the isotorque ramp-and-hold movements in the elbow joint (“movement” task) and during maintaining of a certain position (“position control” task). Analysis of the surface EMGs showed that patterns of the MU responses to unloading in different motor tasks are dissimilar. Depending on the responses to unloading, the examined MUs were classified into two groups. Motor units of the first group, including 14 TBcl MUs and 22 BBcl ones, were characterized as task-independent units. Their spiking was completely inhibited in response to unloading in both motor tasks. The second group (44 TBcl and 3 BBcl MUs) should be considered as a group of the task-dependent units. These MUs were active during unloading in the “movement” task but were fully inhibited in the “position control” task. This specificity in the reactions of MUs during movements may be related to their different functions. Presumably, the former group of MUs is mainly involved in the force control, while the latter one is associated with the control of the movement per se .