Task-dependent differences between mono- and bi-articular heads of the triceps brachii muscle.

We studied motor-unit recruitment and decruitment thresholds in the three heads of the human elbow extensor, the triceps brachii muscle (caput mediale and laterale, both mono-articular heads, and caput longum, the bi-articular head) by means of intramuscular electromyographic-recordings. Two experiments were performed: an 'isometric' and a 'movement' experiment. In the isometric experiment, subjects were asked to increase the elbow extension torque isometrically to a specific level, keep the torque at the level for 10 s, and then decrease the torque again to zero. In the movement experimental subjects moved their forearm from 90 degrees to 110 degrees extension against an increasing flexion torque, kept the latter position for 10 s and then moved their forearm back while the torque decreased. Results for caput longum showed that recruitment thresholds were higher than decruitment thresholds, whereas in caput mediale and laterale no difference in thresholds was found. In caput longum recruitment thresholds were found to be lower in movement conditions than in isometric conditions. The reverse effect was observed in caput mediale, whereas no difference in recruitment thresholds was found in caput laterale. Our results point to a transfer of force from mono-articular muscles in isometric conditions to bi-articular muscles in movement conditions. A similar transfer is found when recruitment and decruitment are compared. This means that the transfer is not only a property of the elbow-flexor muscles, but is a more common trait. A qualitative analysis of firing frequencies at recruitment and at decruitment in both conditions supports our findings.