Changes in recruitment order of motor units in the human biceps muscle

Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasks

“…The methods were similar to those described in detail elsewhere ( 19). A resume and some modifications are given below.…”

“…The results are summarized in Table 1. 'Flexion Units." Motor units that could be recruited only by exerting force in the flexion direction ( 19) were recorded at a lateral site in the muscle belly. A preset level of supination or exorotation did not change the CFL for flexion nor did pronation or endorotation (Fig.…”

“…Units that could be recruited only by a combination of S and E forces, no matter the level of F or extension ( 19), were exclusively found in recordings from the medial side of the BLH, and will be referred to as "supination units" (Fig. 2e).…”

“…Desmedt (12,13) found reversals of recruitment order in that muscle when recording simultaneously at two sites in the muscle. It was found in the human biceps (11, 19) that recruitment of each investigated MU depended either on flexion or on supination and exorotation forces. Also units were found that were recruited when a linear combination of these forces exceeded a certain threshold.…”

“…Motor units recorded in the medial side of the BLH are preferably recruited during isometric supination or exorotation [for a definition of these tasks see ter Haar Romeny et al (19)], whereas laterally recorded MUs are recruited mainly during isometric flexion, independent of supination or exorotation force. Since medial units are attached to the outside of the insertion area of the tendon on the radius they may have a slightly more favorable lever for supination in the semiprone isometric position in which all experiments were done.…”