Figure 11.7 Mean EMG amplitudes of 22 subjects recorded at different positions (1 cm distance between electrode positions) across the upper trapezius from near the clavicle (El.A) to near the scapula (El.H) during an isometric shoulder elevation with increasing force (acromion as the point of force application). The EMG amplitudes were plotted against the amplitude recorded at position D corresponding to the standard position approximately halfway between the clavicle and spina scapula (reproduced from Jensen and Westgaard, 1995).

ABSTRACT

Although it remains controversial, another principle of motor unit regulation may exist. The phenomenon is termed motor unit rotation, i.e., newly recruited motor units replace previously active units, possibly to avoid or postpone signs of fatigue. Indications of motor unit rotation have been obtained during prolonged isometric contractions of the biceps brachii at 10 per cent MVC, whereas the phenomenon was not observed at 40 per cent MVC (Fallentin et al., 1993). Motor unit rotation may not only depend on the required force level; other demands which lead to activation of more motor units such as an increased demand of precision, mental demands, etc. may also reduce the potential for motor unit rotation. Continous long-term activation of even a few motor units in a muscle may be causally related to the development of muscle disorders (Hägg, 1991).