[Dependence of the action potential amplitude of motor units on the recruitment threshold: implications for...

The relationship between the action potential amplitude and the threshold force of recruitment of 275 motor units (MU) recorded from the first dorsal interosseus muscle of 15 healthy subjects was analysed. The number of newly recruited MU decreased exponentially with increasing muscular force. Action potential amplitudes showed a highly significant positive linear correlation with threshold force of recruitment. This relationship is in accordance with the size principle of MU recruitment. Applying the rank correlation method, 88.6 +/- 2.0% (mean +/- standard deviation) of paired comparisons between all MU of one subject followed the normal activation sequence of MU with an increasing action potential amplitude. It is shown that small variations of the distance between the leading-off surface of the electrode and the MU recorded do not affect these findings. These results demonstrate that the force at which a MU is recruited is the most important factor influencing MU action potential amplitude in normal subjects. So far this fact has not been adequately appreciated in clinical electromyography although it is of significance for the assessment of pathological conditions.