Effects of joint immobilization on firing rate modulation of human motor units

We investigated the effects of 6 weeks of immobilization on firing rate modulation in motor units in the first dorsal interosseous (FDI) of human volunteers. The middle finger, index finger and thumb were immobilized for a period of 6 weeks in a fibre-glass cast, which kept FDI in a shortened position. During isometric contraction at 20, 40, 60 and 80 % of maximal voluntary contraction (MVC) (index finger abduction), motor unit action potentials were recorded from the FDI using a tungsten microelectrode, and the relationship between voluntary force and mean firing rate (MFR) was obtained by plotting the MFR of each motor unit action potential train as a function of voluntary force. Four recording sessions were held for each subject: before immobilization, after 3 and 6 weeks of immobilization, and after a 6 week recovery period. As a result of immobilization, FDI volume (as measured by computerized tomography (CT) scanning) decreased, with an accompanying reduction in aggregate EMG activity per day (P < 0.01). The force measured during MVC also decreased (P < 0.05). The slope of the relationship between voluntary force and MFR was significantly decreased after immobilization, as was the range of firing rate modulation (P < 0.01). Maximal MFR, estimated from the relationship between voluntary force and MFR, was decreased (P < 0.05). MFR was also plotted against voluntary force without being normalized with respect to MVC, and the slope of the regression line was decreased (P < 0.05). Voluntary force when the MFR was 15 Hz was estimated from regression equations for the absolute force-MFR relationship, and it was increased after immobilization (P < 0.05). These results suggest that firing rate modulation shows two different adaptations to joint immobilization: a restriction of motoneurone firing to the lower rates and an enhancement of the voluntary force exerted when the MFR is relatively low.