Motor unit behaviour and contractile changes during fatigue in the human first dorsal interosseus

1. In 67 single motor units, the mechanical properties, the recruitment and derecruitment thresholds, and the discharge rates were recorded concurrent ly in the first. dorsal interosseus (FDI) of human subjects during intermit tent fatiguing contractions. The task consisted of isometric ramp-and-Bold contractions performed at 50% of the maximal voluntary contraction (MVC). T he purpose of this study was to examine the influence of fatigue on the beh aviour of motor units with a wide range of activation thresholds. 2. For low-threshold (<25% MVC) motor units, the mean twitch force increase d with fatigue and the recruitment threshold either did not change or incre ased. In contrast, the twitch force and the activation threshold decreased for the high-threshold (>25% MVC) units. The observation that in low-thresh old motor units a quick stretch of the muscle at the end of the test reset the unit. force and recruitment threshold to the prefatigue value suggests a significant. role for fatigue-related changes in muscle stiffness but not twitch potentiation or motor unit synchronization. 3. Although the central drive intensified during the fatigue test, as indic ated by an increase in surface electromyogram (EMG), the discharge rate of the motor units during the hold phase of each contraction decreased progres sively over the course of the task for motor units that were recruited at t he beginning of the test, especially the low-threshold units. In contrast, the discharge rates of newly activated units first. increased and then decr eased. 4. Such divergent behaviour of low- and high-threshold motor units could no t be individually controlled by the central drive to the motoneurone pool. Rather, the different behaviours must, be the consequence of variable contr ibutions froth motoneurone adaptation and afferent, feedback from the muscl e during the fatiguing contraction.