REDUCED SERVO-CONTROL OF FATIGUED HUMAN FINGER EXTENSOR AND FLEXOR MUSCLES

1. In healthy human subjects holding the index finger semi-extended at the metacarpophalangeal joint against a moderate load, electromyograp hic (EMG) activity was recorded from the finger extensor and flexor mu scles during different stages of muscle fatigue. The aim was to study the effect of muscle fatigue on the level of background EMG activity a nd on the reflex responses to torque pulses causing sudden extensor un loading. Paired comparisons were made between the averaged EMG and fin ger deflection responses under two conditions: (1) at a stage of fatig ue (following a sustained co-contraction) when great effort was requir ed to maintain the finger position, and (2) under non-fatigue conditio ns while the subject tried to produce similar background EMG levels to those in the corresponding fatigue trials. 2. Both the unloading refl ex in the extensor and the concurrent stretch reflex in the flexor wer e significantly less pronounced and had a longer latency in the fatigu e trials. Consequently, the finger deflections had a larger amplitude and were arrested later in the fatigue trials. 3. It is concluded that - with avoidance of 'automatic gain compensation', i.e. reflex modifi cations attributable to differences in background EMG levels - the ser vo-like action of the unloading and stretch reflexes is reduced in fat igued finger extensor and flexor muscles.