Rapid slowing of maximal finger movement rate: fatigue of central motor control?

Exploring the limits of the motor system can provide insights into the mechanisms underlying performance deterioration, such as force loss during fatiguing isometric muscle contraction, which has been shown to be due to both peripheral and central factors. However, the role of central factors in performance deterioration during dynamic tasks has received little attention. We studied index finger flexion/extension movement performed at maximum voluntary rate (MVR) in ten healthy subjects, measuring movement rate and amplitude over time, and performed measures of peripheral fatigue. During 20 s finger movements at MVR, there was a decline in movement rate beginning at 7-9 s and continuing until the end of the task, reaching 73% of baseline (P < 0.001), while amplitude remained unchanged. Isometric maximum voluntary contraction force and speed of single ballistic flexion and extension finger movements remained unchanged after the task, indicating a lack of peripheral fatigue. The timing of finger flexor and extensor EMG burst activity changed during the task from an alternating flexion/extension pattern to a less effective co-contraction pattern. Overall, these findings suggest a breakdown of motor control rather than failure of muscle force generation during an MVR task, and therefore that the mechanisms underlying the early decline in movement rate are central in origin.