Effects of fatigue on the temporal neuromuscular control of vastus medialis muscle in humans

The effects of muscle fatigue on the temporal neuromuscular control of the vastus medialis (VM) muscle were investigated in 19 young male subjects. Th e electromyogram (EMG) activities of VM and the force generation capacities of the quadriceps muscle were monitored before and after a fatigue protoco l. In response to light signals, which were triggered randomly, the subject s made three maximal isometric knee extensions. This was then followed by t he fatigue protocol which consisted of 30 isometric maximal voluntary contr actions at a sequence of 5-s on and 5-s off. Immediately after the exercise to fatigue, the subjects performed another three maximal isometric contrac tions in response to the light signals. The effects of fatigue on the tempo ral neuromuscular control were then investigated by dividing the total reac tion time (TRT) into premotor time (PMT) and electromechanical delay (EMD). The TRT was defined as the time interval between the light signal and the onset of the knee extension force. The PMT was defined as the time from the light signal to the onset of EMG activities of VM, and EMD as the time int erval between onset of EMG activities to that of force generation. Followin g the contractions to fatigue there was a significant decrease in peak forc e (F-peak, P = 0.016), an increase in the root mean square (rms)-EMG: F-pea k quotient (P = 0.001) but an insignificant change in the median frequency (P = 0.062) and rms-EMG (P = 0.119). Significant lengthening of mean EMD wa s found after the fatigue protocol [0.0396 (SD 0.009) vs 0.0518 (SD 0.016) s P < 0.001]. The lengthening of EMD in VM would affect the stabilizing eff ect of the patella during knee extension. The faster mean PMT [0.2445 (SD 0 .093) vs 0.2075 (SD 0.074) s, P = 0.042] following the fatigue protocol mig ht have compensated for the lengthened EMD and contributed to the insignifi cant change in the mean TRT [0.284 (SD 0.09) vs 0.259 (SD 0.073) s, P = 0.1 64]. This was probably related to the low level of fatigue (15% decrease in force) and the stereotyped nature of the action such that the effects of t he fatigue on neuromuscular control were likely to have been attributable t o peripheral processes.