Electrical stimulation of human forearm extensor muscles as an indicator of handgrip fatigue and recovery

A set-up for percutaneous electrical stimulation of the forearm extensor muscles and measurement of wrist extension force is described. The frequency-force relationship and pulse duration-force relationship are described together with an experimental protocol showing that brief electrical test stimulations do not produce fatigue. In another set of experiments carried out a few weeks later, the subjects performed handgrip contractions: protocol A at 25% of maximal voluntary contraction (MVC) continuously until exhaustion, protocol B at 25% MVC intermittent (contraction + relaxation = 10 + 2 s) until exhaustion, and protocol C at 25% MVC intermittent until half the time to exhaustion. In all experiments, brief electrical stimulations were used to test the degree of fatigue during and up to 24 h after the experiments. There were marked changes in the force during stimulation at 20 and 100 Hz and these changes did not correlate with the increase in intramuscular temperature. Low frequency fatigue persisted for at least 24 h after protocol A and 1 h after protocols B and C. The significance of this is discussed and it is suggested that low frequency fatigue could be used as a sensitive indicator of muscle dysfunction after low and medium intensity exercise.