Electromyographic evidence of delayed fatigue-induced sarcolemmal excitability impairment in McArdle's disease.

Loss of sarcolemmal excitability is an early sign of fatigue in exercising muscle. This can be determined from a decreasing muscle fibre conduction velocity (MFCV). Underlying causes are still unclear. Three McArdle's disease patients and seven healthy controls performed sustained isometric ischaemic biceps brachii exercise until exhaustion. Each subject participated in three tests: at 40%, at 80% and at 100% of the maximal voluntary contraction (MVC). Analysis was done over the period in which the force level was maintained at 40% and 80% MVC, and over the period during loss of force at all three levels. We found that, in contrast to the occurrence of an immediately starting and ongoing decrease of MFCV in controls, a delayed onset of this decrease was observed in patients with McArdle's disease. Only during the loss of force phase was the MFCV decrease similar in patients and controls. The early occurrence of an MFCV decrease in controls appears to be related to the accumulation of lactic acid, which is virtually absent in the patient group. During force loss, different (additional) mechanisms must be responsible for the MFCV decrease in patients and, most probably, in controls as well.