The influence of force and circulation on average muscle fibre conduction velocity during local muscle fatigue.

Two series of experiments were performed to examine the relationship between force and change in average muscle fibre conduction velocity (MFCV) during local muscle fatigue. The average MFCV was estimated using the cross-correlation method. In the first experiment this relationship was studied with surface EMG of vastus lateralis at force levels from 10 to 100% of maximal voluntary contraction (MVC) with and without occluded circulation. The product of relative force and time was held constant. At 10-20% MVC, MFCV increased slightly under the 2 conditions. Between 30-40% MVC, MFCV decreased, this decline in conduction velocity being significantly greater with occluded circulation. Above 40% MVC the decline in MFCV was larger at higher forces, but without any differences between the ischaemic and non-ischaemic conditions. In the second experiment the relationship between change in force and MFCV was studied during sustained maximal voluntary contractions of biceps brachii. MFCV declined during the first 26-39 s of the contraction, followed by an increase. Since this increase occurred when the force had dropped to 30-50% of the initial maximal force, a partial restoration of blood flow is thought to be responsible for this phenomenon. Because an increase in MFCV was noted, despite a further decline in force, this implies that at high force levels the change in MFCV during fatigue could partly be caused by mechanisms different from those accounting for the force loss. It is concluded that above 40% MVC intramuscular pressure is sufficiently high to cause ischaemia, and MFCV is found to be very sensitive to changes in intramuscular blood flow.