The influence of muscle length on muscle fibre conduction velocity and development of muscle fatigue.

The influence of muscle (vastus lateralis) length on the muscle fibre conduction velocity (MFCV) and on muscle fatigue was studied in 8 healthy volunteers. In experiment 1, the electromyographic (EMG) responses were evoked by electrical stimulation of the motor point and recorded by a surface electrode array aligned along the muscle fibre direction. The MFCV (determined by cross-correlation) was measured at knee flexions of 5 degrees (full extension), 45 degrees, 90 degrees and 120 degrees with 3 different extension torques. The MFCV declined with increasing muscle length and increased with increasing background torque at knee flexions from 5 degrees to 90 degrees. From 90 degrees to 120 degrees knee flexion of MFCV tended to increase. In experiment 2, the EMG activity at a static fatiguing contraction (80% MVC) was measured at 45 degrees and 90 degrees knee flexion. The EMG was measured until the subject gave up contracting the muscle (endurance). The largest increase in the RMS amplitude and the fastest decreases in the mean power frequency (MPF) and MFCV were found at 90 degrees flexion. The MVC at 45 degrees knee flexion was 35% lower than at 90 degrees and the time until endurance was approximately twice as long for the 45 degrees contraction. The results indicate that muscle length is an important parameter for the propagation velocity of action potentials and for the development of static muscle fatigue.