Muscle fiber conduction velocity during isometric contraction and the recovery period

Changes of muscle fiber conduction velocity (MFCV) detected by surface array electrodes during an isometric contraction and the recovery period were evaluated. The location on skin measured for action potentials of muscle fiber in m. biceps brachii was a distance of 5 mm and 30 mm from the end-plate to the distal tendon. The MFCV was evaluated by averaging raw EMG waves. The MFCVs at both locations declined gradually during the loads of sustained isometric contractions of 30, 50, and 70% of the maximum voluntary contraction (MVC). The degree of the decrease of the MFCV was extremely intense during a sustained contraction of 70% MVC. The values of the MFCV at the location of 5 mm from the end-plate in the period close to the exhaustion state showed a significant decrease compared with the values in the initial period during the contractions for the above three kinds of loads, while the decrease of the MFCV at the location of 30 mm from the end-plate was not found to be so significant during the contractions. In the recovery period, the contraction of 5% MVC was maintained, and the resultant MFCVs restored gradually to the value of the initial period. At the location of 5 mm from the end-plate, the MFCVs at 15 minutes after the end of the load were significantly higher than those just after the end of the load. At the location of 30 mm from the end-plate, the increases of the MFCVs during the recovery period did not show significant changes. Changes of the MFCV during the isometric contraction and the recovery period depend greatly on the location of the electrodes measured for the action potentials of the muscle fibers.