Changes of Single Motor Unit Activity During Muscular Fatigue

The purpose of this study was to clarify the relationship between the changes of surface electromyogram (EMG) and the single motor unit (MU) activity with muscle fatigue. It is well known that median power frequency (MDF) shifts to the low-frequency side and the muscle fiber conduction velocity (MFCV) decreases with muscle fatigue.We recorded the activity of the single MUs from the biceps brachii and the flexor carpi ulnaris during sustained contraction. The experiments were performed on four healthy male adults without signs of neuromuscular disease. Surface EMG signals were picked up with an electrode array consisting of four silver wire contacts. Deriving the single MU was enabled by displaying EMG signals on the CRT of a computer and presenting them to the subjects. The changes of the MU activity during muscle fatigue were analyzed by MFCV, MDF, and interspike intervals.As a result, the phenomenon that a single MU activity stopped suddenly in the process of muscle fatigue was observed. The durations of the single MU activity varied depending on MUs. MFCV of the biceps brachii was a level of 3.5 to 4.5 m/s and no large change was confirmed. MFCV of the flexor carpi ulnaris varied from 2.2 to 4.6 m/s. The changes of the muscle fatigue process were also various. MDF of the biceps brachii was a level of 104 to 138 Hz except that one MU increased by 20 Hz or more. MDF of the flexor carpi ulnaris varied from 76 to 178 Hz. The changes of the muscle fatigue process were not large. The interspike intervals of the biceps brachii were about 100 milliseconds, and increased before stopping discharge. The interspike intervals of the flexor carpi ulnaris also increased before stopping discharge.These results suggest that the substitution of MUs and the decrease of firing rate cause the changes to the surface EMG parameters during muscle fatigue.