Noninvasive evaluation of neuromuscular function using electromyography during fatiguing contraction.

The power spectrum is well known to shift to lower frequencies and increased amplitude with progressing of fatigue. The increase in amplitude depends on motor unit recruitment. The frequency shift depends on the reduction in muscle fiber conduction velocity. Neuromuscular function was evaluated noninvasively by observing electromyography (EMG) during fatiguing contraction. We studied the effects of factors such as exercise, aging, and short-term immobilization on EMG during fatiguing contraction and evaluated the feasibility of EMG as a noninvasive index for determining neuromuscular function. Subjects exerted maximum voluntary contraction (MVC) isometrically and conducted 60% MVC fatiguing contraction. Surface EMG was detected using multichannel electrodes, and EMG variables such as average rectified value (ARV), mean power frequency (MPF), median frequency (MDF), and muscle fiber conduction velocity (MFCV) were calculated. We studied the relationship between EMG during fatiguing contraction and muscle performance, such as MVC and endurance time.We studied the effects of fatiguing exercise on EMG using knee flexion in 14 men aged 21 to 29 years (mean: 24.9 years), divided based on twitch interpolation into high and low voluntary activation groups. Four sedentary subjects were included in the low-activation group. MVC was significantly greater and endurance significantly shorter in the high voluntary activation group. Changes in ARV and MPF calculated from EMG detected in the vastus lateralis muscle during fatigue were also smaller in the high voluntary activation group. EMG results during fatigue contraction suggest that the low voluntary activation group, which included sedentary subjects, has low metabolic capacity in the muscle but low voluntary activation during MVC. Endurance was thus prolonged during the fatigue task.We examined the effects of aging on EMG during fatigue in dorsiflexion of the ankle joint in 25 women aged 20 to 77 years (mean: 47.1 years) and divided by age into older (66 to 77 years) and younger (20 to 24 years) groups. MVC was significantly smaller, and ARV, MDF, and the MFCV calculated from surface EMG detected in the tibialis anterior muscle during MVC were lower in the older group than in the younger. A similar trend was seen in the change in EMG during the fatigue task. These results suggest that older subjects have low motor unit activation and that their muscle fiber consists mainly of slow-twitch fibers.We also studied the effects of short-term immobilization on EMG during fatigue contraction, using abduction of the index finger in 10 men aged 20 to 29 years (mean: 24.4 years). MVC decreased and endurance was shortened when the finger was immobilized in a cast for 1 week. The decrease in MDF calculated from surface EMG detected in the first dorsal interosseous muscle was smaller, but the rate of change was greater after immobilization than before. These results suggest that the decrease in metabolic capacity caused by short-term immobilization led to a decline in performance factors such as MVC and endurance.These results further suggest that EMG during fatigue depends on individual factors, such as exercise, aging, and immobilization, reflecting the muscle fiber type and the degree of motor unit activation. We propose that EMG be applied during fatigue as a noninvasive EMG biopsy.