It is well documented that the frequency spectrum of myoelectric signal shifted toward lower frequency during a sustained isometric contraction, and several studies used this changes to quantify the localized muscular fatigability. However only few studies have compared fatigability of different muscles using spectrum analysis.

The aims of the present study were: (1) to compare the patterns of changes of frequency spectrum with increasing force in different muscles (2) to compare the fatigability of several muscles that are known to have different muscle fiber composition during sustained submaximal isometric contraction, and (3) to evaluate the effects of immobilization and strengthening exercise on behavior of frequency spectrum.

Four muscles (biceps brachii, triceps brachii, tibialis anterior, soleus) of control, immobilized and trained group were tested on brief contraction at four different levels (20% MVC, 40% MVC, 60% MVC, 80% MVC) and sustained isometric contraction at 80% MVC (maximal voluntary contraction). The changes of MPF (mean power frequency) and RMS (rectified mean square) voltage analyzed on each study, and the differences of those changes among muscles and groups were evaluated.

The results were as follows:

1) During the graded contraction, the significant range of frequency spectrum was below 150 Hz on the biceps brachii and triceps brachii. On the other hand, the frequency spectrum of the tibialis anterior and soleus were broadened to 400 Hz.

2) The MPF of the tibialis anterior and soleus were increased with increasing force level during graded contraction. On the contrary, the MPF of triceps brachii were decreased with increasing force level. These findings were influenced neither by immobilization nor strengthening exercise.

3) During sustained isometric contraction at 80% MVC, the frequency spectrums from all the muscles examined shifted toward lower frequency and the MPF were decreased. The slope of MPF changes was lowest for triceps brachii and highest for soleus, which means that the triceps has high fatigability and the soleus has low fatigability.

4) The MPF of immobilized group was significantly lower than that of control group on all the muscles studied during graded contraction.

5) The slope of MPF decrease of triceps brachii became significantly less steep by immobilization. On the other hand, the slope of soleus became steeper by immobilization. These findings suggested that the fatigability changed differently by immobilization on different muscles.

6) The strengthening exercise affects significantly neither the pattern of changes of MPF with graded increasing force nor the slope of MPF decrease during sustained isometric contraction.

From the above results, it was concluded that there were differences in fatigability, in mechanism developing additional contratility, and in changes of fatigability by immobilization of different skeletal muscles according to their muscle fiber composition.