In this study, a mechanomyogram (MMG) sensor system for monitoring muscle conditions is developed. The polyvinylidene fluoride (PVDF) film is used as the sensory material of the MMG sensor. The mean power frequency (MPF) of the sensor output, which is related to firing rate of action potential of muscle, and the variance of the sensor output, which is related to the activity of fast fibers and complete recruitment, are calculated from the MMG signal. The fundamental experiments were conducted. First, MMG signal are measured for muscular injury of upper arm. From the results, MMG sensor is available to estimate the recovery of muscle. Next, muscle activities of isometric contraction are studied using the developed MMG sensor before and after muscle fatigue. Experimental results show that contraction of fast fibers is occurred earlier and fast fibers activity increase earlier due to muscle fatigue. Focusing on the time shift of local maximum point in MPF and decrease of growth rate in variance, parameters for evaluation of muscle fatigue degree are proposed.