Muscle fatigue has been studied in various fields. As it accompanied the change of its mechanical characteristics, we had studied the relation between muscle fatigue and its biomechanical characteristics (visco-elasticities) by using biomechanical impedances in case of the isometric contraction with continuous load. It was confirmed, as a result, that the condition of muscle fatigue was estimated from its visco-elasticities. In this study, we adopted the isometric contraction with intermittent load to examine how the rest time affected muscle fatigue. Three sets of 5 minutes 15% MVC (Maximum Voluntary Contraction) load were applied with 1 or 4 minutes interval on an antebrachial flexor muscle. The advancement and the recovery of muscle fatigue were observed in both loaded and unloaded situation. Longer rest decreased the fatiguing speed and caused an obvious recovery. Visco-elasticities were less sensitive to muscle fatigue at its early stage than MFs, which were calculated from surface EMGs.