Establishing a quantitative evaluation index for muscles is required during rehabilitation for assessing muscle state recovery. The authors previously tried to quantify muscle fatigue and defined “Muscle-fatigue time” as the quantitative evaluation index of muscle fatigue based on the use ratio of the muscle fiber type. “Muscle-fatigue time” is the time when the use ratio of the slow muscle fiber (Type I) and the fast muscle fiber (Type II b) intersect. This paper proposed a method for estimating “Muscle-fatigue time” using physical features and muscle loads. In the proposed method, “Muscle-fatigue time” is estimated using the feature common to all muscle types: the use ratio can recur based on the linear equation as a time function. First, in the use ratios of Type I and Type I I b, the initial values were estimated from physical features and the slopes were estimated from muscle loads. Next, each linear regression equation was formulated from the estimated initial values and the estimated slopes. The “Muscle-fatigue time” was estimated by solving the simultaneous regression equations.