Physical exercise-induced fatigue causes a significant decline in functional performance and subsequently increases the risk of sport injuries. The fatigability tolerance is generally increased by, improving muscular strength and endurance using physical exercise training programs. However, a simple quantitative assessment of fatigue remains as an important step in such programs. The nature of fatigue and its impact on gait and posture is studied using inertial sensors. In this approach, the flexion/extension of knees of a subject during stair climbing test (SCTs) before and after performing a specific set of fatigue-inducing exercises is measured. The knee motion data obtained during SCT before after the exercise are compared using dynamic time warping (DTW). The amount of difference is an indication of the degree of fatigue. The method is applied to twenty subjects including both male and female. The result is encouraging and confirms the feasibility and validity of the approach.