Quantitative and non-invasive measurement of exercise-induced fatigue
Prolonged exercise-induced muscular fatigue adversely affects physical performance. The fatigue increases the risk of sport injuries, whereas early fatigue detection and assessment can prevent injuries. An assessment of the most crucial impacts of fatigue on physical performance leads to the development of an accurate, non-invasive and objective muscular fatigue measurement method. The most common manifestation of exercise-induced fatigue is a significant decline in force produced by the muscles that in turn affects motion characteristics. This directly alters body postural behaviour and decreases the amount of kinetic energy produced by the subject. The current non-invasive and objective methods to measure fatigue and analyse motion characteristics cannot provide a comprehensive information about muscular fatigue because of their limited ability to record different aspects of motion. An objective and non-invasive assessment method of exercise-induced fatigue, in which the variations in postural behaviour and kinetic energy are quantified to determine the fatigue effects on the motion characteristics, is proposed. The inertial parameters associated with 23 body segments of 20 healthy subjects (males and females) were measured during stair climbing tests before and after a fatigue protocol. The degree of fatigue was assessed by a score measuring the total differences in kinetic energy and postural behaviour between the first and second stair climbing tests. A Gaussian mixture model was used to classify the body postures and the total variation in postural behaviour caused by the two stair climbing tests was determined. The validity of the method to measure the degree of fatigue induced by this fatigue protocol was ascertained by benchmarking it against the degree of fatigue experienced by the subjects during fatigue protocol and reported in a questionnaire based on the Rate of Perceived Exertion Scale. A Pearson product-moment correlation measuring the linear correlation between the objective and subjective fatigue scores shows a strong correlation of 0.95 for male subjects and 0.70 for female subjects. The findings demonstrate that inertial motion analysis proposed in this study provides a reliable, non-invasive and accurate fatigue measurement tool.