Fatigue analysis of the quadriceps femoris muscle based on mechanomyography

The aim of this study was to assess quantitative muscle fatigue during isometric and isotonic contractions using a mechanomyography (MMG) and a computational geometry technique known as a convex hull algorithm. Twenty-five male volunteers participated in the experiment. The subjects performed knee flexion and extension exercises on an isokinetic dynamometer. A 3-axis accelerometer (x: longitudinal; y: medial-lateral; z: superior-inferior) was placed on the right rectus femoris muscle. The MMG signal was recorded at a sampling rate of 1 kHz. Each subject performed the maximal knee extension 10 times to determine the 10-repetition maximum torque and maximal isometric contraction at a knee angle of 90°. The results showed that positive correlations were observed between normalized torque and x-direction MMG (linear, r²=0.44), z-direction MMG (linear, r²=0.58), and signal vector magnitude (linear, r²=0.44). The decreases in hull area or volume of MMG signal may result from fatigue due to reductions in motor unit firing rate and increase in muscle thickness. This study evaluated muscular function during fatigue exercises by analyzing the electrical activities of activated skeletal muscle fibers and low-frequency lateral oscillations using accelerometer-based MMG.