Effect of fatigue on muscle elasticity in the human forearm using ultrasound strain imaging

The etiology of skeletal muscle fatigue is not well understood partly because techniques portraying muscle performance in vivo are limited by either their invasiveness (e.g., needle electrodes) or poor spatial resolution (e.g., surface EMG). To better characterize effects of FES and muscle fatigue, we captured real-time high resolution dynamics of the human forearm before and after a fatigue exercise using ultrasound strain imaging. A 10 MHz linear ultrasound probe aligned with the fiber axis of the 3rd flexor digitorum superficialis (FDS) provided scans at 3-msec intervals during isometric twitch and tetanic contractions evoked by low and high frequency electrical stimuli (ES). Ultrasound images synchronized with traditional force and EMG were obtained for 5 healthy adults before and after a fatiguing exercise, induced by sustained maximal exertion of the middle finger pressed against a restraint until the initial force decreased by 75%. Immediately after fatigue, twitch and tetanic stimuli generated 55.1% and 19.5% less force, respectively, implying that low frequency fatigue dominated. The force deficit was associated with a decrease in several mechanical properties of the fatigued muscle during twitch contractions, such as transverse peak strain (34 +/- 15%) and half peak strain duration (32.3 +/- 12.5 msec). Changes were not uniform across the imaged section of the muscle, suggesting that boundary conditions or fiber heterogeneity affected the strain profile. Indeed, high stress zones appeared closer to the muscle-tendon junction during isometric contractions. This study provided new insight on the elastic behavior of muscle and potential mechanisms of injury, especially directed at prolonged stimulation and control of a neuromuscular prosthesis.