It is unclear how muscle stiffness changes due to exercise-induced muscle damage or injury. Recent development of novel ultrasound elastography has opened the possibility for objectively quantifying muscle stiffness by imaging the internal propagation of muscle vibration with ultrafast ultrasound technology (Shinohara et al. Muscle & Nerve, 2010). PURPOSE: To examine the changes in muscle stiffness due to exercise-induced muscle damage with the novel ultrasound elastography. METHODS: To induce muscle damage in the biceps brachii muscle, six healthy young adults performed a series of maximal eccentric contractions with their elbow flexor muscles in their left arm. They did not perform this task with their right arm (control). Before, immediately after, and 24 hours after the eccentric contraction task, subjects performed a brief submaximal isometric elbow flexion task in each arm with the elbow joint angle fixed at 90 degrees and the wrist joint in the neutral position. The contraction levels were 0% (rest), 10%, and 25% of maximal voluntary contraction (MVC) that was measured before the eccentric contraction task. Muscle stiffness in the biceps brachii was quantified in kPa from ultrasound elastography images during the isometric task. RESULTS: The reproducibility of muscle stiffness in the right (control) arm was confirmed by the intraclass correlation coefficient of muscle stiffness > 0.91 between measurements and between days, with no statistical difference in the mean values. Muscle stiffness increased with increases in the contraction level in both arms. In the left arm, muscle stiffness was not different (P > 0.05) before and immediately after the eccentric contraction task at each contraction level. At 24 hours after the eccentric contraction task, muscle stiffness during the brief contraction at 25% MVC decreased by > 20% to 77.3 ± 14.8 kPa (P < 0.05) compared with the value before the eccentric contraction task (100.0 ± 14.2 kPa) in the left arm. There was no change at 0% or 10% MVC. CONCLUSIONS: A decline in muscle stiffness during contraction was manifested at 24 hours after exercise-induced muscle damage as detected by the novel ultrasound elastography imaging. The reduction at 25% MVC suggested the delayed alterations in mechanical properties of relatively larger motor units due to muscle damage.
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