We have clarified the relationship between activation patterns during eccentric knee flexion/hip extension exercises and the muscle damage after exercise in hamstring muscles by utilizing electromyography (EMG) and magnetic resonance imaging (MRI). In the eccentric knee flexion exercise, subjects exercised with each unilateral limb at 120 % of 1 repetition of maximum (1RM) and 50 % of 1RM. EMG activity was recorded for the biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) muscles; MRI T2 values and cross-sectional areas (CSAs) of the same muscles were measured at rest, as well as immediately after, and 1, 2, 3 and 7 days after exercise. It was found that EMG of the ST was significantly higher than that of the SM during the exercises at 120 % 1RM. T2 value change in the ST was significantly higher than in the BFlh and SM in both exercises. In the eccentric hip extension exercise, other subjects performed 5 sets of 10 repetitions of hip extension. EMG activity during the exercise was recorded for the BFlh, ST, and SM muscles; MRI T2 values and CSAs of the same muscles were measured at rest, immediately after, and 2 and 7 days after the exercise. It was found that EMG of the BFlh and SM were significantly higher than that of the ST. Immediately after exercise, the T2 value and CSA changes in the SM showed a significant increase. Overall, it was concluded that the ST muscle has a sensitive to intensive eccentric knee flexion load and the BFlh and SM muscles to an eccentric hip extension load.
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