Objective. Joint torque differences between healthy and rehabilitated legs are often measured as a clinical index of recovery\nfrom muscle strain injury. Unfortunately, it should be noted that this is a questionable evaluation measure of the muscle\nafter injury because it is a composite value including related cooperating muscles. Meanwhile, the use of ultrasound\nelastography for the measurement of individual muscle mechanical properties (i.e., muscle hardness) has recently expanded.\nThe purpose of this study was to examine, using ultrasound elastography, the differences in the linear relationship between\nmuscle contraction intensity and muscle hardness during knee extension in athletes who had recovered from grade II\nrectus femoris muscle strain injury through comparison of the healthy and rehabilitated legs. Methods. Six athletes\nparticipated. Rectus femoris muscle hardness, determined during isometric contraction at 10%, 20%, 30%, and 40% of\nmaximum voluntary contraction, was evaluated using ultrasound strain elastography. Results and Conclusion. The results\nindicated that for the healthy legs, the strain ratios, as indicated by muscle hardness, decreased linearly (became harder)\nwith contraction intensity, but the strain ratios for the rehabilitated legs decreased nonlinearly. These results show the\ndanger of judging the recovery period using only the difference between healthy and rehabilitated muscle strengths and the\nimportance of evaluating individual muscles.
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