Human hamstring muscles adapt to eccentric exercise by changing optimum length

Purpose: It is now established that unaccustomed eccentric exercise leads t o muscle fiber damage and to delayed-onset muscle soreness (DOMS) in the da ys after exercise. How-ever, a second bout of eccentric exercise, a week af ter the first, produces much less damage and soreness. The purpose of this study was to provide evidence from muscle mechanical properties of a propos ed mechanism for this training effect in human hamstring muscles. Methods: The eccentric exercise involved 12 sets of 6 repetition "hamstring lowers," performed on specially designed equipment. Hamstring angle-torque curves w ere constructed fur each of 10 subjects (8 male and 2 female) while they pe rformed maximum voluntary knee extension and flexion movements on an isokin etic dynamometer. Testing sessions were performed over the week before ecce ntric exercise, immediately post exerciser and daily, up to 8 d post exerci se. Subject soreness ratings and leg girth measurements were also made post exercise. Sis subjects performed a second bout of eccentric exercise, 6 d after the first. and measurements were continued up to 10 d beyond that. Re sults: There was a significant shift in the optimum angle for torque genera tion (L-o), to longer muscle lengths immediately post exercise(7.7 degrees +/- 2.1 degrees, P < 0.01), indicating an increase in series compliance wit hin some muscle fibers. Subsequent measurements showed increases in leg gir th and some muscle soreness, suggesting muscle damage. The shift in L-o per sisted, even after other injury parameters hail returned to normal, consist ent with a training effect. Subjects also showed fewer signs of muscle dama ge after the second exercise bout. Conclusion: This is the first study to s how a sustained shift in optimum angle of human muscle as a protective stra tegy against injury from eccentric exercise. Implications of this work for athletes, particularly those prone to hamstring strains are discussed.