ECCENTRIC EXERCISE-INDUCED MUSCLE DAMAGE IS DEPENDENT ON MUSCLE LENGTH675

Force generation of skeletal muscle is dependent on joint angle. Elbow flexors generate maximal force when the elbow joint angle is approximately 100 degrees. It has been shown that eccentric actions at a long muscle length resulted in a larger decrease in force and more muscle tenderness compared to those at a short muscle length (Newham et al. 1988), yet the reason for this is unclear. To further investigate the effect of joint angle on the development of muscle damage, the present study compared two maximal eccentric exercise regimens in which the starting position of the eccentric action was different (50 vs 100 degrees) but the range of movement was same (80 degrees). Ten male students (19.8 ± 1.9 yrs) were used as subjects. One arm performed 24 maximal eccentric actions of the elbow flexors at the elbow joint angle from 50 to 130 degrees (50-130) and the other arm at the elbow joint angle from 100 to 180 degrees (100-180). Peak force generated during the eccentric actions in both conditions was not significantly different, but the total work performed during exercise (integrated force) was significantly(p<0.01) larger for the 50-130 compared to the 100-180. Isometric force of the elbow joint angle at 90 degrees (IF), range of motion (ROM), muscle soreness (SOR), CK, circumference (CIR), and B-mode ultrasound pictures (US) of the elbow flexors were measured before and for 5 days after both conditions. MRI of the transverse and longitudinal scans of the upper arm was taken at 4 days after exercise. All measures changed significantly (p<0.01) after exercise for both conditions, however significantly (p<0.01) larger changes in the measures (IF, ROM, SOR, CK) were found in the 100-180 compared to the 50-130 (ex. peak CK: 10,640±2,876 vs 2,891±1,261 IU/L, respectively). CIR and US showed that swelling was more conspicuous in the 100-180 compared to the 50-130. MRI demonstrated that only the brachialis (BR) was damaged for the 50-130 but the biceps brachii (BB) was also damaged for the 100-180. T2 relaxation time of the BB was significantly (p<0.01) larger for the 100-180 (46.1±2.6 sec) compared to the 50-130 (36.8±1.7 sec). These results suggest that length-dependent components play an important role in eccentric exercise-induced muscle damage, especially regarding which muscles are damaged.