The role of passive muscle stiffness in symptoms of exercise-induced muscle damage

We examined whether passive stiffness of an eccentrically exercising muscle group affects the subsequent symptoms of muscle damage. Passive hamstring muscle stiffness was measured during an instrumented straight-leg-raise str etch in 20 subjects (11 men and 9 women) who were subsequently classified a s "stiff" (N = 7), "normal" (N = 6), or "compliant" (N = 7). Passive stiffn ess was 78% higher in the stiff subjects (36.2 +/- 3.3 N.m.rad(-1)) compare d with the compliant subjects (20.3 +/- 1.8 N.m.rad(-1)). Subjects then per formed six sets of 10 isokinetic (2.6 rad.s(-1)) submaximal (60% maximal vo luntary contraction) eccentric actions of the hamstring muscle group. Sympt oms of muscle damage were documented by changes in isometric hamstring musc le strength, pain, muscle tenderness, and creatine kinase activity on the f ollowing 3 days. Strength loss, pain, muscle tenderness, and creatine kinas e activity were significantly greater in the stiff compared with the compli ant subjects on the days after eccentric exercise. Greater symptoms of musc le damage in subjects with stiffer hamstring muscles are consistent with th e sarcomere strain theory of muscle damage. The present study provides expe rimental evidence of an association between flexibility and muscle injury. Muscle stiffness and its clinical correlate, static flexibility, are risk f actors for more severe symptoms of muscle damage after eccentric exercise.