Introduction: In this study, we used a newly developed method for measuring passive length-tension relations of a single human muscle in vivo to quantify changes in the mechanical properties of the human gastrocnemius after eccentric exercise.
Methods: Twelve subjects performed eccentric exercise on the right leg for 1 h by walking backward downhill on a treadmill. Passive ankle torque was measured as the ankle was rotated within its available range, with the knee in eight different angles. Subjects were studied before exercise, 1 h after exercise, and 24 h later, with further measurements at 48 h and at 1 wk in a subset of six subjects. Subjects also rated the level of perceived muscle soreness on a 10-point scale during walking on flat ground. We examined passive tension in the gastrocnemius at a standard length before and at various times after exercise.
Results: Muscle tension increased significantly at this length 1 h after exercise (34.7 ± 7.3%; mean ±SEM), peaked at 24 h (88.4 ± 12.6%), declined at 48 h (45.5 ± 4.4%), and returned to the control level at 1 wk. Stiffness of the gastrocnemius in the sitting and standing postures (i.e., at short and long lengths) was derived from passive length-tension relations. Stiffness increased after exercise, and the relative changes in muscle stiffness were similar in both positions. There was no apparent correlation between stiffness and subjective reports of muscle soreness during walking.
Conclusion: This study provides the first specific measurements of the increase in stiffness of the human gastrocnemius in vivo after a single bout of eccentric exercise. The increase peaks at 24 h and is nearly fully resolved within 1 wk.