Introduction: The repeated bout effect characterizes the protective adaptation after a single bout of unaccustomed eccentric exercise that induces muscle damage. Sarcomerogenesis and increased tendon compliance have been suggested as potential mechanisms for the repeated bout effect by preventing muscle fascicles from being stretched onto the descending limb of the length-tension curve (the region where sarcomere damage is thought to occur). In this study, evidence was sought for three possible mechanical changes that would support either the sarcomerogenesis or the increased tendon compliance hypotheses: a sustained rightward shift in the fascicle length-tension relationship, reduced fascicle strain amplitude, and reduced starting fascicle length.
Methods: Subjects (n = 10) walked backward downhill (5 km·h, 20% incline) on a treadmill for 30 min on two occasions separated by 7 d. Kinematic data and medial gastrocnemius fascicle lengths (ultrasonography) were recorded at 10-min intervals to compare fascicle strains between bouts. Fascicle length-torque curves from supramaximal tibial nerve stimulation were constructed before, 2 h after, and 2 d after each exercise bout.
Results: Maximum torque decrement and elevated muscle soreness were present after the first, but not the second, backward downhill walking bout signifying a protective repeated bout effect. There was no sustained rightward shift in the length-torque relationship between exercise bouts, nor decreases in fascicle strain amplitude or shortening of the starting fascicle length.
Conclusions: Protection from a repeated bout of eccentric exercise was conferred without changes in muscle fascicle strain behavior, indicating that sarcomerogenesis and increased tendon compliance were unlikely to be responsible. As fascicle strains are relatively small in humans, we suggest that changes to connective tissue structures, such as extracellular matrix remodeling, are better able to explain the repeated bout effect observed here.