This study examined eccentric exercise-induced muscle damage and rapid adaptation. Twenty-two male subjects performed 70 eccentric actions with the knee extensors. Croup A (n = 11) and group B (n = 11) repeated the same exercise 4 and 13 days after the initial bout, respectively. Criterion measures included muscle soreness, muscle force generation (vertical jump height on a Kistler platform), and plasma levels of creatine kinase (CK), slow-twitch skeletal (cardiac beta-type) myosin heavy chains (MHC), and cardiac troponin I. Subjects were tested pre-exercise and up to day 4 following each bout. The initial exercise resulted in an increase in CK and MHC, a decrement in muscle force, and delayed onset muscle soreness in all participants. CK and MHC release correlated closely (rho = 0.73, p = 0.0001), both did not correlate with the decrement in muscle force generation after exercise. Because cardiac troponin I could not be detected in all samples, which excluded a protein release from the heart (cardiac beta-type MHC), this finding provides evidence for a injury of slow-twitch skeletal muscle fibers in response to eccentric contractions. Repetition of the initial eccentric exercise bout after 13 days (group B) did not cause muscle soreness, a decrement in muscle reaction force with vertical jump or significant changes in plasma MHC and CK concentrations, whereas in case of repetition after 4 days (group A) only the significant increases in CK and MHC were abolished. The decrement in reaction force with vertical jump did not differ significantly from that after the initial exercise session, but perceived muscle soreness was less pronounced. Performance of one bout of eccentric exercise produced a greater resistance of the exercised muscle to a subsequent bout of eccentric exercise.
Key words
Muscle damage - eccentric exercise - slow-twitch - adaptation - creatine kinase - myosin heavy chains - troponin I
