Effect of heat preconditioning by microwave hyperthermia on human skeletal muscle after eccentric exercise

The purpose of this study was to clarify whether heat preconditioning results in less eccentric exercise-induced muscle damage and muscle soreness, and whether the repeated bout effect is enhanced by heat preconditioning prior to eccentric exercise. Nine untrained male volunteers aged 23 +/- 3 years participated in this study. Heat preconditioning included treatment with a microwave hyperthermia unit (150 W, 20 min) that was randomly applied to one of the subject's arms (MW); the other arm was used as a control (CON). One day after heat preconditioning, the subjects performed 24 maximal isokinetic eccentric contractions of the elbow flexors at 30 degrees.s(-1) (ECC1). One week after ECC1, the subjects repeated the procedure (ECC2). After each bout of exercise, maximal voluntary contraction (MVC), range of motion (ROM) of the elbow joint, upper arm circumference, blood creatine kinase (CK) activity and muscle soreness were measured. The subjects experienced both conditions at an interval of 3 weeks. MVC and ROM in the MW were significantly higher than those in the CON (p < 0.05) for ECC1; however, the heat preconditioning had no significant effect on upper arm circumference, blood CK activity, or muscle soreness following ECC1 and ECC2. Heat preconditioning may protect human skeletal muscle from eccentric exercise-induced muscle damage after a single bout of eccentric exercise but does not appear to promote the repeated bout effect after a second bout of eccentric exercise.