Muscle damage and inflammation after eccentric exercise: can the repeated bout effect be removed?

Area of Science:

• Exercise Physiology
• Muscle Plasticity
• Skeletal Muscle Adaptation

Background:

• The repeated bout effect (RBE) is a well-established phenomenon in exercise physiology, where initial muscle damage from eccentric exercise is attenuated in subsequent sessions.
• The current consensus suggests RBE consistently appears after a few eccentric exercise sessions.
• Specificity in muscle plasticity, particularly opposing adaptations from different exercise types, has not been fully explored in relation to RBE.

Purpose of the Study:

• To investigate whether opposing muscle adaptations induced by alternating concentric and eccentric exercise can attenuate or eliminate the repeated bout effect.
• To challenge the established tenet that the repeated bout effect invariably occurs after eccentric exercise.
• To compare the effects of an alternating eccentric-concentric exercise protocol versus an eccentric-only protocol on muscle damage markers and strength adaptations.

Main Methods:

• Seventeen young men were assigned to either an alternating eccentric-concentric exercise group or an eccentric-only exercise group.
• Both groups underwent 8 weeks of knee extensor resistance training on an isokinetic dynamometer.
• Muscle damage, inflammation, and strength were assessed at various time points (weeks 1, 5, 10, and 14) using physiological and biochemical markers.

Main Results:

• The alternating eccentric-concentric exercise group demonstrated a dramatic reduction in the repeated bout effect, with prior concentric training seemingly reversing muscle to an unaccustomed state.
• The eccentric-only group exhibited a typical manifestation of the repeated bout effect.
• Both training groups showed similar elevations in muscle strength after 13 weeks of training.
• The findings suggest inflammation may not play a critical role in exercise-induced muscle adaptations.

Conclusions:

• The alternating eccentric-concentric exercise protocol successfully overcame the repeated bout effect, challenging existing exercise physiology consensus.
• Muscle plasticity exhibits specificity, where opposing training stimuli can modulate the response to subsequent exercise bouts.
• The similar strength gains in both groups question the significant role of inflammation in mediating exercise-induced muscle adaptations.