Mild to moderate damage in knee extensor muscles accumulates after two bouts of maximal eccentric contractions

This study aimed to determine whether mild to moderate muscle damage accumulates on the knee extensors after two bouts of maximal eccentric contractions performed over two consecutive days.

Thirty participants performed an initial bout of maximal eccentric contractions of knee extensors during the first day of the protocol (ECC₁). Then, they were separated in two groups. The Experimental (EXP) group repeated the eccentric bout 24 h later (ECC₂) while the Control (CON) group did not. Indirect markers of muscle damage (i.e., strength loss, muscle soreness, and shear modulus) were measured to quantify the amount of muscle damage and its time course.

Two days after the initial eccentric session, participants from EXP had a higher strength deficit (- 14.5 ± 10.6%) than CON (- 6.6 ± 8.7%) (P = 0.017, d = 0.9). Although both groups exhibited an increase in knee extensors shear modulus after ECC₁, we found a significant increase in muscle shear modulus (+ 13.3 ± 22.7%; P < 0.01; d = 0.5) after ECC₂ for the EXP group, despite the presence of mild to moderate muscle damage (i.e., strength deficit about 16%).

Although the markers of muscle damage used in the current study were indirect, they suggest that the repetition of two bouts of maximal eccentric contractions with 24 h apart induces additional muscle damage in the knee extensors in presence of mild to moderate muscle damage.