Downhill walking (DW) is a representative exercise that predominantly involves submaximal eccentric contractions of knee extensors, and has been shown to be effective in improving knee extensor strength with relatively short-term and lowtraining frequency. However, DW induces muscle damage when performed unaccustomedly. This study examined whether muscle damage can be avoided by gradually increasing exercise duration in DW training, and whether avoiding/experiencing muscle damage influences training-induced strength gain. Healthy young males performed treadmill DW (gradient: -28%, velocity: 5 km/h, load: 10% of body mass) 1 session/week for 4 weeks with either ramp-up (R-UP: n = 10) ; gradually increasing exercise duration from 10 to, 30, 50, and 70 min from the 1st to 4th session to avoid muscle damage, or constant (CON: n = 12) protocol; 40 min for 4 sessions to induce muscle damage at the 1st session. Total exercise duration was 160 min for both groups. Muscle damage markers were measured before and for 72 h following each session. Before and after the intervention, maximal knee extension torques in eccentric (-60°/s) , isometric (0°/s) , and concentric (60°/s) conditions were measured. R-UP group showed no indications of muscle damage throughout the training period while muscle damage was evident after the 1st session in CON group. Both groups significantly increased maximal knee extension torques at all angular velocities with greater gain in eccentric (R-UP: +19%, CON: +24%) than isometric (+14%, +13%) and concentric (+10%, +12%) strength without significant group-difference. The current results suggest that muscle damage can be avoided by R-UP protocol, and is not a major determinant of the training-induced strength gain.

DESCENTE SPORTS SCIENCE Vol.36/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE