Changes in the relationship between joint angle and torque production associated with the repeated bout effect

A single bout of eccentric exercise induces a protective adaptation against damage from a repeated bout. The aim of this study was to determine whether this repeated bout effect is due to a change in the length-tension relationship. Twelve individuals performed an initial bout of six sets of 10 eccentric quadriceps contractions and then performed a repeated bout 2 weeks later. Eccentric contractions were performed on an isokinetic dynamometer at 1.04 rad . s^-1 with a target intensity of 90% of isometric strength at 70° of knee flexion. Isometric strength and pain were recorded before and after both eccentric bouts and on each of the next 3 days. Isometric strength was tested at 30°, 50°, 70°, 90° and 110° of knee flexion. On the days following the initial bout, there was a significant loss of isometric strength at all knee flexion angles except 110° (bout×angle: P <0.01). On day 2, strength averaged 86% of baseline for 30-90° and 102% of baseline for 110°. Strength loss and pain after the initial bout was contrasted by minimal changes after the repeated bout (pain: P <0.001; strength: P <0.01). The repeated bout effect was associated with a rightward shift in the lengthtension curve; before the repeated bout, isometric strength was 6.8% lower at 30° and 13.6% higher at 110° compared with values before the initial bout (bout×angle: P <0.05). Assuming that torque production at 110° occurs on the descending limb of the lengthtension curve, the increase in torque at 110° may be explained by a longitudinal addition of sarcomeres. The addition of sarcomeres would limit sarcomere strain for subsequent eccentric contractions and may explain the repeated bout effect observed here.