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

Area of Science:

• Exercise Physiology
• Muscle Physiology
• Biomechanics

Background:

• A single bout of eccentric exercise confers protection against subsequent damage.
• The underlying physiological mechanisms of this 'repeated bout effect' remain incompletely understood.
• Changes in the muscle's length-tension relationship are hypothesized to contribute to this protective adaptation.

Purpose of the Study:

• To investigate whether the repeated bout effect of eccentric exercise is associated with alterations in the length-tension relationship of the quadriceps muscle.
• To quantify changes in isometric strength across various knee flexion angles following an initial and a repeated bout of eccentric contractions.

Main Methods:

• Twelve participants completed an initial bout of eccentric quadriceps contractions followed by a second bout two weeks later.
• Isometric strength was measured at multiple knee flexion angles (30°, 50°, 70°, 90°, 110°) before and after each bout.
• Pain perception was also recorded following each eccentric exercise bout.

Main Results:

• The initial eccentric bout caused significant strength loss across most knee angles, except 110°, and notable pain.
• The repeated bout resulted in minimal strength loss and significantly reduced pain compared to the initial bout.
• A rightward shift in the length-tension curve was observed before the repeated bout, with strength decreasing at shorter lengths (30°) and increasing at longer lengths (110°).

Conclusions:

• The repeated bout effect is associated with a shift in the muscle's length-tension relationship.
• Increased torque at 110° knee flexion suggests potential sarcomere addition, which may limit sarcomere strain during eccentric contractions.
• Sarcomere addition offers a plausible explanation for the protective adaptation observed after repeated bouts of eccentric exercise.