Muscle length influence on rectus femoris damage and protective effect in knee extensor eccentric exercise

Area of Science:

• Exercise Physiology
• Muscle Physiology
• Biomechanics

Background:

• Muscle damage and the repeated bout effect (RBE) are influenced by exercise-induced muscle length.
• The rectus femoris (RF) muscle's mechanical properties may vary depending on its length during exercise.

Purpose of the Study:

• To test if knee extensor eccentric exercise causes greater RF damage and RBE in a supine (long muscle length) versus sitting (short muscle length) position.
• To investigate if muscle length effects on damage are more pronounced in the proximal than distal RF.

Main Methods:

• Untrained men performed two bouts of knee extensor eccentric exercise, separated by 4 weeks.
• Groups differed in exercise position: sitting then supine (S group) or supine for both bouts (L group).
• Measurements included maximal voluntary isometric contraction (MVC) torque, electromyography (EMG), muscle soreness, and shear modulus.

Main Results:

• After the first bout, the supine group (L) showed significantly greater hip flexor MVC torque reduction and proximal RF EMG and shear modulus changes than the sitting group (S).
• No significant differences were found between groups for knee extensor MVC torque or any variables after the second bout.
• The hypothesis regarding greater RF damage in the supine position, especially proximally, was supported, but the RBE hypothesis was not.

Conclusions:

• Performing knee extensor eccentric exercise in a supine position induces greater acute muscle damage in the rectus femoris, particularly in its proximal region, compared to a sitting position.
• The protective repeated bout effect does not appear to be influenced by the muscle length during the initial eccentric exercise bout.