Effects of repeated eccentric and isometric contractions with relatively low loading dose on joint stiffness.

BACKGROUND: The purpose of the present study was to compare changes in joint stiffness after repeated eccentric (ECC; leading to muscle fatigue) and isometric (ISO; leading to tendon fatigue) contractions and to elucidate the mechanisms responsible for decline in joint stiffness after repeated stretch-shortening cycle exercises reported previously.

METHODS: Ankle joint stiffness (change in joint torque divided by change in joint angle during braking phase) and electromyographic activity (mEMG) of the plantar flexor muscles during drop jump using the ankle joint were measured before and after ECC, ISO, and control conditions.

RESULTS: Peak ankle joint torque, range of motion, and joint stiffness during drop jump did not change after ECC or ISO. However, relative changes in joint stiffness correlated with those in range of motion, but not peak torque. In ECC and ISO, mEMG during pre-landing, braking, and push-off phases did not change after endurance tests, except for braking phase in ISO. Furthermore, relative change in joint stiffness correlated with that in mEMG during braking phase in ISO, but not in ECC. Under control conditions, no significant changes were noted in the measured variables.

CONCLUSIONS: These results suggest that relative changes in joint stiffness (but not decrease in average values) after ECC and ISO were related to change in range of motion, but not exerted force levels. Furthermore, changes in range of motion after repeated contractions were relevant to those in muscle activation in ISO and other factors (probably muscle stiffness under passive and active conditions) in ECC.