The effect of eccentric exercise on intrinsic and reflex stiffness in the human hand

Objective. The purpose of this work was to determine the effect of strenuous eccentric exercise on joint stiffness and to separate joint stiffness into components due to intrinsic muscle mechanics and delayed reflex muscle activation. Design. Subjects performed 100 maximal eccentric contractions, using the first dorsal interosseus muscle to abduct the index finger while undergoing a 20° displacement of the metacarpophalangeal joint. Joint stiffness was measured 24 h later during 15% and 65% maximal voluntary contraction and during electrical muscle stimulation at 15% of maximal voluntary contraction torque. Background. Joint stiffness can be varied by changing voluntary muscle activation and thereby serves an important role in joint stabilization. Eccentric exercise has been shown to result in muscle fiber injury, reducing maximal muscle force. However, it is not known whether intrinsic muscle stiffness or reflex stiffness is also affected. Methods. Displacements of 3° amplitude were used to estimate joint stiffness about the neutral angle of the index finger. The difference between measurements made during voluntary muscle activation and electrical muscle stimulation was used to obtain reflex stiffness. Results. There was no change in the passive joint stiffness nor was there any change in either the intrinsic or reflex stiffness at 15% maximal voluntary contraction. However, intrinsic stiffness for the electrically stimulated muscle was higher post-exercise than pre-exercise, while active joint stiffness at 65% maximal voluntary contraction (comprising intrinsic and reflex stiffness) was lower. Conclusion. The observed mechanical changes are compatible with the hypothesis that type II muscle fibers are more susceptible to injury than type I muscle fibers, which have higher intrinsic stiffness.