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 strenuou s eccentric exercise on joint stiffness and to separate joint stiffness int o components due to intrinsic muscle mechanics and delayed reflex muscle ac tivation. Design. Subjects performed 100 maximal eccentric contractions, using the fi rst dorsal interosseus muscle to abduct the index finger while undergoing a 20 degrees displacement of the metacarpophalangeal joint. Joint stiffness was measured 24 h later juring 15% and 65% maximal voluntary contraction an d during electrical muscle stimulation at 15% of maximal voluntary contract ion torque. Background. Joint stiffness can be varied by changing voluntary muscle acti vation and thereby serves an important role in joint stabilization. Eccentr ic exercise has been shown to result in muscle fiber injury, reducing maxim al muscle force. However, it is not known whether intrinsic muscle stiffnes s or reflex stiffness is also affected. Methods. Displacements of 3 degrees amplitude were used to estimate joint s tiffness about the neutral angle of the index finger. The difference betwee n measurements made during voluntary muscle activation and electrical muscl e stimulation was used to obtain reflex stiffness. Results. There was no change in the passive joint stiffness nor was there a ny change in either the intrinsic or reflex stiffness at 15% maximal volunt ary contraction. However, intrinsic stiffness for the electrically stimulat ed muscle was higher post-exercise than preexercise, while active joint sti ffness at 65% maximal voluntary contraction (comprising intrinsic and refle x stiffness) was lower. Conclusion. The observed mechanical changes are compatible with the hypothe sis that type II muscle fibers are more susceptible to injury than type I m uscle fibers, which have higher intrinsic stiffness.