Aging effects on passive resistive torque in the rat ankle joint after lengthening contractions

Background. The aim of this study was to evaluate whether aging affects (1) the sensation of joint stiffness after lengthening contractions (LCs); (2) passive resistive torque (PRT) of the ankle joint; (3) and the connectin (titin) isoform composition in the medial gastrocnemius (MG) muscle. Methods. Anesthetized young (9 weeks) and adult (35 weeks) Wistar rats (n = 6 per group) were used in the present study. A single bout of LCs was performed on the MG muscle with a combination of electrically induced tetanic contractions via a skin electrode and simultaneous forced dorsiflexion of the ankle joint (angular velocity 15 degrees/s, 0 degrees-45 degrees). Static PRT of the ankle joint (at 30 degrees and 45 degrees) was measured to evaluate joint stiffness. These parameters were measured until 10 days after treatment. The connectin isoform was measured as muscle extensibility employing low-porosity 2% polyacrylamide gel electrophoresis. Results. The averaged static PRT was significantly higher in adult rats than in young rats. Connectin isoform analysis revealed that the adult group contained larger amounts of beta-connectin than did the young rats. After the LCs, static PRT of the ankle joint gradually increased until day 4 in the young group, whereas the adult group did not show a significant change during the experimental period. Conclusions. We concluded that (1) joint stiffness of adult rats is greater than in young rats, similar to human studies; (2) the increased joint stiffness may be due to connectin isoform composition; and (3) joint stiffness after MG muscle LCs is more apparent in young rats.