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°/s, 0°–45°). Static PRT of the ankle joint (at 30° and 45°) 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 β-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.