Evaluation of the Length-Tension Relationship in an Elderly Population

Abstract

The effects of aging on the muscle length (as inferred by joint angle)-tension relationship was studied in the ankle dorsiflexors of male and female subjects aged 20-40 years (x=25.3; 15d, 159) and 60-80 years (x=68.8; 15d, 159) at 10 joint angles {15°0 through 30°P, in 5° increments). Isometric twitches, voluntary contractions, and 1-sec evoked tetanic contractions {20, 50 & 80 Hz) were measured in the R-tibialis anterior muscle. The resting joint angle for the ankle dorsi flexors was similar between elderly and young adults {13°P ± 3.44). On average, evoked and voluntary torque output increased upon muscle lengthening beyond resting length, and decreased upon shortening. Evoked single twitches of the TA revealed that peak total torque occurred at the extreme of plantarflexion (30°P} in both elderly and young adults. Most importantly, elderly individuals produced similar twitch torque values at all joint angles compared to young adults. Maximal voluntary torque was stronger at the more plantarflexed compared to the dorsiflexed angles, for all subjects, regardless of age, with maximum torque plateauing at 15°P. Elderly subjects demonstrated much reduced MVC torque values compared to young adults at all joint angles (ave.= 18% reduction, p<O.Ol) while maintaining no less than 96% motorunit activation (MUA) . Stimulation of the dorsiflexors at 20, 50, & 80 Hz revealed that the 1-sec peak tetanic torques declined from a maximum at 30°P through to 15°0 for all subjects. Elderly adults produced significantly less tetanic torque at all joint angles compared to young adults (p<0.05). There was no difference between the elderly and young adults in the rate at which the rise in tetanic torque was developed at all joint angles, but elderly adults displayed a significantly greater twitch/tetanus ratio as compared to young adults (p<0.005). In conclusion, these results suggest that there is no age-associated change in the elastic properties of the ankle dorsiflexors, and thus, the length-tension relationship of this muscle group is similar between elderly and young adults.