Increased antagonist coactivation-related hamstring torque reduces maximal knee extension torque in healthy old adults.

摘要

As humans age, the ability to produce maximal voluntary torque decreases due to muscle atrophy (sarcopenia) and it is also known that advancing age alters the neural control of voluntary movement. One manifestation of the neural changes is that old adults execute voluntary movements with heightened antagonist muscle coactivation. Previous studies have examined the roles of increased coactivation but none have examined the effects of coactivation on motor output per se. In a simple knee extension task, it is possible to estimate the torque generated by the knee extensor and hamstring muscle groups using an electromyography- (EMG) driven model. The purpose of this study was to determine if the higher hamstring coactivation during knee extension in old adults would produce proportional reductions in knee extension torque. The hypothesis was that old compared with young adults have higher levels of antagonist hamstring muscle coactivation at all velocities, producing greater reductions of torque during knee extensions in the old adults.;Peak torque was measured in the quadriceps and hamstrings and surface EMG activity were collected from the vastus lateralis, vastus medialis, semitendinosus, and biceps femoris during knee extension at 30°/s, 90°/s, and 150°/s using concentric (shortening) and eccentric (lengthening) contractions of the hamstrings. Outputs of the EMG-driven model were: (1) net concentric knee extension torque (summation of the torque computed from EMG activity of the quadriceps and hamstrings), (2) agonist torque (torque calculated from the EMG activity of the quadriceps), and (3) antagonist torque (torque calculated from the EMG activity of the hamstrings). We compared these three measures of torque during controlled knee extension at 30°/s, 90°/s, and 150°/s between old (mean age 76.2, n=10) and young adults (age 20.7, n=13) using a speed (3) by group (2) ANOVA followed by a Tukey's post-hoc contrast at p 0.05.;Measured peak torque was 31% lower in old vs. young adults. Old adults had greater coactivation during knee extension at all three contraction velocities. The calculated torques revealed that young vs. old adults produced more calculated net concentric torque and that old vs. young adults had significantly greater calculated antagonist hamstring torque production. The difference in calculated net concentric torque was attributed to increased calculated antagonist hamstring torque in old adults, since calculated agonist quadriceps torque was similar between the two groups. There was a significant speed-related increase in calculated antagonist hamstring torque production. In conclusion, while the age-related loss of maximal voluntary torque has been primarily attributed to sarcopenia, the present data suggest that such reductions may also include decreases in torque due to hyperactivity of antagonist muscle that increase the counteracting torques produced by the agonist prime movers.