The effect of knee joint angle on plantar flexor power in young and old men

Human adult aging is associated with a loss of strength, contractile velocity and hence, power. The principal plantar flexors, consisting of the bi-articular gastrocnemeii and the mono-articular soleus, appear to be affected differently by the aging process. However, the age-related effect of knee joint angle on the torque-angular velocity relationship and power production of this functionally important muscle group is unknown. The purpose was to determine whether flexing the knee, thereby reducing the gastrocnemius contribution to plantar flexion, would exacerbate the age-related decrements in plantar flexion power, or shift the torque-angular velocity relationship differently in older compared with young men. Neuromuscular properties were recorded from 10 young (~ 25 y) and 10 old (~ 78 y) men with the knee extended (170°) and flexed (90°), in a randomized order. Participants performed maximal voluntary isometric contractions (MVCs), followed by maximal velocity-dependent shortening contractions at pre-set loads, ranging from 15 to 75% MVC. The young men were ~ 20-25% stronger, ~ 12% faster and ~ 30% more powerful than the old for both knee angles (P < 0.05). In both age groups, isometric MVC torque was ~ 17% greater in the extended than flexed knee position, with no differences in voluntary activation (> 95%). The young men produced 7-12% faster angular velocities in the extended knee position for loads ≤ 30% MVC, but no differences at higher loads; whereas there were no detectable differences in angular velocity between knee positions in the old across all relative loads. For both knee angles, young men produced peak power at 43.3 ± 9.0% MVC, whereas the old men produced peak power at 54.8 ± 7.9% MVC. These data indicate that the young, who have faster contracting muscles compared with the old, can rely more on velocity than torque for generating maximal power.