Introduction: This study examined the effects of age and knee position (fully extended, K0; 90° flexed, K90) on plantar flexor maximal voluntary contraction (MVC) torque and the rate of torque development (RTD) in both sexes. Methods: The following parameters were measured in 32 older (66–81 yr, 17 males and 15 females) and 37 young (20–30 yr, 18 males and 19 females) adults: evoked peak twitch torque, time to peak twitch torque, RTD of the twitch torque, MVC torque, RTD at early (0–50 ms, RTD0–50) and later (100–200 ms, RTD100–200) time intervals during explosive contractions, voluntary activation (VA%) during MVC, root mean square of the electromyogram (RMS-EMG) during MVC and explosive contractions, thickness of the triceps surae, and pennation angle of the medial gastrocnemius. The magnitudes of the differences were interpreted based on Cohen's d (d). Results: Age-related difference in RTD0–50 was greater for females (d = 1.36) than males (d = 1.03) and vice versa for MVC torque and RTD100–200. For young adults, MVC torque, RTDs, and RMS-EMGs of the gastrocnemius but not the soleus were significantly higher in K0 than in K90. For older adults, no differences in voluntary RTDs were observed between K0 and K90, and RMS-EMGs of the gastrocnemius were higher in K90 than in K0, except for that of the lateral gastrocnemius in the early time intervals during explosive contraction. The age-related difference in the effect of knee position for RTD0–50 was higher in females than males, and vice versa for MVC torque and RTD100–200. Conclusion: The results suggested that the effects of age and knee joint angle on the plantar flexor performance were more prominent in the early phase of force production for females and were more apparent in the later phase and maximal force for males.
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