The magnitude and character of resistance-training-induced increase in tendon stiffness at old age is gender specific.

Human tendon mechanical properties are modified with loading. Moreover, there are indications that the training response in the tendon is gender specific. The aim of the current study was to examine whether in vivo patella tendon stiffness (K) differentially alters with training in older males compared with females. We also aimed to identify which endocrine pathway underlies the responses. Maximal knee extensor forces were also monitored to determine the training effect on muscle function. Fourteen healthy, habitually active older persons (seven males aged 74.0 ± 1.2 years (mean±SEM) and seven females aged 76.7 ± 1.2 years) were tested at baseline and after 12 weeks of weekly, progressive resistance training. With training, percentage increase in quadriceps maximum voluntary isometric force (MVC) was similar in males (2,469.6 ± 168.0 to 3,097.3 ± 261.9 N; +25.3 ± 6.1% (p < 0.01)) and females (1,728.8 ± 136.3 to 2,166.5 ± 135.8 N; +30.4 ± 15.1% (p < 0.05)), respectively. K increased more in males (338.0 ± 26.6 to 616.9 ± 58.7 N/mm; 79.8 ± 4.2% (p < 0.001)) compared to females (338.9 ± 31.0 to 373.2 ± 25.8 N/mm; +13.0 ± 3.7% (p < 0.001)). Interestingly, a pattern was found whereby below ~40% MVC, the females showed their greatest degree of K changes, whereas the males showed their greatest degree of K change above this relative force level. This gender contrast was also true at a standardised force level (1,200 N), with 5.8 ± 0.4% vs. 82.5 ± 1.8% increments in the females (i.e. value change from 380.3 ± 14.1 to 402.4 ± 13.3 N/mm) and the males (i.e. value change from 317.8 ± 13.8 to 580.2 ± 30.9 N/mm), respectively (p < 0.001). While circulating levels of both IGF-I and IL-6 did not alter with training, IGFBP-3 showed a significant training effect (19.1 ± 4.8%, p < 0.001) and only in the male sub-group (p = 0.038). We show here that with training, in vivo older females' tendon is less dramatically modulated than that of males'. We also show that the relative forces, at which the greatest adaptations are exhibited, differ by gender, with a suggestion of endocrine adaptations in males only. We thus propose that both training and rehabilitation regimens should consider gender-specific tendon responsiveness, at least in older persons.