Estrogen-induced Changes In Achilles Tendon Behaviour Are Not Reflected In Leg Stiffness

Abstract

Recent evidence suggests that circulating blood estrogen influences Achilles tendon (AT) mechanical properties in vivo (Bryant et al., 2007) However, it is uncertain whether the changes observed in AT stiffness (KAT) are reflected in leg stiffness (KLEG) during repetitive ground contacts. PURPOSE: To examine whether estrogen-induced changes in KAT are reflected in KLEG. METHODS: Twenty females (Age=28.0±4.2yr; Ht=1.67±0.68m; Mass=61.6±6.8kg) who had been using the MOCP for at least 12 months together with 20 matched, non-MOCP users (Age=31.9±7.3yr; Ht=1.63±0.51m; Mass=62.5±5.9kg) participated in this study. Non-MOCP users were tested at the time of lowest (menstruation) and highest (≈ovulation) estrogen whilst MOCP users, who exhibited constant and attenuated estrogen levels, were tested at Day 1 and Day 14 of their cycle. At each test session, maximal isometric plantarflexion efforts were performed on a calf-raise apparatus whilst synchronous real-time ultrasonography of the triceps surae aponeurosis was recorded. KAT (N·mm-1) was calculated from the load-displacement relationship at maximal plantarflexion force. KLEG (N·m-1) was determined from ground reaction force data as subjects hopped at 2.2Hz on a force plate. RESULTS: Repeated measures ANOVA revealed a significant (p < 0.05) main effect of subject group with significantly higher AT stiffness (18.4%) in the MOCP users compared to the non-MOCP users. In contrast, no significant differences were identified between test sessions or subject groups for KLEG. CONCLUSIONS: Increased KAT in MOCP users may be attributed to augmented exerciseinduced collagen production given that a long-term reduction in blood estrogen levels increases fibroblast activity and Type I collagen mRNA production (Lee et al., 2004). However, despite inherent differences in AT mechanical properties, KLEG of the MOCP users was similar to that of the non-MOCP users. Maintenance of KLEG in MOCP users may be a neuromechanical adaptation (referred to as "muscle tuning") whereby the activation patterns of the triceps surae muscles are modified to preserve the energy dissipating properties of the soft tissues. These results demonstrate that intrinsic differences in tendon behaviour can be modulated during a repetitive hopping task via neuromechanical adaptations.