E-12 Free Communication/Slide - Bone 2: May 30, 2008 8:00 AM - 10:00 AM ROOM: 108
Evidence suggests that estrogen may decrease exercise-induced increases in tendon collagen synthesis (Miller et al., 2007), prompting speculation that circulating blood estrogen influences longer-term tendon mechanical properties. Support for this hypothesis would have important physiological implications for both eumenorrheic women and those with constant and attenuated estrogen levels; that is, women using the monophasic oral contraceptive pill (MOCP).
PURPOSE: To elucidate the effect of circulating levels of estrogen on the strain properties of the human Achilles tendon (AT) in vivo.
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 (¡Öovulation) 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. AT strain was calculated using the following equation:
Where: Li = initial tendon length and Lf = final tendon length
RESULTS: Repeated measures ANOVA revealed a significant (p < 0.05) main effect of subject group with significantly higher Achilles strain rates (25.5%) in the non- MOCP users compared to the MOCP users.
CONCLUSIONS: Lee et al., (2004) reported that increasing estrogen concentration reduced ligament fibroblast activity in response to mechanical loading and decreased Type I collagen mRNA production. Hence, a long-term reduction in blood estrogen levels due to MOCP administration is thought to have augmented exercise-induced collagen production in the AT thereby making the tendon less complaint. Decreased AT strain may alter the capacity of the muscle to harness elastic strain energy. If the effects of estrogen on tendon behaviour are consistent throughout the female body, these results provide a rationale for the lower rate of joint injuries in MOCP users as a reduction in tendon compliance would enhance joint support.