Author(s): EPRO, G.1, SUHR, F.2,3, KARAMANIDIS, K.1,4, Institution: LONDON SOUTH BANK UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 2451

INTRODUCTION:
Both muscle and tendon adapt to mechanical loading (1), with muscles demonstrating faster tissue renewal rates than tendons (2). In young male adults we recently observed a persistent reduction in tendon stiffness and signs of molecular damage accumulation within the tendon in response to a frequent high strain cyclic loading exercise (3). Given, that hormonal fluctuations across female menstrual cycle may potentially affect tendinous tissue, we aimed to examine the effects of consecutive daily repetitive high strain cyclic loading on triceps surae (TS) muscle-tendon unit (MTU) mechanical properties in young male and young female adults, and determine whether tendon’s adaptive response is affected by different serum levels of estradiol.
METHODS:
11 young healthy male (26±6 yrs) and 20 female adults (24±4 yrs) underwent 8 consecutive days of isometric resistance exercise for ankle plantarflexors using high Achilles tendon (AT) strain cyclic loading by exercising three times a day (90% MVC; ~2h rest between sessions within day). Female group consisted of both eumenorrheic females and females using oral contraceptives, with the first being trained either in high or low estradiol level of their hormonal cycle. Serum estradiol levels were determined every day along their menstrual cycle. At baseline and every second day of the exercise intervention TS MTU mechanical properties were assessed using synchronized dynamometry, ultrasonography and motion capture. Baseline levels were determined as an average of two measurement sessions on separate days.
RESULTS:
At baseline female adults demonstrated on average ~20% lower AT stiffness and maximal AT force in relation to males leading to no sex-related difference in maximal tendon strain during MVCs. Following the 8 consecutive days of high AT strain cyclic loading, both groups showed similar relative increase in maximal AT strain (~16%) along with a corresponding ~20% decrease in AT stiffness (P<0.05). Moreover, the rate and magnitude of relative changes in maximal AT strain and stiffness over the 8 days cyclic loading exercise were not affected by menstrual cycle in eumenorrheic females. Accordingly, no significant correlation was detected between the average estradiol levels and relative or absolute changes in tendon stiffness or strain.
CONCLUSION:
This study showed that frequent high-magnitude and volume of tendon strain cyclic exercise can lead to a reduction of tendons tolerance to resists deformation in response to mechanical loading irrespective of the sex and female hormonal fluctuation. Hence, suggesting that fluctuations in the hormonal levels across the female menstrual cycle do not necessarily affect the homeostasis and adaptive response of the tendinous tissue under frequent high loading.

REFERENCES:
1) Arampatzis et al. 2007 JExpBiol
2) Heinemeier et al. 2013 FASEB J
3) Epro et al. 2023 JExpBiol