The Effect of Estrogen on Passive Mechanical Properties in Human Skeletal Muscle

Evidence from preclinical experiments suggests that circulating estradiol (E2) reduces elastic modulus of connective tissue and muscle fibers. Clinical research also supports the notion that E2 tends to increase connective tissue volume, while reducing elastic modulus in connective tissue in humans. However, whether these changes in humans are due to changes in extracellular matrix (ECM) abundance or extend to the composite muscle tissue (CMT) containing myofibers and ECM, remains unclear. Given E2’s known effects on collagen synthesis, cross-linking and intramyofibrillar factors, it is not clear how these interrelated factors (and the sometimes contradictory effects of E2 with respect to stiffness) may impact human CMT modulus. Therefore, the overall objective of this study is to assess the impact of circulating E2 on passive tissue mechanics and collagen abundance in human CMT. We hypothesize that E2 will enhance collagen abundance of CMT while paradoxically reducing passive modulus in the same structures. METHODS: To test this hypothesis, we compared muscle function of 2 female groups with distinct estrogen levels: 2 young eumenorrheic (EU) females and 2 young females on implanted hormonal contraceptive (HC) devices (Nexplanon). Skeletal muscle biopsies from the vastus lateralis of each participant were obtained. For passive mechanics measurement, tissue was stored in 50% glycerol solution until the day of experiment. On the day of experiment, bundles of ~5-10 fibers were mounted between a force transducer and length motor and sarcomere lengths were initially set to 2.4μm. Then, the bundles were subjected to a series of seven stretches, each 8% initial length (Lo) and held for 2min to allow for stress-relaxation. The total strain applied to each bundle equaled 156% Lo. The Young’s Modulus (YM), defined as the slope of the resultant stress-strain curve, was the outcome measure of interest and it represents passive mechanical properties. After experiments, the stretched bundles were blotted dry and mounted in Optimal Cutting Temperature compound (OCT) for Immunohistochemistry (IHC). Relative abundance of collagen was calculated as the fractional area of the field of view occupied by collagen in a transverse muscle tissue cross section. Small sample size precluded formal group-level statistical analyses, but group differences were explored graphically with linear regression analysis performed to assess associations between E2 and tissue properties. RESULTS: Preliminary results revealed that one of the females on HC had high serum E2, therefore, HC and EU groups had similar values for their passive YM and collagen abundance. When separated by estrogen status evaluated by serum E2 (high vs low), passive YM was relatively higher in the high E2 group compared to the low E2 group. Subsequent regression analyses indicated trends towards an association between serum E2 and passive modulus (R2 = 0.812, p = 0.09), and serum E2 and collagen abundance (R2 = 0.981, p = 0.01). DISCUSSION: Our findings supported prior evidence in literature linking E2 to enhanced collagen synthesis in connective tissue. However, we found surprising enhancement of CMT stiffness with E2, opposite our hypotheses and prior observations in ligament and tendon. Further experiments will increase sample size to provide statistical power that is necessary to adequately test our hypotheses. These data will help shed insights on mechanisms that explains increased soft tissue injury risk in young females. FUNDING: Wu Tsai Human Performance Alliance This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.