Graduation Year

2026

Document Type

Campus Only Senior Thesis

Degree Name

Bachelor of Arts

Department

Neuroscience

Reader 1

Jenna Monroy

Reader 2

Ashley Heers

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Terms of Use for work posted in Scholarship@Claremont.

Abstract

Female musculoskeletal physiology remains critically understudied despite the substantial mechanical and hormonal changes that occur across reproductive states. This study investigated how pregnancy and lactation influence skeletal muscle mechanics by quantifying passive, active, and stretch-related force production in ex vivo soleus muscles from virgin, pregnant, and lactating C57BL/6 mice. Passive and active stress, residual force enhancement (RFE), peak and total isometric stress, and extra active stress (exAS) were measured across a series of controlled muscle stretches and circulating estradiol concentrations were assessed via an ELISA assay. Reproductive state significantly affected passive stress, with lactating muscles exhibiting consistently higher passive tension than both virgin and pregnant muscles at extended lengths, indicating increased baseline stiffness in the postpartum state. Active and peak isometric stresses did not differ significantly among groups, suggesting preserved contractile capacity across reproductive conditions. RFE also did not change significantly, suggesting that history-dependent titin-based force enhancement is maintained during pregnancy and lactation. However, lactating muscles produced significantly higher stretch-activated force (exAS) across multiple lengths, revealing an augmented mechanical response to active lengthening unique to the postpartum state. Pregnant and lactating muscles exhibited elevated estradiol levels compared with virgin muscles, however high variability in hormone measures suggests that estradiol concentrations alone are not sufficient to explain differences in muscle stresses across conditions. Together, these findings indicate that reproductive state primarily modulates the elastic and strain-responsive components of muscle function rather than its active contractile machinery. Increased passive stiffness and elevated stretch-activated force in lactation suggest postpartum remodeling of titin or extracellular matrix elements, contributing to a mechanically distinct maternal muscle phenotype. These results highlight the dynamic nature of female muscle physiology and the necessity of incorporating reproductive state into musculoskeletal research.

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