Graduation Year

2026

Document Type

Open Access Senior Thesis

Degree Name

Bachelor of Arts

Department

Biology

Reader 1

Marion Preest

Reader 2

Jennifer Armstrong

Rights Information

2025 Lily G Colberg

Abstract

Type II muscle fibers, also known as fast-twitch muscle fibers, are reliant on aerobic glycolysis and are crucial for high-force contractions, mobility, and rapid movements, such as fast reflexes. Heavy resistance training has been found to increase type II muscle fiber concentration in young, middle-aged, and elderly individuals. However, there have been few studies conducted on female-only sample populations. In women, the impact of heavy resistance training on type II muscle fibers is affected by the progressive decline of estrogen that defines the transition throughout the four female reproductive stages: premenopause, perimenopause, late-transition menopause, and postmenopause. Menopausal estrogen depletion causes a decrease in both muscle mass and strength, in addition to a lessened ability to gain muscle. This diminished muscle mass is primarily attributed to a reduction in type II muscle fiber size. To better understand the effect of heavy resistance training and female reproductive stage on type II muscle fibers, this study proposes a 20-week exercise intervention. Four sample populations of premenopausal, perimenopausal, late-transition menopausal, and postmenopausal women will participate in the exercise protocol. 2,500 participants will be recruited for each reproductive stage group (N=10,000). The protocol will include a 2-week orientation and strength tests, followed by the first heavy resistance training block (weeks 2-10), a mid-protocol rest week (week 10), and a second heavy resistance training block (weeks 11-20). The training blocks will consist of four lower body exercises. Muscle fiber biopsies of the participants’ dominant vastus lateralis muscle will be taken prior to and after the 20-week training protocol using the suction-modified Bergström technique. Muscle biopsies will be histochemically treated and stained for myosin ATPase activity. Type II fiber diameter will be measured, and fibers will be counted to determine type II fiber cross-sectional area and concentration. A two-way ANOVA will be run to determine potential effects of reproductive stage and heavy resistance training on type II muscle fibers. It is expected that the premenopausal group, which maintains the highest estrogen levels, will experience the greatest increase in type II fiber cross-sectional area and concentration as a result of the training protocol. As estrogen levels decrease, type II fiber adaptations will increase, but to a lesser degree. The smallest increase is expected to be seen in the postmenopausal group. Understanding how heavy resistance training can mitigate the effects of decreased estrogen levels on type II muscle fibers across reproductive stages is crucial for female health. The results of the proposed study can be used to advise clinical decisions, educate women on how to exercise to support healthy aging, and will meaningfully contribute to the field of muscle physiology.

Download

Share

COinS