It doesn’t take much scratching at the surface in the field of biology to reaffirm just how much the human body is a beautifully complex machine, and the muscle system is a perfect example of this. Skeletal muscle contains both slow- and fast-twitch fibers that differ by ratio: slow-twitch fibers use energy slowly and support sustained movements; conversely, fast-twitch fibers use energy quickly and are required for quick and powerful movements.
Much of the previous research on skeletal muscle hasn’t focused on type-specific changes in muscle fibers. This is largely because protein analysis of muscle fibers can be expensive and time consuming. But, recently, scientists developed a protocol using freeze-dried muscle samples that allowed them to study slow and fast-twitch fibers separately.
A newly published study using this approach revealed that fast and slow-twitch muscle fibers actually respond differently to exercise training. Using muscle samples of nine healthy males, researchers looked to identify exercise-related changes to individual muscle fibers both before and after training. Subjects were exposed to two experimental phases separated by 12-weeks of endurance training. Researchers performed protein analysis on their freeze-dried samples and identified over 4,000 different proteins in the samples.
More importantly, researchers identified 273 slow-twitch and 172 fast-twitch muscle proteins that were expressed differently following exercise, suggesting that muscle adaptation to exercise training occurs in a fiber-specific manner. According to the authors, exercise training upregulates fiber-specific proteins related to metabolism, oxygen uptake, and muscle growth, all of which contribute to muscle maintenance. These findings can be used to develop targeted therapies for musculoskeletal disorders including sarcopenia, which is muscle loss due to age.
Muscle loss is considered a normal part of aging, but it doesn’t have to be. According to scientists, sarcopenia can begin as early as age 30 and cause us to lose muscle at a rate of 3-5% per decade. Most people have an even number of slow and fast-twitch fibers, but as we age, slow-twitch fibers remain relatively stable and may even increase, while fast-twitch fibers are quickly lost. Sarcopenia puts an individual at increased risk for age-related injuries and accidents like falls, sprains, and strains; some research even suggests that a loss of skeletal muscle is correlated with physical disability, poor quality of life, and death.
Until scientists can develop therapies to spare fast-twitch muscle in adults, knowing that each type of muscle fiber responds differently to exercise can help you diversify your training routine and mitigate some of this loss. Slow-twitch fibers respond well to endurance training (e.g., running, walking), but if you want to stimulate fast-twitch fibers, be sure to include things like heavy squats or deadlifts, sprints, bench press, and medicine ball training into your routine.