Ever wonder why some people seem built for marathons while others excel at sprinting or heavy lifting? Part of the answer lies in muscle fiber type. Your muscles are made up of two main fiber types — slow-twitch (Type I) and fast-twitch (Type II) — and understanding how they work can help you set realistic expectations for your training.

Slow-twitch fibers are aerobic: they use oxygen efficiently to fuel endurance activities like distance running. Fast-twitch fibers are anaerobic, powering short, intense bursts of strength or speed, such as sprinting, but they tire out much faster. Here are three things worth knowing about how these fibers work and what you can (and can't) do to influence them.

1. Genetics largely determine your fiber type.

Most people are born with roughly an even split — about 50% slow-twitch and 50% fast-twitch fibers. The exact ratio varies by individual and comes down largely to genetics. Muscle groups also differ depending on their job in the body. The core muscles, for instance, work constantly to keep you upright and stable, so they're made up mostly of fatigue-resistant slow-twitch fibers.

2. Training can shift some fibers but won't change your fundamental type.

Your dominant fiber type helps predict whether you'll naturally excel at endurance- or power-based activities, but research on how much training can change that remains mixed. Many muscle fibers are actually "hybrids," showing traits of both fiber types, and evidence suggests these hybrids can be nudged toward one type or the other through consistent training. A fiber fully converting from one type to the other, though, is unlikely. Elite athletes often have up to 80% of one fiber type, but it's hard to know how much of that comes from training versus what they were simply born with.

3. Fiber composition shifts as you age.

Age-related muscle loss typically becomes noticeable around age 50, but not all fibers decline at the same rate. Slow-twitch fibers tend to hold up better, with noticeable loss often not appearing until age 60. The good news: regular resistance training strengthens and builds both fiber types, making it one of the best tools for countering age-related muscle loss.

The bottom line: don't overthink your fiber type when choosing how to stay active. A balanced mix of muscle fibers means you're equipped to enjoy a wide range of activities. And ultimately, the best exercise program is the one you enjoy enough to stick with.

Sources

• American Council on Exercise - A Step-by-step Approach to Helping Your Clients Successfully Lose Weight and Tone Up
• Mayo Clinic - The many benefits of resistance training as you age

Lori Rice, M.S., is a nutritional scientist and author with a passion for healthy cooking, exercise physiology, and food photography.