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Training Muscle Fibers

Whether you’re training for recovery from an injury, to improve your performance in a sport or for general strengthening, it is always important to keep your goals in mind and train appropriately. The FITT (Frequency, Intensity, Type and Time) principle helps direct our training programs and tells us that it’s not just about finding the right exercise, but performing it according to your goals. This principle is even more important when you consider muscle fiber types.

There are two main types of muscle fibers which are commonly referred to as fast twitch(Type II) or slow twitch (type I). The fast twitch fibers can be further broken down into Type IIa and Type IIb. The basic distribution of fast and slow is typically guided by genetics and is variable from muscle to muscle, but can be modified through training regimens.

Type I fibers, or slow twitch muscle fibers, get their name from their slow contraction ability. They have a strong vascular supply meaning they get plenty of blood and oxygen to function aerobically which allows them to be used for long periods of time before they fatigue.

These are the muscle fibers that you use when you perform long endurance activities such as jogging or riding a bike for a few miles. To train these muscle fibers specifically you want to perform exercises with lower weights so that you can do multiple repetitions (12-20) and you typically want your movements to be slower rather than quick and explosive.

Type II muscle fibers are known as the fast twitch that can be split into type IIa and Type IIb. Type IIb fibers are the opposite of type I fibers in that they are able to produce force quickly, have poor blood supply and rely on anaerobic metabolism (no oxygen) for energy production. While they are capable of producing the same force as type I, they can do so at a much faster rate which allows us to perform actions such as sprints, jumps, and rapid movements such as throwing a ball.

This fiber type is unable to quickly replenish energy stores and therefore fatigues in a short period, usually within seconds. You might use these muscle fiber types when lifting heavy loads at the gym for 2-5 repetitions or for doing quick explosive movements such as box jumps.

Type IIa are a hybrid between Type I and IIb in that they are able to more rapidly produce force than at type I fiber, but are still aerobic and are slower to fatigue than type IIb. They can be useful for activities such as a 400 meter sprint or lifting 8-12 repetitions in the gym with a moderate to high load. This muscle fiber can be most easily influenced by training regimen and can mimic Type I or IIb to help maximize performance. It can allow you to maintain strong muscle strength while maintaining good endurance and vice versa.

It's important to train with this principle in mind. If you’re training to jump high enough to dunk vs training to complete a half marathon, your program should reflect that. You may be training squats or lunges for both goals, but jumping would require far more explosive power and emphasis on Type IIb fibers whereas a half marathon would require emphasis on type I fibers. There is no one size fits all program for people, but keeping in mind what you’re training for can help you decide how to best tailor your training regimen.. Always be sure to scale things slowly and stay within your means so that you don’t end up injuring yourself unintentionally while achieving your goals!


Author: Edgar Vargas, PT, DPT

Dr. Edgar Vargas, PT, DPT received his Bachelor of Science degree in Kinesiology with emphasis on Exercise Science from California State University, Long Beach graduating Cum Laude. He later received his Doctorate in Physical Therapy from Chapman University. He has clinical experience in a variety of inpatient and outpatient settings including orthopedic and neurologic. Through his treatments, Edgar aims to treat the patient as a whole by listening to and understanding the individual needs of the patient. He seeks to educate each patient so that they may become confident in managing their conditions.Outside of the clinic, he spends his time rock climbing, traveling, and relaxing with his family and friends.

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