Maximizing Motor Units
Muscle Fiber Types
If you are a physical therapist, strength coach, personal trainer, or other related professional, you need to understand the different muscle fiber types and what they are responsible for. This information is also helpful as an athlete so that you can tailor your training program to target specific muscle fibers.
Generally speaking, there are three main fiber types in muscle; Type I, Type IIA, and Type IIB (see note at bottom of post). These fiber types and their properties have been outlined in a chart below.
As you can see, Type I muscle fibers are utilized when low forces and a high oxidative demand is required. These are your endurance fibers.
Type IIA fibers are utilized when high forces are generated at intermediate speeds. These are your strength fibers.
Type IIB fibers are utilized in high velocity conditions when medium forces are required. These are your power fibers. If you’re at all familiar with the NSCA guidelines for endurance, strength, hypertrophy, and power, you will see that those guidelines were based on the properties of each muscle fiber type.
It would require writing an entire book for me to explain the various ways to utilize and train each fiber type to its full potential. Instead, let’s highlight a few important points that you should understand when working with patients and athletes.
Each Muscle Serves a Purpose
All muscles contain a certain proportion of the three fiber types. However, not every muscle contains the same ratio of each fiber type. For example, postural muscles such as the multifidus tend to have more Type I fibers because its job is to produce low forces through a long period of time (endurance). A large limb muscle such as the hamstring will have a higher proportion of Type IIA and Type IIB fibers based on its need to develop strength and power with activities like jumping and sprinting. That’s not to say that the hamstring doesn’t have Type I fibers as well, it does. In fact, the ratio of fibers from one individual to another will vary highly based on genetics and training methods.
Genes versus Training
There is plenty of evidence in the literature indicating genetic influence on muscle composition. Some people are born with a higher proportion of Type I muscle fibers, and others have been given more Type IIA or IIB fibers. This explains why some athletes are naturally more gifted with endurance sports and other athletes are naturally more gifted at producing high forces such as jumping. Training, however, can greatly influence the properties of muscles. For example, intense endurance training will elicit more mitochondria formation and thus improve the oxidative capacity of Type I fibers. Additionally, resistance training with high loads can increase fiber size of Type IIA muscle fibers which leads to hypertrophy and strength gains. The elaborate interplay between genetics and training method is complex but it is important to understand that both factors influence the athletes performance.
Understanding Muscle Fiber Recruitment
Activities that require smaller forces such as walking or jogging may use many different muscles, but only serve to recruit Type I fibers. In order to recruit more motor units, the activity needs to have a high force demand or a high velocity component. See the image to the right. As the force demands of the activity increase, the percentage of muscle fibers recruited also increases.
Only with maximal effort activities are the largest muscle fibers utilized.
Fiber recruitment order is based on size and is always the same. Smaller fibers are recruited before larger fibers. As the force demand increases, larger fibers will be called upon. It is not possible to recruit large fibers before smaller fibers. The image below also explains this phenomenon. The image shows an activity of moderate-high level force being performed. This activity will require full recruitment of all small muscle fibers, moderate recruitment of medium sized fibers, and little to no recruitment of the largest muscle fibers.
Clinical Application
The following two examples should help you better understand the concept of graded recruitment.
- Let’s say the activity is getting up from a chair. A 25-year-old male may need to recruit about 40% of his lower extremity muscle fibers to perform the task. Those fibers will always be recruited based on need to produce force. An 88-year-old male may need to recruit 80% of his lower extremity muscle fibers to complete the same task because his muscles are not capable of producing the same amount of force without recruiting larger fiber types.
- Now let’s talk about an athlete going for a jog versus doing sprints. A recreational runner goes outside and jogs for 30 minutes. His cardiovascular system may have been challenged and his legs may be tired. However, during his entire jog, he was only recruiting 45% of his muscle fibers. The next day he decides to go do several sprints instead. He does five 80-yard maximal effort sprints. During those sprints he recruited 100% of his muscle fibers because he incorporated a maximal effort activity along with a power (velocity-dependent) activity.
Key Concepts
- Every exercise you prescribe to a patient must have a purpose or goal
- The external force provided needs to be sufficient enough to recruit the targeted muscle fibers
- If your goal is to improve power, you MUST incorporate a velocity component
- Training should mimic sport or event. Utilize the SAID principle (specific adaptation to imposed demands)
- If the patient needs to handle high forces in his/her sport, you must incorporate those in the training
- Jogging or activities with high repetition are not able to target the largest muscle fibers
- In order to effectively train all muscle fibers, there must be a substantial force demand
When programming a training regimen for your athletes, clients, and patients, be sure that you are utilizing these concepts and training the muscles in a way that best mimics the specific demands of their sport.
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