You can maximize your clients’ training and performance by identifying appropriate loading zones in relation to relative muscle fiber recruitment. First, let’s review the various types of muscle fibers.

It is generally accepted that there are two main types of muscle fibers: type I (slow twitch) muscle fibers and type II (fast twitch) muscle fibers. Type II fibers can be further categorized into type IIa and type IIb. Each fiber type contracts in a unique way and will influence how muscles respond to training and physical activity.

Type I – often referred to as slow-twitch oxidative fibers. 
Type I muscle fibers are efficient at using oxygen to generate fuel (known as ATP) for continuous, extended muscle contractions for long durations of exercise. Type I fibers fire more slowly than type II fibers and can maintain workloads for a long time before fatiguing. Type I fibers are great at helping athletes perform endurance activities such as marathons and long – distance cycling.

Type I fibers are characterized by low-force/power/speed production and high endurance and are optimally recruited at low intensities. Workloads under 40% of 1RM recruit type I fibers.

Type II – often referred to as fast-twitch muscle fibers. 
Type II muscle fibers use anaerobic metabolism to create fuel and generate short bursts of strength or speed. However, they fatigue more quickly. Type II fibers generally produce the same amount of force per contraction as type I fibers, but they are able to fire more rapidly. Fast-twitch fibers are an asset to sprinters who need to quickly generate a lot of force. Type II fibers can be categorized into type IIa and type IIb.

Type IIa fibers – often referred as fast-twitch oxidative muscle fibers or intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. Workloads of 40-75% of 1RM predominately recruit type IIa fibers.

Type IIb fibers – often referred to as fast-twitch glycolytic muscle fibers. 
These fibers use anaerobic metabolism to create energy. They excel at producing quick, powerful bursts of speed and are the most explosive fiber types. Type IIb muscle fibers are rapid firing and have the highest rate of contraction of all the muscle fiber types. These fibers also have a much faster rate of fatigue.

Type IIb fibers are characterized by high-force/power/speed production and low endurance. Workloads greater than 75% of 1RM predominately recruit type IIb fibers.

This table summarizes the characteristics of each muscle fiber type.
________________________________________________________________________
Fiber type Type I Type IIa Type IIb
________________________________________________________________________
Contraction time slow fast very fast
Fatigue resistance high intermediate low
Activity type aerobic long anaerobic short anaerobic
Force production low high very high
Oxidative capacity high high low
________________________________________________________________________
(Adapted from http://www.coachr.org/fiber.htm)

Implications for Training
Muscles contain a genetically determined mixture of both slow and fast fiber types. The only direct way to assess the fiber-type of a muscle is to perform an invasive muscle biopsy in which a needle is stuck into the muscle and a few fibers are plucked out and examined under a microscope.

There is evidence that both the structure and metabolic capacity of individual muscle fibers can adapt to different types of training. Muscle fibers cannot be changed from one type to another, but training can change the amount of area taken up by a fiber type in the muscle. In other words, there can be a selective hypertrophy of fibers that result from the type of training.

For example, an athlete can have a 50/50 mix of fast-twitch (FT) and slow-twitch (ST) fibers in a muscle, but because FT fibers normally have a larger cross-sectional area than ST fibers, 65% of that muscle’s area might be FT and 35% might be ST. Following a weight training program to improve muscular strength, the number of FT and ST fibers will remain the same (still 50/50); however, the cross-sectional area will change. The ST fibers will atrophy (get smaller) and the FT fibers will hypertrophy (get larger).

Depending on the specific intensity used in training, the muscle can change to a 75% FT area and a 25% ST area. The change in area will lead to greater strength but decreased endurance capabilities. In addition, the athlete will gain mass, as measured by the circumference of the muscle because the mass of FT fibers is greater than that of ST fibers.

Conversely, if the athlete trains for muscular endurance (lower loads, more reps), the FT fibers will atrophy while the ST fibers hypertrophy, causing a greater area of ST fibers. The area of the muscle, which began at 65% FT and 35% ST before training, can change to 50% FT and 50% ST following training. The endurance capabilities of the muscle will increase and its strength will decrease. The athlete will lose some muscle mass, again because ST fibers are lower in mass than FT fibers. The decrease in mass can be observed by a smaller circumference of the muscle.

No matter what the workout intensity, type I, or slow-twitch fibers, are recruited first. If the workout intensity is low, these fibers may be the only ones that are recruited. If the workout intensity is high, such as when lifting heavy weights or performing intervals on the track, type I muscle fibers are recruited first, followed by type IIa and type IIb fiber.

Join master trainer and kinesiotherapist, Paul Chek, as he shares more in-depth information on how to optimize muscle fiber recruitment through training specificity.

For more information on specific training for muscle fiber types and how to maximize training variables for optimal results, check out Paul’s CE course Program Design: Choosing Reps, Sets, Loads, Tempo, and Rest Periods.