Clearing up “Muscle Confusion:” Understanding Muscle Fiber Differences and Training for Results

Clearing up “Muscle Confusion:” Understanding Muscle Fiber Differences and Training for Results

“Muscle Confusion,” the term gets tossed around as if it is some sort of scientifically proven concept that guarantees magical results, whether those results be fat loss or muscle gain. Muscle confusion is key right? Wrong, skeletal muscles are not the brain; skeletal muscles are incapable of thought, rationalizations, and problem-solving. Confusion of the skeletal muscles is a physiological impossibility. The only confusion going on here is the attempt to sound knowledgeable without having an educated understanding of muscle physiology and response to exercise stimulation should influence designing a training program.

There is more to creating an effective exercise program than just getting sweaty and out of breath. The information about to be discussed, from a scientific understanding of human physiology and exercise, is a very simplistic view. Any undergraduate student in a kinesiology or exercise science program could readily explain the science. Understanding the science allows for the art of proper and practical application within program design. A program is designed and executed with a purpose. A properly designed program will make a person better, not create confusion.

To avoid confusion, the discussion will cover the two primary types of muscle fibers: slow-twitch and fast-twitch. The discussion will cover basic anatomical differences, the different systems, providing energy to the muscle fibers, and the necessary ways to stimulate the muscles to maximize fitness levels and build a better, healthier body. After all, if it does not make the body better, why bother?

Before getting into the specifics of the muscle fibers, a quick point needs to be made the chemical substrate Adenosine Triphospate (ATP). It is the driver of all actions within all 430 of skeletal muscles in the human body. Regardless of what type of muscle fiber is working or for how long, it requires ATP for movement. The different energy systems that work within the different fiber types all working to replenish the levels of ATP to allow for continued movement. Many of the calories that are eaten and diverted into the muscles are put to work creating more ATP to fuel continued movement.

Type One Muscle Fibers, aka, slow-switch muscle fibers are excellent at resisting fatigue. This a result of having high numbers mitochondria and dense capillary concentration in comparison with Fast-Twitch fibers. Slow-twitch fibers are fueled by the oxidative system for the breakdown and utilization of ATP. The oxidative system is slowest at using ATP. In practical application, this meanest that Type One muscle fibers are great for endurance-based activities. If the exercise lasts longer than two minutes for cardiovascular exercise or for more than 20 repetitions in a strength-training exercise, slow-twitch fibers are the primary drivers of movement and the muscle fibers being improved with the training stimulus.

The Type Two, aka, Fast-Twitch fibers are capable of developing higher levels of force and higher velocities of movement. In short, fast-twitch fibers work at higher intensities, both with heavier weights and faster speeds. These can be broken into two subgroups: Fast-Twitch B and Fast-Twitch A fibers.

Fast-Twitch B Fibers work on the opposite end of the exercise spectrum from slow-twitch fibers. The fast glycolytic energy system fuels these fibers for very high intensity, short duration activities. These muscle fibers have the fewest mitochondria, lowest, capillary density, and low aerobic enzyme activity making it the first and fastest energy system to be utilized, but also the slowest to recover. This why a sprint only lasts a few seconds while it is possible to walk for hours. For these fibers to be utilized, physical activity must be so intense that it lasts 10 seconds or less. Strength-training for less than 6 repetitions, sprinting, and plyometric exercises train the Fast-Twitch B fibers.

Fast-Twitch A fibers bridge the gap between Slow-Twitch Fibers and Fast-Twitch B fibers. The A fibers can be thought of as the medium intensity fibers. Being fuelled by the oxidative glycolytic energy system, the mitochondria levels and aerobic enzyme activity falls in between that of the slow-twitch and fast twitch B fibers. The A fibers are performing the work in cardiovascular exercise that lasts from 10 seconds up to two minutes, think intervals, and in strength training done for 7-19 repetitions.

So, with a basic understanding of the muscle fibers, how should this influence the design and emphasis of a training program? It would be easy if these different types of muscle fibers existed in equal amounts; a program could then be designed to spend an equal amount of time training each muscle fiber type for a well-balance, physically capable and health body. They do not. The slow-twitch fibers comprise roughly 45% of the total skeletal muscle within the body; Fast Twitch B, 20% and Fast Twitch A, 35%.

Assuming that we are talking about the average person who simply wants to be healthy and look good, without training for a high level of sport-specific performance, a training program should break up its time along the same lines. As an example, let’s assume a person has time to train one hour a day, five days per week. This translates into 300 minutes of exercise per week.

If a person wants visible results this is a reasonable time commitment. Less time will improve health but not the reflection in the mirror.

45% of 300 is 135. So to adequately train the slow-twitch fibers a person would need to spend 135 minutes doing endurance based training, preferably cardiovascular exercise. That equates to two hours and fifteen minutes of steady-pace cardio per week. 35% of 300 is 105. So 105 minutes should be spent training the Fast Twitch A fibers using medium intensity exercise. The Fast Twitch B fibers should be emphasized in 20% of training which would translate into 60 minutes of high intensity sprinting or strength training per week.

Five Hours (300 minutes) of Exercise Per Week
Muscle Fiber Type	Energy System	Duration/Intensity	Recovery Ratio	Time Per Week
Slow Twitch (Type I)	Oxidative System	Anything done longer than two minutes or 20 reps	N/A	135 minutes
Fast Twitch B (Type IIB)	Fast Glycolytic	1-10 second or less than 6 reps	1: 4-5	60 minutes
Fast Twitch A (Type IIA)	Oxidative Glycolytic	10 seconds-two minutes or 7-19 reps	1: 2-3	105 minutes

The simplest approach to properly balancing training is to pick one primary goal for the workout. This is especially important for high intensity exercise. For the beginner following this example, training one hour a day, five days per week, it would be best to dedicate one full 60 minute workout to high intensity training with the Fast Twitch B fibers than to try and mix it in with the other days. Two 60 minute workouts could be dedicated to training the Slow Twitch fibers while spending one 60 minute workout training the Fast Twitch A fibers. One workout would be split between the remaining training time dedicated to the Fast Twitch A fibers and Slow Twitch fibers.  See the table below for a sample outline of a training week.

Day	Fiber Type	Duration
Sunday	Fast Twitch B	60 minutes of sprinting and heavy strength training 1-6 reps
Monday	Slow Twitch	60 minutes of steady pace cardiovascular exercise
Tuesday	None	Rest
Wednesday	Fast Twitch A	60 minutes 7-19 reps strength training
Thursday	Slow Twitch	60 minutes of steady pace cardiovascular exercise
Friday	Fast Twitch A/Slow Twitch	45 minutes of 30 second-2 minute intervals and 15 minutes of steady pace cardiovascular exercise
Saturday	None	Rest

Variation is training stimulus is necessary in order to maximize the fitness potential of the human body. However, it should never be confusing. Variations in training should always be deliberate, methodical, practical, and performed with passion.