High-intensity exercises sequentially recruits all the different types of muscle fibers in your body, starting with the smaller motor units made up of slow-twitch fibers, also known as Type I. These are the muscles involved in aerobic metabolism (i.e. steady state running). They recover quickly and have a lot of endurance; perfect for long distance runners. Next in line are the intermediate fast twitch fibers or Type IIA, followed by the stronger, fast-twitch fibers known as Type IIB. The main difference between a slow twitch motor unit and a fast twitch motor unit is the fast twitch motor unit controls more muscle fibers or cells and these cells are bigger; thus producing more force.
Your fast-twitch fibers are largely glycolytic and store a lot of glucose. When these muscles are recruited (through high intensity exercises), it creates the stimulus needed to grow muscle. At the same time, it enlarges the glucose storage reservoir in the muscle, which in turn enhances your insulin sensitivity. Normalizing your insulin is one of the primary health benefits of exercise, and this is particularly true in the case of high-intensity exercise.
Long, slow distance running does not do this as efficiently, which is due to the gradual recruitment of slow-twitch motor units. Since they recover quickly, you’re merely recruiting that one group over and over again while exercising, instead of tapping into the fast twitch fibers as you would with high intensity exercise. As a result, your intermediate and fast-twitch fibers can actually begin to atrophy! Aside from losing muscle mass, you’ll also experience an earlier onset loss of insulin sensitivity. Another downside to long distance running is the highly catabolic effect, or breakdown of your muscles. High-intensity exercise, on the other hand, promotes muscle building, and that stimulus is what triggers the body to make an adaptive response to hold on to muscle, unlike aerobic exercise.
So what constitutes high intensity exercise? It’s training that focuses on performing exercises to the point of momentary muscular failure. Whether you choose to run hill repeats, 400’s on the track or lift weights at the gym; the fundamental principles of high intensity training are that exercises should be brief, infrequent, and intense. When exercises are performed with a high level of effort, it is thought that it will stimulate the body to produce an increase in muscular strength and size.
Boone A. Ebel
Certified Fitness Trainer
Specialist in Performance Nutrition
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