Muscles are made up of individual muscle fibers. And like muscles themselves, not all muscle fibers are the same.
While a variety of types of muscle fiber have been identified, they are generally classified as being either slow-twitch or fast-twitch and each one has different functions that are important to understand when it comes to movement and exercise programming.
WHAT DETERMINES HOW MANY OF EACH MUSCLE FIBER TYPE AN INDIVIDUAL HAS?
Genetics
You are genetically programmed to having a certain percentage of each muscle fiber based on your parents’ genes. However, identifying whether you are fast or slow-twitch dominant would require an invasive muscle biopsy.
Therefore, if you find that you tend to enjoy more endurance-based activities and that they are relatively easy for you, you probably have a greater number of slow-twitch fibers.
Otherwise, if you really dislike going for long runs, but enjoy playing sports that rely on short bursts of explosive movements, or if you like weight training because it is relatively easy, you are probably fast-twitch fiber dominant.
Hormone Levels
The amount of hormone in the blood will affect the fiber type of an individual and how big the fibers are. Hormone levels naturally fluctuate throughout a person’s lifetime, so some changes in fiber type and distribution can occur as we grow and mature.
Males and females also have different levels of certain hormones produced.
Some of these hormones are ‘catabolic’, that is they stimulate muscle breakdown, while others are ‘anabolic’, that is they stimulate the growth and repair of muscle tissue.
So depending on our age, gender, and the type of training we do, we can cause an increase or decrease in the production of certain hormones. The result of this is either an increase or decrease in the slow or fast-twitch muscle fibers.
Training Undertook
There is evidence to show that fibers adapt to the type of training they are exposed to.
For example, if a person with predominantly slow-twitch ‘endurance’ fibers is trained predominantly with explosive power movements and heavyweights their slow-twitch fibers would overtime begin to behave more like fast-twitch fibers.
Age
Age is also a factor for our muscle fibers. Aging causes a loss in lean muscle mass, with a decline in our fast-twitch fibers, but there is also an increase in our slow-twitch fibers.
Loss of lean muscle mass can contribute to age-related metabolic dysfunctions, body composition changes, even an increased risk of falls. Resistance training can combat this decline!
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