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Endurance

PES-2By Stacey Penney, MS, NASM-CPT, CES, PES, FNS

Looking to build endurance? What about power? Do dreams of being an all-star hitter or marathon runner need to be dashed if twitch ratios aren’t ideal? Not necessarily. The types of muscle fibers targeted in different types of training programs can impact performance goals.

As most of us may recall from our physiology studies, there are two main types of human skeletal muscle fiber types, type I and type II, or slow-twitch and fast-twitch, respectively. Fast-twitch are further classified into type IIa and type IIx. They differ in both their biochemical and contractile properties.

Slow-twitch, Type I

These muscle fibers have high concentrations of mitochondria and myoglobin, and although they are smaller than the fast-twitch fibers, are surrounded by more capillaries (1,2). This combination supports their capacity for aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal exercise activities. Type I fibers produce less force, are slower to produce maximal tension (lower myosin ATPase activity) compared to type II fibers, but they are able to maintain longer-term contractions, key for stabilization and postural control (1,2).

Fast-twitch, Type II

Our fast-twitch, type II muscle fibers are further divided into type IIx and type IIa. Typically, these have lower concentrations of mitochondria, myoglobin, and capillaries compared to our slow-twitch fibers and are quicker to fatigue (1,2). These larger-sized fibers are also able to produce a greater and quicker force, an important consideration for power activities (1,2).

Related Articles
Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises.
J Strength Cond Res. 2012 Aug;26(8):2293-307
Authors: Wilson JM, Marin PJ, Rhea MR, Wilson SM, Loenneke JP, Anderson JC

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