IT’S PROBABLY BEEN YEARS SINCE YOU SAT THROUGH AN ANATOMY or physiology class, and chances are you didn’t save your textbooks. But a basic understanding of how your musculature works, grows and thrives can make your time in the gym more intentional and productive. Here, then, is a comprehensive overview of the need-to-know facts about building muscle, preventing injury and living healthier.
There are three kinds of muscle tissue in the human body:
Skeletal muscle gets the lion’s share of attention and is your main focus when it comes to exercise. This tissue is attached to your bones and is responsible for maintaining your posture and moving your body around in space.
Cardiac muscle is controlled by the heart’s sinoatrial node and produces rhythmic contractions that cause your heart to pump.
Smooth muscle tissue is found in your skin, blood vessels, and organs and along the tracts of the urinary, respiratory and reproductive systems. It allows for functions such as digestion, peristalsis, and blood circulation.
Both cardiac and smooth muscles are involuntary, meaning you do not have conscious control over their actions. Skeletal muscle is voluntary, meaning you can contract and extend it at will to run, jump, sit upright, or type.
SKELETAL MUSCLE FIBERS: TYPE I VS. TYPE II
Skeletal muscles contain a mix of different fibers, and the exact ratio depends on the specific muscle in question as well as the individual person. There are two basic kinds of fibers — Type I and Type II — which function in different ways. Type I fibers are often called “slow-twitch” because they are slow to produce maximal tension and generate less force than Type II fibers. But what they lack in speed and power, they make up for in endurance. They are more resistant to fatigue than Type II fibers and are crucial when doing an endurance exercise such as distance running or similar sustained steady-state cardio.
Type II muscle fibers are also called “fast-twitch” because they contract quickly and produce bursts of power. These fibers are further categorized into Type IIa and Type IIb (also called Type IIx). Type IIb fibers produce the most force and fatigue the quickest. They are primarily engaged in actions such as a 50-yard dash or a single, max-effort barbell snatch. Type IIa fibers are a mix of Type I and Type IIb, and they are slightly less powerful than Type IIb but boast greater endurance. These are engaged in longer duration yet still quick activities such as a fast 200-meter run.
TRAINING FOR TYPE
Just as you can voluntarily contract your skeletal muscle, you also can control its shape and growth in order to achieve a certain look or level of performance. For instance, Type II fibers are larger than Type I, so if a bigger booty or badass biceps are among your fitness goals, program your workouts to target those fibers. Endurance-based athletes such as long-distance cyclists should ostensibly target their Type I muscle fibers, and power-based athletes such as CrossFitters who do endless box jumps and short sprints should focus on Type II.
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