5 Things You Need to Know About Muscle Mass

STACK Expert Steve Green explains muscle anatomy so athletes can understand how to build muscle mass.

Increasing muscle mass is the goal of many athletes' off-season workout plans because it can directly correlate to increased performance on the field or court. But do you actually know what muscles are and how they grow? Having a basic understanding of muscle mass will guide your training, creating more effective workouts.

What is Muscle?

What is Muscle?

Muscle is what keeps the body moving and upright. The human body has three types of muscle—cardiac, smooth and skeletal. Cardiac muscle, as you can guess, is heart muscle, responsible for the contractions of the heart that push blood through the body (think cardiovascular exercise). Smooth muscle lines the walls of the intestines, blood vessels and even muscles located in the eyes. Skeletal muscle, the muscle most athletes are concerned about, is attached to bones and skin. Skeletal muscles, which are attached to bones by tendons, help us maintain our posture.

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When muscles are contracted, the tendons transmit force to the bones, facilitating locomotion or movement. Although the tendons transmit force during contractions, the muscles behave in different ways. Muscle contraction places tension on muscle fibers. Contraction has two variables: length and tension. If tension increases but muscle length stays the same (e.g., by holding a heavy dumbbell), it is called an isometric contraction. If muscle length changes (e.g., curling a heavy dumbbell), it is also referred to as an isometric contraction.

Isotonic contractions are have two subsets: concentric, or shortening of the muscle, and eccentric, or lengthening of the muscle. In our dumbbell example, curling the dumbbell up is a concentric action, while lowering it back down is eccentric. Skeletal muscle, the most common type, is often referred to as a "voluntary" muscle, since we control when and how to use it.

How Muscle Grows Physiologically

How Muscle Grows Physiologically

Broadly speaking, muscle growth, or hypertrophy, is an increase in skeletal muscle size. Muscle hypertrophy has two components, sacroplasmic hypertrophy and myofibril hypertrophy. Sarcoplasmic hypertrophy occurs when muscle fibers increase glycogen, water, collagen and mitochondria. These are non-contractile elements, but they increase the size of the fiber.

Myofibril hypertrophy refers to an increase in the number of contractile elements within a muscle fiber. Myofibrils are the most basic unit of muscle, and they contain the proteins actin and myosin. During muscle contraction, actin and myosin slide past each other, causing myosin to release the ADP that was attached to it, powering muscle contraction.

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Both forms of hypertrophy occur when an external stimulus is applied to the muscles. The stimulus for athletes is generally strength training. When you lift weights, you place tension on your muscles. This tension, often referred to as muscle contraction, depletes glycogen stores and damages myofibrils. In the recovery phase, your body recognizes that more glycogen and myofibrils are needed, and responds accordingly. Thus, muscles grow larger to accommodate the increased needs of the body.

Types of Muscle Fibers

Types of Muscle Fibers

When muscle fibers break down and are repaired, the muscle grows larger. The body has several types of muscle fiber, which perform different duties: Type I, Type IIA and Type IIX (also referred to as Type IIB).

Type I fibers are referred to as slow twitch fibers. Their twitch speed and force are small, so they don't generate much power. However, Type I fibers are highly resistant to fatigue. A long-distance runner activates mostly Type I fibers when competing.

Type IIX fibers are fast twitch fibers. Their twitch speed is fast and their force is large. But although they can generate substantial amounts of power, they fatigue quickly. Sprinters use their Type IIX fibers when they compete.

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Between these two fiber types are Type IIA fibers. They have fast twitch speed, generate moderate force and are highly resistant to fatigue. Middle-distance runners use Type IIA fibers to get them across the finish line.

The volume of muscle fibers varies from person to person. Genetics plays a large role in determining the mix of muscle fibers in an individual. Highly trained athletes tend to display higher amounts of one type of of fiber. Elite endurance athletes have more Type I fibers. The fastest sprinters have more Type IIX fibers.

How Can You Train Muscle Fibers?

How Can You Train Muscle Fibers?

Think of training as muscle recruitment. Type I fibers are used in everyday activities and longer endurance events. Type I fibers are the first ones recruited when stimuli are placed on the body. If you are performing a low-intensity exercise, your body will realize that additional fibers are not needed and will let your Type I fibers handle all the work. When you increase the intensity of your workout with greater speed, heavier weight or both, your body realizes it needs help and recruits Type IIA and Type IIX fibers. In more practical terms, Type I fibers are recruited in light endurance workouts, such as steady state cardio. Type IIX fibers, the last to be recruited, are called into action when you life heavy weight or apply lots of force.

Type IIX fibers are the largest of the fiber types, and comprise the muscles that grow the largest. Lifting heavy weight spurs these fibers into action. When they recover, they increase in size, providing the mass many athletes want. This serves to visually explain why distance runners are often leaner than wrestlers, weightlifters and other athletes requiring more power.

Role of Nutrients

Role of Nutrients

Workouts provide the physical growth, but proper nutrition is another key aspect of building muscle. Workouts serve to tear down and build up muscle. Muscle repair is done with the nutrients we take in through our diet. The body runs on the macronutrients fat, protein and carbohydrate. All three are important, but to rebuild muscle, the body needs protein. A rule of thumb is that the body needs 1.5 grams of protein per pound of body weight.

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Athletes looking to gain muscle mass need to consume more calories than they expend. Rigorous workouts take a lot out of your body, and it's important to eat enough calories to properly recoup the losses. Anything less can stunt muscle growth. Ideally, your calorie intake should break down as 20-30 percent from protein, 40-60 percent from carbohydrates and 20-30 percent from fats; but the percentages may vary, depending on your individual goals.

Building muscle is relatively simple in theory, but difficult to implement in practice. Athletes who want to increase muscle mass should research and experiment with different weight and diet programs until they find one that fits their needs. As athletes begin to focus more on muscle growth, the gains will come, serving asinspiration to push themselves harder.

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