Muscular tissues are formed of elongated cells called muscle fibers. They use ATP, a chemical compound that is produced through cellular respiration (which we will likely cover at a later date in detail), to generate force. Muscular tissue produces movements, maintains posture, generates heat and provides protection. We have three different types of muscular tissue in our bodies, skeletal muscle, cardiac muscle, and smooth muscle.

Smooth muscle is an involuntary tissue, meaning you don’t voluntarily contract it. It’s found in the iris of your eyes where it dictates the size of your pupil. It’s also found in the walls of blood vessels, in the airways to your lungs, in your stomach, intestines, gallbladder, urinary bladder and the uterus. Smooth muscle constricts blood vessels and airways, and propels food through the gastrointestinal (GI) tract. It contracts the urinary bladder and gallbladder. These are things you do not need to think about, but your body continually does.

Cardiac muscle is an involuntary tissue that is only found in the heart wall. It functions to contract and relax, to pump blood to all parts of the body. You also don’t have to consciously think to make your heart beat – but it happens!

Skeletal muscle is a voluntary muscle, because you can choose to contract or relax it. It’s more often than not attached to bones in your body and is the main producer of your body’s gross movements. It’s skeletal muscle you are aiming to make significant changes to during strength training. Skeletal muscle allows you to lift more, and is what you see bulging under an athletes skin. As previously mentioned, skeletal muscle weighs more than fat tissue. Your biceps, hamstrings, abs (short for abdominal muscles) and pecs (short for pectoralis muscles) are all somewhat familiar examples of skeletal muscles.

When I first considered what muscle tissue actually was and what my muscles actually were, I realized that when looking at a thick cut steak or slice of roast beef, the red tissue is the skeletal muscle while the white marbling it, is adipose (or fat).

Skeletal Muscle Fiber Types

Ever wonder why a chicken breast is white while it’s leg is dark? The job of the muscle in a chicken leg is to hold sustained contractions to keep the chicken standing erect, or to contract repeatedly all day long in order to assist the chicken in walking or running. The job of a muscle in the chicken breast is to use periodic short bursts of more powerful contractions to flap wings or to press its beak into food or other peck-able things, as necessary. These jobs are quite different from each other – sustained continuous contractions verses less frequent short, powerful contractions. The muscle fiber types differ to facilitate the different jobs.

Skeletal muscle is composed of three different fiber types, and because those who get to name science things are known for their creativity 3 different types are called: type 1, type 2a & type 2b fibers. Type 1 muscle fibers are considered slow twitch muscle fibers and are responsible for creating energy using mainly what’s called aerobic respiration. Type 2a muscle fibers are fast twitch muscle fibers, which create energy using a combination of both aerobic and anaerobic respiration. Type 2b muscle fibers are very fast twitch muscle fibers, which create energy using mainly anaerobic respiration. Respiration is simply the process of creating energy. Aerobic or anaerobic clarifies whether or not that process uses oxygen. We will discuss respiration, the processes in which our muscles create energy in detail later.

For now know that that mitochondria are present in greater quantities in type 1 muscle fibers than in type 2 muscles fibers. Mitochondria are like little powerhouses, meaning they create energy using glucose from the foods you eat and oxygen from the air you breathe. Large quantities of mitochondria using large quantities of oxygen for aerobic respiration are what cause type 1 muscle fibers to appear to be red (or dark) in color, like that of a chicken leg. This is just like the oxygenated blood in your arteries is redder than the non-oxygenated bluish-looking blood in your veins. Type 2 muscle fibers like that of chicken a breast have far less mitochondria and use less oxygen. This causes the muscle to appear to be light or white in color.

For a large part, your genetics will determine whether or not you have more type 1, 2a or 2b muscle fibers. This is why some people seem more naturally able to lift large weights, while others seem like natural born marathon runners. Luckily, because of the nature of type 2a muscle fibers, with the ability to create energy aerobically or anaerobically pretty much equally, and the way our body selectively strengthens the things and processes that we use, over those things we don’t use (think of the saying ‘use it or lose it!’), the types of exercises you do can help strengthen and make more efficient any of the processes your body uses to make energy. A natural born weightlifter can still run, while a natural born marathon runner can still lift large quantities of weight with enough time and practice. They just need to work a little harder at refining and strengthening different muscle fiber types in order to strength whichever process their body does not prefer.