Muscle Mass 101
The muscles of the body enable us to live life as we know it. They enable us to lift, carry, push, pull, walk, run, scratch, and hug. They can get stronger, weaker, bigger, smaller, tear, heal, tighten, and stretch. These many types of movements and features are incredible complex, and muscles are equally advanced in order to do them as accurately as we can. Learning the intricacies and physiology of the muscles is integral to knowing how to use them to our advantage through exercise and fitness.
Types of Muscle
There are 3 main types of muscles throughout the body: smooth, cardiac, and skeletal. Smooth muscle is what lines many of the internal organs, and cannot be contracted voluntarily. Cardiac muscle forms our heart, and also cannot be controlled. Despite the lack of control over it, this muscle can be strengthened through training. The primary muscles used during exercise are called skeletal muscle, because of their role in moving and controlling the bones, and are attached through connective tissue called tendons.
Skeletal muscle is composed of cells that form chains called myofibrils, which are protein filaments made up of molecules of actin, myosin, and titin. These filaments overlap, and can work to pull together and contract, shortening the total length, as well as to relax and expand. These filament contractions and relaxations need energy to occur, which comes from the breakdown of ATP molecules. Myofibrils come together to create full muscle fibers, or myocytes. These fibers also contain glycogen which helps provide more energy for exercise, along with myoglobin that stores needed oxygen. Muscle fibers are surrounded by fascia, and are bundled together repeatedly by more fascia, forming the complete muscle as we know it.
The collective action of these muscle fibers create contractions and relations across the entire muscle. Each fiber can only be switched on or off, so muscle action does not use every muscle fiber. Groups of muscle fibers contract, recruiting more to join as needed to use additional strength. While muscles can contract which leads to movement, they can also create isometric contractions, which do not cause any movement to the muscle.
Muscle Fiber Types
Despite their function to contract and relax, muscle fibers are not all equal. They can be divided into Type I and Type II muscle fibers, also known as slow twitch and fast twitch, respectively. Each complete muscle usually contains a mixture of both fiber types. The exact proportions are different depending on the muscle group, and can vary from person to person. The composition of Type I and Type II fibers in each muscle may be also affected by training and exercise.
Type I muscle fibers are referred to as slow twitch because their myofibrils contract slower. They are aerobic, which means that they use oxygen for their energy transfer process. This helps them work at lower intensities, and for longer durations without fatigue. Because of these features, they are ideal for their help in maintaining posture and joint stability.
Type II muscle fibers are called fast twitch because of their short bursts of strength and speed. Because of this ability to act quickly, they cannot use this power for long periods of time and fatigue sooner. They also access ATP faster, utilizing quick burst of energy. Because of this, Type II fibers are not always activated, unless the muscle is exerting a large amount of force. For these reasons, Type II fibers are responsible for a majority of the body’s movement, as well as contributing to muscle size and definition.
Further getting into the specialization of these fast twitch fibers, there are Type IIa and Type IIb fibers. Type IIb fibers are anaerobic, and can only store energy for short, quick, powerful bursts. Type IIa, however, get their energy both from aerobic processes, as well as through ATP, delivering both rapid force and longer lasting strength. Type IIa fibers are also structurally larger than others. When muscles contract, they first activate smaller Type I, and only recruit Type II when the first fatigue or are not strong enough. Also, their larger size also makes them play a major role in muscle growth.
Because of the unique composition and action of their fibers, muscles can adapt to change in size over time. They have the ability to grow or shrink, depending on the activity level of contractions and relaxations. Exercise and activity can spark more myofibril protein chains to be created, which increases the size of the muscle fibers. Because muscle fibers are composed primarily of protein molecules, they need adequate protein intake in order to form and grow. As a result, if not enough protein is consumed, muscle growth may be stopped. Furthermore, muscles can shrink in size and strength through a process known as atrophy. Atrophy can occur when there is not enough activity and stress applied to the muscle fibers through exercise. While this can also happen as a result of injury or disease, the most common cause is inactivity. Decrease from disuse is not permanent, and usually takes less time to rebuild size and strength than it did to gain the original level.
Exercise is a complicated process using the entire body, with muscle as a key player. Knowing about the composition and function of the body’s muscles are key to understanding