Types of Muscles

The human body has three types of muscle: cardiac, smooth, and skeletal. Cardiac muscle is found in the heart; smooth muscle in the hollow organs of the body (i.e., blood vessels, intestinal tract); and skeletal muscle is voluntary and attached to bones.


The body contains over 450 voluntary skeletal muscles, comprising between 40 and 50 percent of the total body weight. Skeletal muscle performs 3 major functions: (1) motion, (2) postural support, and (3) heat production during cold stress.


Skeletal muscles are composed of muscle cells, nerve tissue, blood, and connective tissues. The fibrous connective tissue surrounding the muscle group is the epimysium. The bundles of muscle fibers called fasciculi are encapsulated by the perimysium connective tissue. Each individual muscle fiber is surrounded by the endomysium. This intramuscular network of connective tissues coalesces and becomes continuous with the dense connective tissue of the tendons at each end of the muscle. Tendons attach the muscles to the bone. One end of the muscle is attached to a bone that does not move (origin), while the opposite end is fixed to a bone (insertion) that is moved during muscular contraction.

To examine a muscle from the largest to the smallest features, one would first examine the whole muscle group, then the bundle of muscle fibers or fasciculus, next the muscle fiber or muscle cell, then the hundreds of threadlike protein filaments called myofibrils that are made up of contractile units called sarcomeres, and finally the sarcomeres containing the myofilaments, named actin (thin ones) and myosin (thick ones). These contractile proteins are found in a special arrangement to one another, each myosin is surrounded by 6 or more actins. The actin filaments are attached to the Z line. The Z lines are the boundaries for the sarcomeres.

Muscle Fiber Types

  1. Slow-twitch type I red fibers – Physiologically adapted for endurance activity with a high capacity to use oxygen (used during long distance running or cycling)
    • Slow contraction time
    • Low peak tension
    • Large blood supply
    • Small muscle fiber diameter
    • First to be recruited
    • Has a small motor neuron
  2. Fast twitch type II white fibers – Physiologically adapted for fast contractions with a low capacity to use oxygen (used for sprinting)>
    • High peak tension (strength)
    • Smaller blood supply
    • Large muscle fiber diameter
    • Poor endurance
    • Last to be recruited
    • Has a large motor neuron

Sliding Filament Theory

Muscle contraction is a complex process where chemical energy is converted to mechanical energy. According to the Sliding Filament Theory, the muscle shortens and develops tension as the result of actin sliding over the myosin filaments. Filament sliding occurs as the result of numerous cross bridges, extending ‘arms,’ from myosin and attaching on the actin filaments. These cross bridge connections are made, broken, and remade again many times before the muscle achieves its contracted length. The energy for contractions comes from the breakdown of adenosine triphosphate (ATP; a high energy compound).

Motor Units

Each skeletal muscle fiber is connected to a fiber coming from a nerve cell. These nerve cells are called motor neurons and extend outward from the spinal cord. Stimulation from motor neurons initiates the contractile process. The motor unit consists of the motor neuron and all the muscle fibers it innervates. The number of muscle fibers in a motor unit varies from about 5 to 2,000. A high muscle fiber to nerve ratio is associated with gross movements requiring considerable force, whereas a low fiber to nerve ratio exists where very precise low force is required of a muscle.

Muscle is made up of two kinds of fibers, fast twitch and slow twitch, which are unique in their functioning and structure. Each motor unit is composed of only one type of muscle fiber. Unlike slow twitch muscle fibers, fast twitch muscle fibers are innervated by larger motor neurons. Motor units are preferentially recruited during exercise following a ‘size principle.’ Smaller motor neurons associated with slow twitch fibers are recruited first. Larger motor neurons associated with fast twitch fibers are recruited last.

All‑or‑None Law

A stimulated muscle or nerve fiber contracts or propagates a nerve impulse completely or not at all. In other words, a minimal stimulus causes the individual muscle fiber to contract to the same extent as a stronger stimulus. While this law of physiology holds true for the individual muscle fibers and motor units, it does not apply to the muscle as a whole. Therefore, the muscle is able to exert forces of graded strength by varying the number of motor units contracting at any given time or the frequency in which they contract.

Types of Muscular Contractions

Isokinetic (iso-constant, kinetic-speed) muscular contraction is performed at a constant speed. Set resistance is not met, rather the speed of movement is controlled. Therefore, it is possible to exert a continual, maximal force at a constant speed. Special equipment (e.g., Cybex) contains a speed control apparatus, so speed remains constant no matter how much tension is produced by the muscle. For example, the arm stroke in swimming and manual resistance training are isokinetic type contractions.

Isometric (iso constant; metric length) or static contraction develops tension, but does not change the length of the muscle. The muscle does not shorten because the external resistance against which the muscle is pulling is greater than the maximal tension (internal force) the muscle can generate. Examples of isometric contractions are pushing against an immovable object, or postural muscles maintaining a static body position while standing or sitting.

Isotonic (iso constant, tonic tension) or dynamic contraction occurs when the muscle shortens with varying tension while lifting a constant load. The force developed by the muscle varies as the lever arms become shorter or longer. If the internal force generated by the muscle exceeds the external force and the muscle shortens, it is called a concentric contraction and considered positive work that is going against gravity, i.e., lifting a barbell. An eccentric contraction is when the muscle lengthens while producing tension. This controlled elongation of muscles back toward their original resting length is called negative work because it is assisted by gravity.

Roles of Muscles

A prime mover or agonist is a muscle that effectively causes a certain joint movement, i.e., quadriceps for leg extension. An antagonist is when a muscle’s concentric contraction causes a movement directly opposite of the movement caused by another muscle, i.e., a knee flexor. For example, the hamstring group is an antagonist to the quadriceps; the actions of the two muscle groups are opposed. Flexor muscles decrease joint angles, while extensor muscles increase joint angles. Fixator or stabilizer muscles will contract isometrically to prevent movement of a bone, i.e., during push ups the abdominals contract isometrically to maintain a stable trunk. Synergist muscles help with intended movements. The contractions assist the prime mover, as several muscles act together to accomplish the movement. Neutralizer muscles contract to prevent any undesirable movement. The concentric contraction of most muscles causes more than one movement. The neutralizing muscle contracts to allow only the desired movement. In general, the interaction of the muscles serves to move a specific bone and to stabilize another bone or joint.

Warm Up

Before beginning your daily weight training program, it is necessary to spend time warming up. It is not wise to rush into your workout without adequate preparation, as you will subject yourself to possible injury.

Two types of warm up programs are generally utilized: the general and the specific. The general warm up consists of large muscle activities such as jogging, riding a stationary cycle, rope jumping, and jumping jacks, etc. The general warm up should be approximately 5 to 15 minutes long. The specific warm up is necessary when training with heavy weights or prior to playing a sport such as racquetball or tennis. The specific warm up should always follow the general warm up in all exercise programs. The specific warm up, as it applies to weight training, is performing a light set prior to training with heavier weights.

Beneficial Effects of Warm‑Up before Exercise

  1. Greater oxygen supply to the muscles
  2. Increases the release of oxygen within the cells
  3. Increases cellular metabolic chemical reactions
  4. Decreases muscle viscosity, improving mechanical efficiency and power
  5. Increases speed of nervous impulses
  6. Increases blood flow to muscles
  7. Improves cardiorespiratory response to sudden and strenuous exercise
  8. Reduces injuries to muscles, tendons, ligaments, and connective tissues

Key warm‑up principles

  1. A slow, gradual warm up, consisting of calisthenics, stretching, and/or slow jogging, always should precede exercise.
  2. Warm up should last at least fifteen minutes.
  3. Only a few minutes should elapse between the completion of the warm-up and the activity.
  4. Warm up exercises should prepare the heart, lungs, and muscles to adequately meet the demands placed on them during rigorous exercise.
  5. The type of warm up should be specific to the type of activity that is to follow.
  6. Stretching increases flexibility and reduces the resistance of muscles.
  7. General body warm up exercises increase body temperature and gradually stimulate the heart.  In certain cases, electrocardiograms have shown abnormalities in individuals who did not warm up prior to rigorous exercise.


  1. Flexibility is defined as the range and the extent of the movement of a joint.
  2. The joint capsule contributes approximately 47 percent to the range of motion, the muscles contribute 41 percent, the tendons contribute 10 percent, and the skin contributes 2 percent.
  3. Emphasis on increasing or decreasing flexibility must rest with the skin and muscle tissue.
  4. Men have larger muscles than women, which tends to reduce their range of movement.
  5. Good flexibility will reduce the possibility of aches, pains, and inflammation associated with joints that are stressed through rigorous activity.

The importance of stretching

Stretching involves extending and holding muscles for a specific time to increase range of movement in a joint. Two kinds of stretching exercises are static and ballistic

  1. Static stretching consists of stretching the muscle slowly and gradually for a period of six to eight seconds followed by relaxing the muscle for several seconds. Static stretching is more beneficial because it does not impose unnecessary strain on the muscle or joint.
  2. Ballistic stretching involves rapid bouncing and jerking movements. Ballistic stretching may cause injury to soft muscle and joint tissue. It is not as effective as static stretching; therefore, it is generally not recommended.

Stretching Exercises

The following stretching exercises include all of the major muscle groups in the body.

Lower‑leg and heel stretch.  Stand approximately three feet away from a wall and place feet several inches apart. Place outstretched hands on the wall, keep feet flat, and lean gradually toward the wall.

Back stretch.  Lying on your back, bring both knees to your chest. Grasp both thighs and pull the knees toward your chest.

Groin stretch.  Sit on the floor with the soles of your feet touching in front of you. Gradually and gently push down on your knees as far as comfortable using your hands.

Quadriceps stretch.  Lying on your side, flex your right knee and grab your right ankle with your right hand. Gradually move the hips forward until a good stretch is felt on the thigh.

Hamstring stretch.  Sit on a table with one leg extended across the table and the opposite leg hanging over the side of the table. Bend forward at the waist and reach toward the toes of your extended leg.

Lower‑back stretch.  Sitting on the floor with your legs extended out in front of you, force your knees flat against the floor. Grab behind your knees and slowly pull your head down toward your knees.

Cool down

Just like the warm-up before your workout, the cool down should follow the workout. A cool down period is essential after your training program to allow the body to recover from exercise. As your training program becomes more strenuous, it will become a necessity to allow for a cool down. During the cool-down, you will use some of the same exercises performed during the general warm up but at a much more casual pace, like walking, easy jogging, calisthenics, stretching, or stationary cycling. The cool down period should last approximately 5 to 15 minutes.

Guidelines for Cool Down

  1. Your heart rate and breathing will return close to normal (less than 100 beats per minute).
  2. Usually, five minutes is enough time.
  3. Ending your workout with flexibility exercises is more effective because warm muscles allow for a greater range of motion.
  4. This period improves return of blood to the heart.

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