How Muscle Fiber Contraction Works

women doing bicep curls

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Muscle contraction occurs when a muscle fiber or group of fibers is signaled by the brain via nerves to activate and increase the tension within the muscle. It is also called muscle fiber activation. Your body has three different types of muscles and they contract in three different ways.

Muscle Fibers

Your muscles are made of bundles of muscle fibers that contain thousands of smaller structures called myofibrils, where the actual contraction occurs. There are two types of protein filaments within myofibrils—actin and myosin. Actin forms thin filaments. Myosin forms thick filaments and is a "molecular motor" that converts chemical energy into mechanical energy, which creates force and drives motion.

These two proteins, myosin, and actin interact with one another by sliding past each other, creating tension during muscle contraction. These molecular mechanisms are not fully understood. This theory is referred to as the "sliding filament model," and is used as the basis for understanding muscle contraction by biologists. 

The sliding filament theory explains that when a muscle is activated and movement occurs, these two interlocking filaments can grab onto each other and pull, which causes the myofibril to shorten. This shortening is called a muscle contraction.

There are three types of muscle cells in the human body:

  • Skeletal muscle: These muscles drive voluntary movement. They are the type that you are most likely to envision when you are discussing muscles, such as the muscles of your arms and legs.
  • Cardiac muscle: These are the cells of the heart that pump your blood. Their contractions result in your heartbeat. When you exercise, you make your heart beat faster and therefore the cardiac muscle must make more frequent contractions.
  • Smooth muscle: These cells are responsible for involuntary movements of the body, lining hollow organs such as the intestines, stomach, and bladder, plus the blood vessels and respiratory tract. They perform important functions in digestion, elimination, and blood circulation.

Three Types of Muscle Contraction

There are three ways a muscle fiber can be activated. Two allow for movement in the muscle and one simply creates tension, without joint movement. The three contraction types include:

  • Concentric muscle contraction (shortening): In weight training, this is the lifting phase of an exercise. For example, lifting the weight in the bicep curl or doing sit-ups. The muscle bulges as it is shortened in the contraction. Activities that result in concentric muscle contraction include sprints, running uphill, cycling, climbing stairs, and even getting up out of a chair. Concentric contraction involves joint movement and builds strength.
  • Eccentric muscle contraction (lengthening): In weight training, this is the phase in which the muscle returns to the starting position of the exercise. In the bicep curl, it is when you lower the weight to the starting position. In doing a sit-up, it's when you lower your torso back to the floor. You use eccentric contraction for walking down stairs, braking while running, and in Pilates exercises.
  • Isometric muscle contraction (static): Isometric exercises are those that involve no joint movement. An example is pushing against a wall. Isometric exercises are often used in rehab programs to help the muscle rebuild. Isometric exercises are helpful early in rehabilitation or physical therapy, especially when joint movement is painful.
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  • Kenney WL, Wilmore JH, Costill DL. Physiology of Sport and Exercise. Champaign, IL: Human Kinetics; 2015.

By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.