Muscle Dynamics: More Than Just Flexing


Muscles are made up of cells called muscle fibers due to their elongated shape, these cells help move your body. Myofilaments, the smaller structures found within muscle cells contain key proteins; which include actin, myosin and titan. When the body contracts causing movement it is responding to bonds that have formed and broken between these proteins.

While joints are the fulcrums in your body and bones are the levers, muscles and tendons are the cables. Muscles operate like a telescoping drawbridge, with thousands of fibers lengthening and shortening in response to nerve impulses sent by the brain. Each muscle has a specified number of motor units, a collection of fibers stimulated by one nerve. The calf muscle, for example, has approximately 600 motor units that receive messages from the brain; each unit in turn controls about 1800 fibers. The muscles of the eye, consider a precision unit, has nerves that control only 3 to 5 fibers, providing for a much more refined movement.

Muscles are encased in tight membrane-like structures known as fascia and are divided into different types of fibers that are recruited for different intensities of activity. While fibers vary greatly there are 2 primary categories, slow-twitch and fast-twitch. Slow-twitch, known as oxidative fibers, are designed for low-intensity endurance activities such as walking, biking and even long distance running. Slow-twitch fibers fatigue slowly and act slowly, but they generate abundant amounts of ATP, as they function using aerobic metabolism. Fast-twitch fibers, also known as glycolytic fibers, are explosive and are utilized for short bursts of high-intensity activities, like sprinting, weightlifting and jumping. Although these fibers fatigue more rapidly they allow for powerful muscular contractions. Fast-twitch muscle fibers utilize anaerobic metabolism for energy production.

Different muscles groups have varying percentages of the 2 fiber types; slow-twitch and fast-twitch. Some muscles are designed for endurance, like the muscles in the back that are responsible for holding the trunk in an upright posture. Other muscles, like the biceps brachii muscle of the upper arm, are built for quick explosive activities.

Muscles are categorized by functionality. Flexor muscles decrease the angle between 2 adjoining limbs. When contracting the bicep the angle at the elbow joint between the forearm and the humerus or upper arm decreases. The hamstrings are flexors of the knee; they close the angle between the lower leg and the thigh.

Extensors are muscles that increase the angle between 2 bones. The triceps muscle extends the elbow joint and the quadriceps extend the knee joint. Other muscles are biarticular and perform different functions at 2 different joints, such as the hamstring and gastrocnemius or calf muscle. Biarticular muscles are generally more prone to injury, especially when movement involves the muscle stretching at one end while simultaneously contracting on the other.

Muscles perform additional functions as well, as in pronation and supination. These are rotational motions that allow the forearm to turn the hand palms-up or palms-down and allow your ankle and feet to roll in and out. Adductor muscles move the extremities, like the arms and legs, toward the centerline of the body. Conversely, abductors move extremity limbs away from the centerline. Muscles can also cause the rotation of a joint, such as at the shoulder and hip.

Bursae is another form of tissue that plays a key role in movement. Bursae are liquid-filled sacs located in or near the joints of the body. They ensure soft tissue, usually tendons, glide easily where they pass over bones. However, when a tendon becomes inflamed as a result of injury or overuse, or when the bursa itself is subjected to stress or a blow, the sac swells with liquid and, rather than easing motion, it begins to obstruct it. This leads to an increase in inflammation as the site of injury or trauma becomes more pronounced and the spaces in the joint become tighter and more painful. Bursitis is commonplace in the foot; it manifests as bunions and near the Achilles tendon behind the ankle.

Located within both muscles and joints are tiny sensory organs called proprioceptors which act as a conduit delivering information between the muscle and the brain. The information provides the brain information about movement awareness and is often referred to as kinesthetic awareness. Kinesthetic awareness enhances coordination and agility. There are 3 important types of proprioceptors; muscle spindles, which provide information to the brain about the length of the muscle and the rate at which that length is changed; the Golgi tendon, located at the junction between the muscle and the tendon which signals muscles to relax or give up when the degree of resistance is too great; and joint receptors, which tell the brain about the joint’s position and the amount of stress it is bearing.

After an injury proprioception diminishes. Decreased proprioception increases the likelihood of a reoccurrence and injury overall. Ankle sprains are a good example of this, even sprains in which the soft tissue is completely resolved, a reoccurrence can occur. The proprioceptors is 1 mechanism of the body’s warning system for impending injury. A foot and ankle brace can help improve proprioception after an ankle injury by cueing the brain to heed sensory signals from the area.

Proprioception can, to some degree, be learned and unlearned. Athletes who were once highly coordinated lose their kinesthetic awareness after an injury, but can relearn it by performing proprioceptive drills. One should remain both cautious and aware of re-injury during the rehabilitation process.

Fatigue causes a decline in proprioceptive ability and injury awareness. Exercising, sports training or playing through fatigue can increase the incidence of injury greatly. Age is also another natural cause of a decrease in proprioception and has been linked to falls among the elderly. Daily exercise may prevent age-related decreases in proprioception, but ultimately with age muscle recovery and injury awareness will decline.

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