What Happens When We Stretch?

Sarcomeres are the basic functional units of a muscle cell that cause contraction.  A stretch is just the lengthening of a muscle’s sarcomeres.  Sarcomeres consist of microfilaments called actin and myosin which “contract” when myosin crawls across thin actin filaments, causing them to overlap.  They stretch by simply thinning out longitudinally as myosin slides back across actin. Once the sarcomeres have completely extended, the connective tissue is second to give.  Collagen fibers elongate and align in the direction of tension.  This realignment of muscle and connective fibers is what allows scarred muscle tissue rehabilitation and induces the feeling of relief.

Explanatory Video: http://www.youtube.com/watch?v=WVuW560nRII

However, before that feeling of relief, you have to get through the pain.  The pain is caused by the stretch reflex. The stretch reflex is the result of your body’s instinctual reaction to a stimulus from the proprioceptors in the nerve endings of your tendons and muscles. These proprioceptors of your musculoskeletal system relay a signal to the spine that the muscle is being stretched and immediately causes a contraction of the muscle called the stretch reflex.  There are two types of stretch receptors:  

“Muscle Spindles”

  • Muscle Spindles run parallel to normal muscle cells and consist of two forms; Nuclear Chain Fibers and Bag Fibers.  When either of these receptors is stimulated by a stretch, they fire a signal which causes a stretch reflex. The difference between the two fibers is the rate at which they fire that signal. 
  • Nuclear chain fibers are long and fire at a slowly increasing rate, which allows for the gradual muscle tightening during a stretch. This is called the static component of the stretch reflex. 
  • The dynamic component is caused by Bag fibers. They are shorter with a bulbous middle section that is excessively flexible and elongates rapidly upon stimulation. This central elongation fires a quick contraction that fades as the rest of the fiber compensates for the initial hyperextension of the middle. 
  • Practice and Flexibility work can decrease the stretch reflex and sensitivity of proprioceptors over time. 

The muscle spindles cause the muscle contraction during stretching, but the Golgi Tendon organ is responsible for the most interesting and ultimately enjoyable part of the stretch.

“Golgi Tendon Organs”

                The stretch receptors in your tendons, called the Golgi Tendon Organs, serve to protect the tendons from tearing as well as provide basic sensory input.  Once they sense a sudden increase in tension, they fire a signal until a threshold is reached, and the body ceases to contract; forcing the muscle to relax and avoid injury.  This fascinating safety mechanism is called the lengthening reaction.  This is why when you hold a stretch for a long period of time; it eventually causes the stretched muscle to relax. 


Read More:   http://web.mit.edu/tkd/stretch/stretching_2.html#SEC16

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