There are 3 types of muscle tissue in the human body. Cardiac muscle is found only in the heart. Smooth muscle is found in other internal organs, such as the intestinal tract, uterus, bladder, and blood vessels. Skeletal muscle is attached to the bones and allows you to breathe, to walk across a room, to pick up a piece of paper, or climb a mountain.

Skeletal muscles are composed of long, thin cells arranged in bundles, much like a bundle of drinking straws. These bundles are wrapped in a slippery sheath, called fascia, that allows them to slide against one another. Then these small bundles are arranged in larger bundles and wrapped again to form muscles.

The bones of the skeleton are held together with ligaments. Muscles are arranged across this framework to allow movement. The tendons at either end of skeletal muscles secure them to the bones. Muscles are described as originating on one bone before crossing a joint to insert on another bone. Muscles can be stretched and contracted rapidly, and repeatedly, without damage. While the body of a muscle is capable of considerable stretch, tendons and ligaments that are stretched more than 6% of their length may tear. Over-stretching ligaments and tendons may result in loose, unstable joints.

Muscles resist stretching. This resistance can help keep joints from bending the wrong way, but muscles that aren’t stretched may shorten and pull the skeleton out of alignment. If you spend a lot of time sitting the muscles connecting your thigh to your torso may shorten. When you stand up you may feel the shortened muscles pulling on your back.

If a part of the body is immobilized for an extended period, such as after breaking a bone or sitting in a chair all day, muscles may resist returning to normal activity. With continued inactivity the fascia can lose some of its slipperiness making it even harder to stretch.

Skeletal muscles work in sets – as the muscles on the front of your thigh relax and stretch, those on the back of your thigh contract to bend your knee. To straighten your leg, the muscles on the front of your thigh contract, while those on the back relax.

Quick, jerking movements sometimes result in muscle tears. Minor tears may cause soreness that lasts for a few days. More serious tears of muscles, tendons, or ligaments may require surgery and leave lasting disabilities. For this reason ballistic, or quick, bouncing, exercises are not recommended for stretching muscles. Instead, exercises using slow and steady movements, held for at least 30 seconds, are better for letting muscles relax and stretch.

For further information:
Types of muscle tissue
https://www.nlm.nih.gov/medlineplus/ency/imagepages/19841.htm

CURRENT CONCEPTS IN MUSCLE STRETCHING FOR EXERCISE AND REHABILITATION
Phil Page, PT, PhD, ATC, CSCS, FACSM
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3273886/

Stretching and Flexibility
http://web.mit.edu/tkd/stretch/stretching_3.html

The Fuzz Speech by Dr Gill Hedley

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