YOUR HEALTH - MEDICINE 101

MUSCLES
One of the most amazing things about the human body is the incredible variety of movement that we are capable of. Think about it for a second - we can skate an olympic gold programme, run the Boston marathon, pull a fire engine, climb Mount Everest - and all of these achievements are due to our remarkable complement of muscles.

The world muscle comes from the Latin MUS - meaning mouse. I guess the early anatomists thought that the bulk of the muscle, the belly, resembled the body of a mouse, and the tendon resembled a tail.

Though they are capable of spectacular feats, a muscle's life is, well boring - it only does one of two things - it contracts (or shortens) or it relaxes (or lengthens). Muscles can shorten by as much as half, and they do so in a very interesting way. Microscopically, muscles consist of sheets of two different proteins - myosin and actin - arranged in fine layers, one on top of the other. When your brain sends down a signal to contract, small projections on the myosin sheet change their shape, causing the lower sheet of actin to slide over it. Here's what a real muscles looks like - this one is from the front leg of a pig. See the thick belly of the muscle here. It's composed of thousands of these little sheets of protein all running in the same direction. Muscles usually pass over a joint as is seen here and contraction of the muscle causes the joint to flex. Muscle tissue is bright red because it contains lots of oxygen - containing protein.
Muscles are attached to bone by structures called tendons. They're tough, white gristle-like bands that fix the contracting part of the muscle to the bone. Tendons are very strong - the fibres are plaited - and a tendon one inch across can hold 18000 lbs, but tendons don't contract at all - only the muscle belly does that. Tendons sometimes have to travel over quite a long distance to attach to the bone - like tendons on the back of your hand, coming all the way from the back of the forearm.
We are allowed such variety of movement simply because muscles and their tendons are wrapped around joints in every conceivable way. Sometimes, in spots where a tendon might rub or fray, a structure called a bursa is found. A bursa is simply a small, fluid filled sac, and it functions to reduce friction. Inflmation in a bursa is called bursitis.
All of the wonderful motions that are ours as humans are a result of this simple arrangement of sliding sheets of muscle protein.