All muscles will get tired when overused either too intensively or for too long without rest.When muscles get tired they will cause pain and feel weak. After a while they will tighten and shorten, thus compounding the problem by restricting both movement and blood supply.
Every muscle in the body has a purpose. They all operate some movement or part of a movement.
What is a 'pulled' muscle?There are 5 ways to 'pull' a muscle or 'strain it'.
1. By overstretching a muscle or muscle group. For example, imagine trying to put a baby seat into the back of seat of your car. The baby chair is light enough but the action requires a lot of stretching of the muscles as we reach forward with the chair. Then the muscles must contract to lift us back out of the car. An awkward movement, simple and everyday. The muscles can first strain through overstretching, then strain again when they over-contract.
2. By sudden movements and falls. Take the example above and imagine that in the middle of placing the baby seat somenoe honks their horn at you, you suddenly jerk, hit your head on the car roof and shout an appropriate obscenity. Some hours later you are flat on your back in agony.
3. Falls and other trauma can obviously pull and even tear muscle fibres. Even a small amount of 'damage' will spread to the neighbouring areas compounding the problem.
4. Pre-existing problems can be re-activated when we injure ourselves. Sometimes even when the area of injury is far removed from our 'old' problem areas.
5. Posture is king. Whilst slouching in the sofa we are usually putting our spine, neck and ribcage into extreme positions which soon proves to have been a painful mistake. We eventually reach the bent over old person appearance, the couch-potato look.