Muscles move joints, to which they are joined, by shortening and pulling one end of the muscle closer to the other end. A muscle is made up of lots and lots of muscle fibres which are held together in bundles. The muscle fibres are activated by the brain sending electrical impulses to each muscle fibre via the nerves. These impulses cause the muscle fibre to contract and shorten so moving the joint over which the muscle works. If a muscle can not work, due to injury or illness, each little muscle fibre gets thinner and the whole muscle gets weak. Muscles are capable of both power and endurance. If one washes one's hair in the shower or blow it dry one needs endurance to hold arms up for a period of time. Loading heavy suitcases onto a roof rack or putting them away in the top cupboard requires power to be able to lift them up. Power is for doing movements against resistance.
Moving limbs, walking up stairs, getting up from a chair lifting a heavy shopping bag all calls for power. Muscle endurance allows the muscle to work at a lower level for long periods. The muscles that help us to maintain a certain posture need endurance. Having been in bed for a week with flu one would feel a bit weak at the knees when first up. This is partly because the Quadriceps have not been working and have become weak. Gradually asking the legs to stand and do more walking the muscles quickly regain some of their strength and endurance. Lets assume that the muscles that bend the index finger have 1000 muscle fibres. To beckon with the finger might ask say 10 fibres to work. Wanting to lift an empty bucket on the one finger might use 100 fibres. Whereas lifting a bucket that was full of water would ask most of the thousand fibres to work. By trying to lift a little more water each time would eventually train the muscle to lift a full bucket of water. When a muscle is damaged the muscle fibres are torn and the connective tissue around the muscle is also damaged
See Connective Tissue Manipulation.
When the fibres are damaged fluid seeps out of torn fibres and causes localised swelling. This works like glue when squeezed from a tube. The glue tries to repair the damage and in its enthusiasm sticks everything to everything else. The individual fibres of the muscle, which are normally free to glide slightly one on the other, become stuck together and are irritated by the pull of even the slightest contraction in the muscle. It is important that the ends of the muscle fibres stick together but not that they stick along their whole length. Sprains and strains are not helped by "working through them" or by exercising and 'keeping it moving'. It is advisable to seek the help of a Chartered Physiotherapist as soon as possible. In the meantime resting and keeping the leg up if its a lower limb problem will allow the body to do its own repair job much quicker than if you try to carry on as though nothing had happened.