First aid for bones, and muscles

Humans have three kinds of muscle tissue: skeletal muscles – striated, contract quickly and strongly, under voluntary control; cardiac muscle – also striated, not under voluntary control; smooth muscles – in the walls of the intestine, vessels and bladder, contract slowly and are involuntary. Skeletal muscles are wrapped in fascia, attach to bones by tendons and are supplied with vessels and nerves; there are over 600 muscles. Muscles work in pairs: when the flexor (e.g. the biceps) contracts and the extensor (triceps) relaxes, the elbow bends; the reverse straightens it. Contraction uses energy, which comes from breakdown of nutrients. In long work, performance falls temporarily – this is fatigue; it is restored by rest. Nerve impulses reach muscles: receptors send impulses to the spinal cord, and motor nerves carry them to the muscle; voluntary movement is controlled by the cerebral cortex.