These are the most common types of contraction. The muscle actually shortens in length as it develops tension. There are two ways in which this happens. – Isotonic action – the muscle shortens or contracts freely. E.g., the biceps contract when curling a weight. – Isometric action – the muscle shortens or contracts only a little before it is stopped from contracting further by an immovable resistance. E.g., trying to lift a weight you cannot move. The origin of a muscle is where the muscle joins the stationary bone. The insertion is where it joins the moving bone. When a muscle contracts, the insertion moves towards the origin. Muscle and their main actions: 1. Deltoids (three muscles) – Raises your arm forward, backwards and sideways at the shoulder. 2. Biceps – Bends your arm at the elbow. 3. Abdominals (four muscles) – Pulls in the abdomen. Flexes the spine so you can bend forward. 4. Quadriceps (four muscles) – Straightens the leg at the knee. Keeps it straight when you stand. 5. Pectorals (two muscles) – Raises your arm at the shoulder. Draws it across your chest. 6. Latissimus dorsi (lats) – Pulls your arm down at the shoulder. Draws it behind your back. 7. Trapezius – Holds and rotates your shoulders. Moves your head back and sideways. 8. Triceps – Straightens your arm at the elbow joint. 9. Gluteals (three muscles) – Pulls your leg back at the hip. Raises it sideways at the hip. Gluteus maximus is the biggest of these muscles. 10. Hamstrings (three muscles) – Bends your leg at the knee. 11. Gastrocnemius – Straightens the ankle joint so you can stand on your tip toes. Muscles are made up of cells called muscle fibre. There are two different types of fibres: slow twitch and fast twitch. Slow-twitch fibres contract slowly, without much force, but do not tire easily and are suited for endurance activities. Fast-twitch fibres contract much faster and with much more force, but tire quickly. They are suited to activities that needs burst of strength and power, such as sprinting and weightlifting. TYPES OF MUSCLE ACTION The type of resistance the muscle meets determines the type of muscle action. There are two types of muscle action: 1. Concentric 2. Eccentric Successful sporting action depends on our muscles working together to produce skilful movement. The muscles used depend on the activity or the phases of activity. Muscles work by shortening or contracting. The muscles that move your bones when an activity is performed are the voluntary muscles. A voluntary muscle usually works across a joint. It is attached to both bones by tendons. The fibres of the tendons are embedded in the periosteum of the bone. This anchors the tendon and spreads the force of contraction. All muscles contract and develop tension. They work in pairs or groups because a muscle can only pull, it can’t push. For example, the biceps and triceps work together. To bend the arm the biceps contracts, the triceps relaxes. To straighten it, the triceps contracts, the bicep relaxes. This is called antagonistic muscle action. The other main pair of muscles that work together are the quadriceps and hamstrings. The muscle that contract or shorten is called the prime mover or agonist. The relaxing muscle is the antagonist. There are other muscles, called synergists, that contract at the same time as the agonist to help it work smoothly. The state of partial contraction of muscles is called muscle tone. Groups of fibres take turns to contract so muscles don’t get too tired. Even when you are standing still, muscles are partially contracted. Muscle tone is important in maintaining the posture of the body and keeps the body ready for action. There are three types of muscles in the body: voluntary, involuntary and cardiac. Voluntary muscles are attached to bones, they work when you want them to. Voluntary muscles are also called skeletal muscles or striped muscles. Involuntary muscles are found in the walls of the internal organs: stomach, gut, bladder and blood vessels. It works on its own. Contractions in the walls of the blood vessels helps to keep blood flowing. Cardiac muscle is a special involuntary muscle that forms the wall of the heart. It works without stopping, each contraction is a heartbeat. TYPES OF MUSCLES MUSCLE FIBRES ECCENTRIC MUSCLE ACTION CONCENTRIC MUSCLE ACTION ORIGIN AND INSERTION MUSCLE TONE This occurs when the muscle is trying to contract while it is actually being lengthened by stretching. E.g., the quadriceps muscles at the front of the thigh when going downhill.