1. Describe movement and maintenance of posture in terms of antagonistic muscle action. Gross and microscopic structure of skeletal muscle. The ultrastructure of a myofibril. The sliding-filament theory of muscle contraction. Muscles as effectors. The structure, location and general properties of slow and fast skeletal muscle fibres.

2. Antagonistic muscle action Muscles exert a pulling force when contracted but cannot push so an antagonistic (opposite) pair of muscles is needed e.g. biceps and triceps Flexors- muscles that bend a limb Extensors- muscles that straighten a limb Isometric contraction- antagonistic muscles in the leg have to both contract a little to exert the right amount of tension to remain upright

3. Skeletal muscle Muscle attached to the bones of the body Also known as voluntary muscle because the person has control over when these muscles are contracted and therefore when they bring about movement Because of its appearance it is also know as striated or striped muscle

4. Structure of skeletal muscle Muscle fibre is surrounded by plasma membrane called the sarcolemma Cytoplasm in muscle fibre is called sarcoplasm and contains many mitochondria It is not made up of individual cells but is more like several cells joined together, so it has many nuclei Myofibrils cause the muscle to appear striped

5. The structure of a myofibril Muscle fibres are made up of myofibrils They contain two different types of filaments: Thick filaments, myosin Thin filaments, actin

6. Sliding-filament theory When muscles contract the myofibrils get shorter. This happens because the filaments slide over each other

7. Slow and fast muscle fibres Slow-twitch fibres- used during aerobic exercise such as long distance running. They appear red because they contain a lot of myoglobin which is a pigment that stores oxygen Fast-twitch fibres- used for intensive, short length exercise such as sprinting. Are whiter because they contain much less myoglobin. They produce their ATP from anaerobic respiration

8. Questions 1. What are skeletal muscles also know as? 2. What are the two different filaments myofibril contain? 3. Which of these is thick and which is thin? 4. Draw a quick diagram of the structure of a myofibril 5. Name two differences between slow-twitch and fast- twitch muscles 6. Why do we need antagonistic pairs of muscles?