Anterior and posterior view of superficial muscles © Cengage Learning Australia 2011

The Muscular System Functions of Muscle: Allows movement Maintains posture Permits essential body functions (e.g. heart beat, breathing, digestion) Types of Muscle: Skeletal (striped, striated) – allows voluntary movement Smooth – involuntary e.g. intestines, blood vessels Cardiac – controls the heart muscle (myogenic) http://www.youtube.com/watch?v=raGl8bLkaAw&fe ature=related

How muscles work Movement is made possible when muscles contract and pull on the bones to which they are attached. Muscles work in pairs - when one muscle contracts, the other relaxes. This is called reciprocal inhibition E.g. quadriceps and hamstrings work in pairs Muscle creating movement (contracting) is the agonist (prime mover) Muscle that relaxes is the antagonist E.g. during a bicep curl: bicep is the agonist, tricep is the antagonist © Cengage Learning Australia 2011

STRUCTURE OF SKELETAL MUSCLES A tendon is the connective tissue that usually attaches muscle to bone. The tendon crosses a joint and attaches itself to 1 or more bones.

The origin of a muscle = the point at which it attaches to a fixed bone (muscles do not attach to other muscles). The structure that the origin is attached to does not tend to be moved by the contraction of the muscle. The site of the origin tends to be more proximal towards the head and have greater mass than what the other end attaches to. The opposite end of the muscle is called the insertion. The point of insertion tends to be more distal, and have less mass than the site of origin. It is the end that tends to move.

For example: The bicep curl The bicep originates at the scapula. It inserts at the radius. During the bicep curl, the bicep contracts and shortens. The tendon that inserts on the radius pulls on the bone. This causes the forearm to bend (flexion). At the same time, the tricep relaxes, allowing the forearm to flex. The bicep is the agonist and the tricep is the antagonist. http://www.youtube.com/watch?v=aXdkzwJITsc

Skeletal muscle is made up of many muscle fibres Muscle fibres are arranged in bundles and are made up of many myofibrils They are arranged parallel to each other & run the length of the muscle fibre Myofibrils are made up of a chain of sarcomeres, linking together like the carriages of a train

Structure of a Sarcomere

Z Lines – found at either end of the sarcomere Actin – the thin protein filament attached to the Z line Myosin – the thick protein filament attached to cross bridges (A band = the length of myosin) Cross bridges – tiny projections on myosin filament that reach towards the actin filaments H zone – the gap between the two actin filaments I band – the distance between the actin filaments between two sarcomeres

Sliding Filament Theory

Sliding Filament Theory Message reaches motor end plate – transmitted to the muscle fibre Stimulates the myosin crossbridges to attach and pull on the actin Once the crossbridge exerts its pulling action, it detaches then swings back to reattach further along the actin filament Makes actin slide into the centre of the sarcomere, shortening the myofibril Attaching and detaching occurs at different time so that tension is maintained in the muscle fibre Video: http://www.youtube.com/watch?feature=endscreen&v=PaN FWYO1lb4&NR=1

Types of Muscle Contraction There are three types of muscular contraction, classified by the movement they cause. These are listed below in order of occurrence in everyday activity, from most common to least common: • isotonic (concentric and eccentric) • isometric • isokinetic lsotonic contraction Occurs whenever the muscle length changes through a range of motion or action. When a constant load (weight) is being moved, differences exist in the amount of force applied at various joint angles. Concentric = muscle shortens when it contractions Eccentric = muscle lengthens when it contractions Isometric contraction Occur when tension is developed but no change results in the length of the muscle. Isometric contractions hence involve little, if any, change in muscle length while tension is developed. lsokinetic contraction Tension developed is maximal throughout the entire range of motion and is common with hydraulic fitness equipment. The amount of force applied by the machine always equals the amount of force applied by the muscle. © Cengage Learning Australia 2011

Nervous control of muscular contractions Motor neurons convey nerve impulses from the brain to muscles A motor neuron and the fibres it controls/stimulates are known as the motor unit Sensory neurons convey nerve impulses from muscles, organs and cells to the brain © Cengage Learning Australia 2011

The all or nothing principle states: It is not until an electrical threshold is surpassed that all of the fibres linked to a motor unit will fire together and maximally. Gross movements requiring major muscle involvement require more motor units than precise/ fine movements. Fibres will be recruited according to the activity demand and this is known as preferential recruitment. © Cengage Learning Australia 2011

Muscle fibre arrangement © Cengage Learning Australia 2011

Fast & slow twitch fibres Muscles are made up of two different types of fibres: • Red, type I, slow-twitch fibres (ST), best suited to aerobic, endurance work such as triathlons. • White, type II, fast-twitch fibres (FT), best suited to short-duration, high intensity anaerobic work, for example the bursts of power and speed required to sprint. © Cengage Learning Australia 2011