CSEC PHYSICAL EDUCATION LECTURE SERIES
Every movement that takes place in the body depends on muscles. They work by contracting or shortening. The cardiac muscles in the heart contract to pump blood out of the heart, involuntary muscles in the artery walls contract to squirt the blood along, and voluntary muscles, which will be the main focus, work when needed.
A voluntary skeletal muscle contains nerves that carry messages to and from the brain. Therefore, a muscle contracts when messages from the brain race along the nerve fibres, telling them to contract. It relaxes when the messages tell the fibres to lengthen again.
Movement is produced because muscles usually work across joints. They are attached to bones by strong cords called tendons. One end (origin) is usually attached to a fixed bone and the other end (insertion) is attached to the movable bone.
For example, the bicep muscles' origin is attached to the scapula, which is the fixed bone, moves across the elbow joint and inserts on the radius, which is the movable bone. On contraction, the insertion moves toward the origin. In other words, the radius will be pulled upwards.
Muscles work in pairs or groups. They have to work in pairs because muscles can only pull; they can't push. For example, the biceps and triceps work together to execute the arm curl movement; to flex the arm, the biceps contract, the triceps relax. To straighten it, the triceps contract and the biceps relax.
Working in unison
Large numbers of pairs of muscles are needed to work together in the different ways for even simple body movements. The muscles take on different roles, depending on the movement that is performed. They can work as:
n Flexors - contracting to bend a joint;
n Extensors - contracting to straighten a joint;
n Prime movers or agonists - contracting to start a movement (biceps muscles perform this role in an arm curl);
n Antagonists - relaxing to allow the movement to place (the triceps muscles perform this role in arm curl);
N.B.: the biceps and triceps will swap places as the prime mover and antagonist when arm is straightened.
n Fixators - contracting to steady parts of the body to give the working muscles a firm base (the deltoid performs this role in the arm curl);
n Synergists - reducing unnecessary movement when a prime mover contracts. They can also fine tune the movement (the brachialis in the fore arm perform this role in the arm curl).
When a prime mover contracts, the antagonist muscle will keep some fibres contracting to exert a 'braking' influence to stop the prime mover moving the joint so hard that the antagonists are damaged. Sometimes, this system fails. For example, when a sprinter is running flat out, he may tear the hamstring and quickly come to a painful stop.
The muscles we use depend on the activity, whether it requires muscles in the upper body or lower body, to work together for short periods, or both at different phases of the activity, or most of the muscles of the body vigorously for longer periods (such as wrestling).
All muscles contract and develop tension, however, the type of resistance the muscles meet will determine the type of muscle action.
There are three main types of muscle contractions:
n Isotonic concentric - the muscles shorten as they contract and the ends of the muscles move closer together, eg., the biceps when doing pull-ups. Most sporting movements are of this type.
n Isotonic eccentric - the muscles lengthen as they contract under tension, the ends of the muscles move further apart, e.g. the biceps work in this way when the body is lowered from a pull-up position.
n Isometric contraction - the muscles stay the same length as they contract. There is no movement so the ends of the muscle stay the same distance apart. E.g. Shoulder muscles work in this way during the tug-of-war activity and the stabilising muscles that hold parts of the body steady as other parts move in many sporting movements.
When we perform sporting activities, we move our limbs in many different directions to affect the type of movements needed to execute various skills. Special words are used to describe the movements:
n Extension - the limbs straighten at the joint, e.g., reaching to catch a ball (netball).
n Flexion - the limbs bend at the joint e.g., bending the trail leg at the knee when clearing hurdles.
n Abduction - the limbs move away from the mid-line of the body.
n Adduction - the limbs move towards the mid-line of the body.
n Rotation - circular movement. Part of the body turns while the rest remain still. E.g., rotation of the hip to play a shot in golf.
n Circumduction - the end of a bone moves in a circle, e.g., bowling in cricket
n Inversion - a lifting of the medial border of the arch combines with a medial bending of the front of the foot.
n Eversion - a slight rising of the lateral border of the foot combines with a slight lateral bending of the front of the foot.
n Pronation - rotation of the forearm so that the palm turns medially.
n Supination - rotation of the forearm so that the palm turns laterally.
Next week's lesson: Principles of movement to enhance performance.