Jennifer Ellison Brown: Muscle contraction and types of movement
Every movement that takes place in the body depends on muscles. They work by contracting.
The cardiac muscles in the heart contracts 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, which 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.
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 and the triceps relax. To straighten it, the triceps contract and the biceps relax.
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:
- Flexors contracting to bend a joint
- Extensors contracting to straighten a joint
- Prime movers or agonist contracting to start a movement (biceps muscles perform this role in arm curl)
- Antagonist relaxing to allow the movement to take place (the triceps muscles perform this role in arm curl)
The biceps and triceps will swap places as the prime mover and antagonist when arm is straightened.
- Fixators contracting to steady parts of the body to give the working muscles a firm base. (the deltoid perform this role in the arm curl)
- Synergist reducing unnecessary movement when a prime mover contracts. They can also fine tune the movement (the brachialis in the forearm performs 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 antagonist 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 (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:
- Isotonic concentric the muscles shorten as they contract and the ends of the muscle moves closer together e.g.; the biceps when doing pull ups. Most sporting movements are of this type.
- Isotonic eccentric the muscles lengthen as they contract under tension, the ends of the muscles move further apart. e.g. the bicep works in this way when the body is lowered from pull-up position.
- 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. Shoulder muscles work in this way during the tug-o-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:
- Extension The limbs straighten at the joint, eg, reaching to catch a ball (netball)
- Flexion The limbs bend at the joint, eg, bending the trail leg at the knee when clearing hurdles.
- Abduction The limbs move away from the midline of the body.
- Adduction The limbs move towards the midline of the body
- 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.
- Circumduction- The end of a bone move in a circle, e.g., bowling in cricket
- Inversion A lifting of the medial border of the arch combines with a medial bending of the front of the foot.
- Eversion A slight rising of the lateral border of the foot combines with a slight lateral bending of the front of the foot.
- Pronation Rotation of the forearm so that the palm turns medially.
- Supination Rotation of the forearm so that the palm turns laterally.