The role of the muscular system

The muscular system works closely with the skeletal system to enable movement in the body. Together, these systems form the musculo-skeletal system, which combines muscles and bones to produce actions.

Muscles are specialised tissues that contract to generate force and movement. They attach to bones and pull on them to create motion at joints. Without muscles, the skeleton could not move on its own.

Types of muscles in the body

• Voluntary muscles - These are under conscious control and attach to the skeleton to help move the body.
• Involuntary muscles - These operate automatically without conscious effort, controlling internal organs and processes like blood flow in vessels to supply oxygen to voluntary muscles.
• Cardiac muscle - This is a specialised type of involuntary muscle that forms the heart. It never fatigues, allowing the heart to pump blood continuously, delivering oxygen to voluntary muscles during exercise.

During physical activity, voluntary muscles require oxygen to function effectively. The heart's cardiac muscle contracts to pump oxygenated blood to these muscles, supporting movement and performance.

Functions of key voluntary muscles

Voluntary muscles each have specific functions, often related to movement at particular joints.

Main voluntary muscles and their actions

• Biceps - Causes flexion at the elbow, such as when lifting weights.
• Triceps - Causes extension at the elbow, such as during a basketball shot.
• Pectoralis major - Causes adduction and horizontal flexion at the shoulder, such as in a tennis forehand stroke.
• Hip flexors - Cause flexion of the leg at the hip, such as lifting the knee during running.
• Gluteus maximus - Causes extension of the leg at the hip, such as pushing the body forward in running.
• Hamstrings - Causes flexion at the knee, such as bringing the foot back before kicking a ball.
• Quadriceps - Causes extension at the knee, such as in a martial arts kick.
• Gastrocnemius - Causes plantar-flexion at the ankle, such as standing on toes in dance.
• Tibialis anterior - Causes dorsi-flexion at the ankle, such as during a snowboarding turn.
• Deltoid - Causes flexion, extension, abduction, or circumduction at the shoulder, such as in swimming strokes.
• Latissimus dorsi - Causes extension, adduction, or rotation at the shoulder, such as in butterfly swimming.
• External obliques - Cause rotation or flexion at the waist, such as preparing to throw a discus.

How antagonistic muscle pairs work

Muscles can only pull; they cannot push. To enable movement in two directions at a joint, muscles work in pairs called antagonistic pairs. In these pairs, one muscle contracts to pull while the other relaxes, and they switch roles for the opposite movement.

Tendons are strong, fibrous tissues that attach muscles to bones. At a joint, a muscle is connected to two bones via tendons, but only one bone typically moves during the action.

Key terms in antagonistic muscle action

• Agonist - The muscle that contracts to produce the movement.
• Antagonist - The muscle that relaxes to allow the movement.

Antagonistic pairs at major joints

Types of muscle fibres and their suitability for activities

All muscles consist of fibres, which are specialised cells that contract to produce force. There are two main types: slow twitch and fast twitch.

Slow twitch fibres

Type I (slow twitch) fibres contract slowly but can sustain activity for long periods without fatiguing. They are ideal for low-intensity, aerobic activities that require endurance, such as distance running.

Fast twitch fibres

Fast twitch fibres contract quickly and generate high force but fatigue faster than slow twitch fibres.

Types of fast twitch fibres:

• Type IIA - These are used in anaerobic work but can improve fatigue resistance through endurance training.
• Type IIX - These generate the greatest force but fatigue very quickly. They are perfect for short, intense bursts of anaerobic activity, such as a 60-meter dash.