1.1 Explain the interrelationship between the skeletal and muscular systems and movement

1. Structure and function of both systems

1.1 The skeletal system: structure and function for movement

The skeletal system includes bones, cartilage, joints and ligaments. Together, these structures support the body and make movement possible.

The skeleton is often organised into two main regions:

• The axial skeleton forms the central axis of the body (skull, vertebral column, ribs and sternum). It mainly provides support and protection.
• The appendicular skeleton includes the limbs and the shoulder and pelvic girdles. It is most directly involved in movement, because its long bones and synovial joints allow large ranges of motion.

Bones vary in shape, and the shape links to function:

• Long bones (for example, the femur) act as strong levers for movement.
• Flat bones (for example, the scapula or parts of the pelvis) provide broad surfaces for muscle attachment and protection.
• Short bones (for example, carpals and tarsals) help with stability and fine control.
• Irregular bones (for example, vertebrae) often combine protection with controlled movement.

Bone structure also supports movement and load-bearing. Most bones have a dense outer layer of compact bone for strength. Many also contain spongy bone inside, which is lighter and helps absorb forces. Many bones contain bone marrow (important for blood cell formation and energy storage).

The skeletal system has several functions. The key ones linked to movement are:

• Support: gives shape and posture, and provides a stable structure for muscles to pull against.
• Protection: shields organs that are essential for movement and survival (for example, skull and brain, ribs and lungs).
• Movement: forms lever systems through bones and joints that muscles can move.
• Mineral storage: stores calcium and phosphorus, which are important for normal muscle contraction and nerve function.
• Blood cell formation: produces blood cells in bone marrow, supporting oxygen transport and immune function.

Example: The femur is designed for load-bearing and force transfer. In high-impact activities such as landing from a jump, it can tolerate very large forces relative to body weight. This is why it is thick, long, and strongly supported by surrounding muscles.

1.2 The muscular system: structure and function for movement

The body has three main types of muscle: skeletal, cardiac, and smooth. Only skeletal muscle is directly responsible for voluntary body movement because it attaches to bones and can be consciously controlled.

Skeletal muscles are organised to pull on bones across joints:

• Muscles attach to bones via tendons (strong connective tissue that transmits force).
• Most skeletal muscles cross at least one joint.
• When a skeletal muscle produces force, it pulls the bone at the insertion towards the origin.
  • The origin is usually on the more stable bone (often closer to the trunk).
  • The insertion is usually on the bone that moves more during the action.

Skeletal muscles support movement in two main ways:

1. Producing movement by generating force that changes joint angle (for example, bending the elbow).
2. Controlling and stabilising movement by holding joints steady or slowing movement (for example, controlling the descent in a squat).

Example: During a bicep curl, the biceps brachii attaches to the scapula and the radius. When it produces force, it pulls the forearm upwards, creating elbow flexion.

1.3 How they work together to produce movement

The skeletal and muscular systems depend on each other. Movement only happens when all of these occur together:

1. A joint allows movement: bones meet at a joint, and the joint structure determines what movements are possible.
2. Bones act as levers: the moving bone behaves like a lever that rotates around the joint.
3. Muscles apply force via tendons: muscle force is transferred to bone through tendons, creating rotation at the joint.

This helps explain two key ideas:

• The same muscle can create different outcomes depending on joint position and the bones involved.
• Joint design matters. A joint that only allows one plane of movement cannot produce multi-directional movement, no matter how strong the muscles are.

Example: The knee is mainly a hinge joint, so it mainly performs flexion and extension. Even though the thigh has large, powerful muscles, the knee does not normally allow rotation in the way the hip does because its structure limits that movement.

2. Major bones and synovial joints

2.1 Major bones involved in movement

“Major bones” are the key bones that form the main lever systems and joint surfaces used in movement, especially in the limbs and the trunk.

The main bones commonly referenced in movement analysis include:

• Head and trunk
  • Cranium and mandible
  • Vertebrae (cervical, thoracic, lumbar)
  • Sternum and ribs
• Shoulder girdle and upper limb
  • Clavicle and scapula
  • Humerus
  • Radius and ulna
  • Carpals, metacarpals, phalanges (hand)
• Pelvic girdle and lower limb
  • Pelvis
  • Femur
  • Patella
  • Tibia and fibula
  • Tarsals, metatarsals, phalanges (foot)

These bones meet at joints, forming the lever system that muscles act upon.

Example: In sprinting, the pelvis, femur, tibia and foot bones form long levers. This helps convert muscle force into large, fast movements, especially at the hip, knee and ankle.

2.2 Synovial joints

A joint (articulation) is where two or more bones meet. Some joints are immovable (for example, skull sutures) or slightly movable (for example, between vertebrae). The joints that allow the greatest range of movement, especially in the limbs, are synovial joints.

A synovial joint is designed for frequent, low-friction movement. Its key features work together:

• Articular cartilage covers the ends of bones where they meet. It reduces friction and helps absorb forces.
• Synovial membrane lines the joint capsule and produces synovial fluid.
• Synovial fluid lubricates the joint, reduces friction, and helps nourish cartilage.
• A joint capsule surrounds the joint, holding structures together and helping maintain joint integrity.
• Ligaments connect bone to bone and limit excessive movement, improving stability.
• Tendons connect muscle to bone and transfer muscle force across the joint to create movement.

Synovial joints can be both mobile and stable, but there is usually a trade-off. Highly mobile joints often need more stabilisation from surrounding soft tissue, especially muscles.

Example: The shoulder is very mobile, which helps with throwing and overhead actions, but it relies heavily on surrounding muscles to stabilise the joint during fast movement.

2.3 Types of synovial joints and the movements they allow

Synovial joints are classified by the shape of the joint surfaces and the movements they permit:

• Hinge joints: movement mainly in one plane, producing flexion and extension. e.g: Knee and elbow.
• Ball-and-socket joints: movement in multiple planes, including rotation. e.g: Hip and shoulder.
• Pivot joints: rotation around a longitudinal axis. e.g: The atlas and axis in the neck during head turning.
• Condyloid (ellipsoid) joints: movement in two planes, typically flexion and extension plus abduction and adduction, with limited rotation. e.g: The wrist joint.
• Saddle joints: movement in two planes with a greater range than condyloid joints, allowing strong functional movement without full axial rotation. e.g: The thumb joint at the base of the thumb.
• Plane (gliding) joints: small sliding movements between flat surfaces. e.g: Small joints between carpals in the wrist and tarsals in the foot.

The type of synovial joint sets the boundaries for what joint actions can occur. Muscles can only produce movements that the joint structure allows.

3. Joint actions

NB: In the syllabus, the dot point specifically mentions “flexion and extension” under “examples”. It does not mention any of the other common joint actions. You might want to clarify with your teacher if they want you to learn these other common joint actions.

3.1 Flexion and extension

Joint actions describe the direction of movement at a joint. They are usually described from the anatomical position (standing upright, arms by the sides, palms facing forward).

Flexion and extension are key joint actions because they occur at many major synovial joints:

• Flexion decreases the angle between two bones (bending).
• Extension increases the angle between two bones (straightening).

Example: In a bicep curl, the upward phase is elbow flexion and the lowering phase returns towards elbow extension.

Flexion and extension are also central for lower-body movement. In a squat, the descent involves hip flexion and knee flexion. Standing up involves hip extension and knee extension. This is why squats are useful for analysing how multiple joints and muscle groups work together.

3.2 Other common joint actions

Beyond flexion and extension, there are other joint actions is used often in movement analysis.

Each joint action occurs because muscles cross a joint and produce force through tendons. The specific action depends on joint structure, joint position, and which muscles are active.

4. Major muscles

To explain movement, you need to link a joint action to the muscle(s) that produce it. Most major joints are controlled by muscle groups that work across the joint via tendons, creating movement by pulling on bones.

The following muscles are commonly used in movement analysis because they contribute strongly to major joint actions. These muscles rarely work alone. For accurate movement explanation, link the muscle action to the relevant joint and joint action, and then consider how other muscles assist or stabilise the movement.

4.1 Shoulder and upper limb

4.2 Trunk and posture

4.3 Hip and lower limb

5. Characteristics and functions of muscle fibres

A muscle fibre is a single muscle cell. Skeletal muscles contain a mixture of fibre types, but the proportion varies between muscles and between individuals. Fibre type helps explain differences in speed, force, and fatigue during movement.

5.1 Slow-twitch muscle fibres (Type I)

Slow-twitch muscle fibres are specialised for endurance and sustained activity.

Key characteristics

• Contract more slowly
• Produce lower peak force
• Are highly fatigue-resistant
• Have high oxygen supply and high myoglobin content (often described as “red” fibres)
• Have many mitochondria, supporting aerobic energy production

Function in movement

Slow-twitch muscle fibres are effective when a muscle must produce force continuously without tiring, especially for posture and steady movement.

Example: During a long-distance run, slow-twitch muscle fibres contribute heavily because they can repeatedly contract over long periods with reduced fatigue.

5.2 Fast-twitch muscle fibres (Type II)

Fast-twitch muscle fibres are specialised for powerful, rapid movement.

Key characteristics

• Contract quickly
• Produce high force
• Fatigue more rapidly
• Rely more on anaerobic energy pathways for rapid energy supply (though fibre subtypes vary)
• Have less myoglobin and fewer capillaries than slow-twitch muscle fibres (often described as “white” fibres)

Function in movement

Fast-twitch muscle fibres are used more when a movement needs high force in a short time, such as sprinting, jumping, or explosive lifting.

Example: In a 100 m sprint, fast-twitch muscle fibres are strongly recruited because the movement depends on rapid, powerful contractions.

5.3 Subtypes of fast-twitch muscle fibres

Fast-twitch muscle fibres include subtypes (often described as Type IIa and Type IIx), which differ in how quickly they fatigue. Type IIa fibres tend to have more endurance than the most explosive fast fibres, making them useful for repeated high-intensity efforts. Type IIx fibres are the most explosive subtype, producing very high force and power but fatiguing quickly, so they are most useful for short, maximal efforts such as sprint starts or jumps.

5.4 Performance implications

Fibre type influences the type of performance you are naturally suited to, but it is not fixed in a simple way. Genetics influences fibre proportions, and training can change fibre characteristics (for example, improving fatigue resistance and efficiency).

Example: A strong endurance training base can improve the fatigue resistance of fast-twitch muscle fibres used repeatedly in intermittent sports (such as football codes), supporting repeated high-intensity efforts across a match.

6. Types of muscle contractions

Muscles can produce force while shortening, lengthening, or staying the same length. The type of contraction helps explain whether a muscle is creating movement, controlling movement, or stabilising a joint.

6.1 Isotonic concentric

An isotonic concentric contraction occurs when a muscle shortens while producing force. It is common in the phase where the body or an object is lifted, accelerated, or pushed.

Example: In a bicep curl, biceps brachii performs an isotonic concentric contraction as the forearm lifts the weight (elbow flexion).

6.2 Isotonic eccentric

An isotonic eccentric contraction occurs when a muscle lengthens under tension while producing force. This usually happens when a muscle controls or brakes a movement against an external force (often gravity).

Isotonic eccentric contractions are important for deceleration, landing control, and injury prevention because they help the body absorb forces safely.

Example: When lowering the dumbbell in a bicep curl, biceps brachii lengthens under control, producing an isotonic eccentric contraction to slow the descent.

6.3 Isometric

Isometric contractions occur when a muscle produces force but does not change length, so there is no visible joint movement. Isometric contractions are important for stability and posture.

Example: In a plank, the trunk muscles contract isometrically to keep the body aligned without bending at the spine or hips.

Most sporting movements combine contraction types across phases.

Example: In a vertical jump, leg muscles often work eccentrically during the dip, concentrically during take-off, and then eccentrically again during landing, while other muscles stabilise isometrically.

7. Muscle relationship

7.1 Agonist and antagonist roles

Movement is controlled by muscle groups that work in coordinated roles:

• The agonist (prime mover) is the main muscle producing the joint action.
• The antagonist opposes the action. It usually lengthens to allow movement, and it can also help control and smooth the movement, especially when braking or decelerating.

These roles can switch depending on the direction of movement.

Example: In elbow flexion (lifting phase of a bicep curl), biceps brachii is the agonist and triceps brachii is the antagonist. When straightening the elbow (elbow extension), the roles switch.

This relationship helps explain how movement can be both powerful and controlled. If the antagonist is completely inactive, movement can become unstable or poorly coordinated.

7.2 Stabiliser role

A stabiliser (fixator) holds part of the body steady so the main movement can occur efficiently. Stabiliser muscles often work isometrically to create a stable base.

Stabilisers are important for joint protection and effective technique because many joints need stability for force to transfer safely.

Example: During a push-up, trunk muscles act as stabilisers to prevent the hips and spine from sagging, allowing the chest and arm muscles to generate force efficiently.