What you'll learn

• How the mammalian skeleton supports, protects and helps move the body.
• How a synovial joint is adapted for smooth, low-friction movement.
• How tendons, ligaments, cartilage and muscles have different roles.
• How antagonistic muscles and lever systems produce movement at joints.

The big picture: movement needs a framework

Movement in mammals depends on the musculoskeletal system: the skeleton, muscles, tendons, ligaments, cartilage and joints working together.

A skeleton is a supporting framework of the body. Mammals have an endoskeleton, meaning the skeleton is inside the body and is made mainly of bone.

Bone is a living tissue. It is rigid because it contains mineral salts, especially calcium compounds, but it also contains collagen fibres that help resist fracture.

The skeleton has several important functions:

• Support — it gives the body shape and holds softer tissues in position.
• Protection — for example, the skull protects the brain and the rib cage protects the heart and lungs.
• Movement — bones act as levers, and muscles pull on them to move the body.
• Blood cell production — bone marrow produces blood cells.
• Mineral storage — bone stores minerals such as calcium.

Joints: where bones meet

A joint is a place where two or more bones meet. Some joints allow very little movement, such as those between skull bones. The joints most important for running and movement are synovial joints, such as the elbow, knee, hip and shoulder.

Synovial joints are adapted to allow movement while reducing damage from friction and impact.

The key parts of a synovial joint

Articular cartilage is smooth connective tissue covering the ends of bones. It reduces friction and helps absorb shock.

The synovial membrane lines the inside of the joint capsule and secretes synovial fluid, a slippery fluid that lubricates the joint.

The joint capsule surrounds the joint and helps hold the bones together.

A ligament is a tough band of connective tissue that joins bone to bone. Ligaments stabilise the joint and help prevent dislocation.

A tendon is a tough band of connective tissue that joins muscle to bone. Tendons transmit the pulling force from a contracting muscle to the skeleton.

Muscles and tendons: turning contraction into movement

A skeletal muscle is a muscle attached to the skeleton that can contract to produce movement. Skeletal muscle is usually under voluntary control, meaning you can consciously control it, although many movements also involve reflexes and learned motor patterns.

When a muscle contracts, it shortens and generates tension. This pulling force is transmitted through a tendon to a bone.

Muscles attach to bones at two main points. The origin is the attachment point that usually remains more fixed. The insertion is the attachment point that usually moves when the muscle contracts.

For example, when the biceps contracts, it pulls on its tendon and moves the forearm at the elbow joint.

Antagonistic muscle pairs

Because muscles pull rather than push, movement in opposite directions requires opposing muscles.

At the elbow, the main antagonistic pair is the biceps and triceps.

• The biceps is a flexor because it causes flexion at the elbow.
• The triceps is an extensor because it causes extension at the elbow.

Flexion means bending a joint so the angle between bones decreases.
Extension means straightening a joint so the angle between bones increases.

Elbow flexion

During elbow flexion:

• The biceps contracts and shortens.
• The triceps relaxes.
• The biceps tendon pulls on the radius.
• The forearm moves upwards, decreasing the angle at the elbow.

Elbow extension

During elbow extension:

• The triceps contracts and shortens.
• The biceps relaxes.
• The triceps tendon pulls on the ulna.
• The forearm moves downwards, increasing the angle at the elbow.

Bones as levers

A lever is a rigid structure that rotates around a pivot. In the body:

• The bone acts as the lever.
• The joint acts as the pivot, also called the fulcrum.
• The muscle contraction provides the effort.
• The load is the object or body part being moved.

At the elbow, the forearm acts as a lever. The elbow joint is the pivot. The biceps provides the effort when lifting the forearm, and the load may be the weight of the forearm plus anything held in the hand.

Many body levers are useful because they give a large range of movement and high speed at the end of the limb. However, this often means the muscle must produce a large force.

Moments and lever calculations

The moment of a force is its turning effect around a pivot. It depends on the size of the force and the perpendicular distance from the pivot.

$$moment=F\times d$$

where $F$ is force in newtons, N, and $d$ is perpendicular distance from the pivot in metres, m. Moment is measured in newton metres, N m.

How to describe movement clearly

When you are explaining movement at a joint, aim for a linked sequence rather than isolated facts.

For example, for elbow flexion:

1. The biceps contracts and shortens.
2. The triceps relaxes.
3. The biceps tendon transmits force to the radius.
4. The radius and ulna move around the elbow joint.
5. The elbow joint bends, decreasing the angle between the upper arm and forearm.

Linking this to exercise

During exercise, muscles contract more frequently and with greater force. This increases demand for ATP, the immediate energy source used by cells. It also increases demand for oxygen and glucose, and increases production of carbon dioxide and heat.

That is why this topic links closely to later ideas in Run for your Life, including ventilation, circulation, respiration, thermoregulation and homeostasis.