Movement and locomotion: muscles, bones and joints

Learning objectives

By the end of this lesson, you should be able to:

• distinguish movement from locomotion
• explain how muscles, bones and joints work together
• describe antagonistic muscle action at the elbow and knee
• identify the main types of joints and their movements
• explain the roles of tendons, ligaments and cartilage

Movement and locomotion

Movement is a change in position of the whole organism or of part of its body. Locomotion is movement that carries an organism from one place to another. A person bending an arm shows movement, but not locomotion. Walking, swimming and flying are forms of locomotion.

Movement is important for obtaining food, escaping predators, finding mates, dispersing offspring and responding to environmental changes. In mammals, the skeleton provides support and acts as a system of levers, while skeletal muscles provide the pulling force.

How the musculoskeletal system works

A skeletal muscle is attached to bones by strong connective tissue called a tendon. When the muscle contracts, it becomes shorter and pulls on the bone. The bone moves around a joint, which acts as a pivot. Muscles can pull but cannot actively push a bone back to its starting position. For this reason, many movements are controlled by pairs of muscles that produce opposite actions.

A ligament connects one bone to another and helps stabilise a joint. Cartilage covers the ends of bones in many joints, reducing friction and helping distribute pressure. Synovial fluid lubricates the joint cavity.

Antagonistic muscles at the elbow

The biceps and triceps form an antagonistic pair at the elbow.

• Bending (flexion): the biceps contracts and the triceps relaxes. The biceps pulls the forearm towards the upper arm.
• Straightening (extension): the triceps contracts and the biceps relaxes. The triceps pulls the forearm away from the upper arm.

The muscles do not push against each other. One contracts to produce a movement while its antagonist relaxes; the reverse occurs for the opposite movement.

At the knee, the quadriceps group helps extend the leg, while the hamstrings help flex it. These muscle groups also act in an antagonistic manner.

Types of joints and movement

A joint is where bones meet. Synovial joints are freely movable and include several forms:

A typical synovial joint contains articular cartilage, a joint capsule, a synovial membrane and synovial fluid. Ligaments strengthen the joint. The shoulder allows a wide range of movement but is less stable than the hip. The hip has a deep socket that provides greater stability for bearing body weight.

Levers in the human body

A lever consists of a rigid bar, a pivot (fulcrum), an effort and a load. In the body, bones act as levers, joints act as fulcrums and muscles provide effort. The arrangement determines the speed, range and force of movement.

When the biceps bends the elbow, the elbow joint is the fulcrum, the biceps supplies the effort and the weight of the forearm or an object in the hand is the load. This is a third-class lever because the effort lies between the fulcrum and the load. Third-class levers favour speed and a wide range of movement rather than force multiplication.

WASSCE examination focus

Use precise terms: flexion decreases the angle at a joint; extension increases it. A tendon joins muscle to bone, while a ligament joins bone to bone. If asked to explain bending of the elbow, state which muscle contracts and which relaxes. Do not say that muscles push bones.

Quick check

1. Distinguish movement from locomotion.
2. What is the function of a tendon?
3. Which muscle contracts when the elbow is bent?
4. What type of joint is found at the shoulder?
5. Why do skeletal muscles commonly work in antagonistic pairs?

Answers: 1. Movement is a change in position of an organism or body part; locomotion moves the whole organism from place to place. 2. It attaches muscle to bone and transmits the pulling force. 3. The biceps. 4. Ball-and-socket. 5. Muscles pull but do not push, so an opposing muscle is needed to reverse a movement.