International A Level Biology / Muscles, movement and the heart

How muscles, tendons, ligaments and the skeleton produce movement revision notes

Edexcel International A Level Biology (YBI11), chapter 21: Muscles, movement and the heart, specification point 7.9. Free lesson.

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Muscles pull on bones to create movement, while tendons connect muscles to bones and ligaments keep joints stable. Muscles work in opposing pairs, and synovial joints are adapted to move smoothly.

How the bones and muscles move the arm

Bones are rigid, so each acts as a lever. The joint is the pivot around which the lever moves. When a muscle contracts, it shortens and pulls on a bone. A tendon carries that pull from muscle to bone. The humerus is the upper arm bone. The radius is the forearm bone on the thumb side, and the ulna is the other forearm bone. The biceps is at the front of the upper arm. It pulls the forearm up to bend the elbow. The triceps is at the back of the upper arm. It pulls the other way to straighten the elbow. Muscles can only pull, not push. A different muscle is…

How tendons and ligaments differ

Tendons and ligaments are both tough connective tissues, but they join different structures and have different jobs. Their attachment points explain how they help movement and keep joints safe. Tendons join muscle to bone and pass the muscle's pull to the bone. They are inelastic, so they do not stretch and waste the pull. Ligaments join bone to bone across a joint. They hold the bones in place, keeping the joint stable, and are slightly elastic so the joint can move.

How an antagonistic pair moves a joint

An antagonistic pair is two muscles that work against each other. Since a muscle can only pull, one contracts while the other relaxes and is stretched. A flexor bends a joint, making its angle smaller. An extensor straightens it, making its angle bigger. At the elbow, the biceps is the flexor and the triceps is the extensor. To hold a joint still, both muscles contract at the same time and pull against each other. If only one contracts, it moves the joint.

How a synovial joint moves smoothly

The elbow and knee are synovial joints, which move freely. Their parts reduce friction and absorb shock, so the bone ends can move without wearing against each other. Cartilage covers the ends of the bones. It reduces friction and absorbs shock. Synovial fluid fills the space between the bone ends and lubricates the joint, helping the surfaces slide easily. The synovial membrane lines the joint and secretes synovial fluid. The capsule encloses the joint and holds the fluid in. Ligaments join the bones across the joint and keep it stable.

Applying the muscle-pair idea to the knee

The same rules apply to other joints. At the knee, the quadriceps and hamstrings form an antagonistic pair. Identify each muscle by the movement it causes, not by the names of the arm muscles. The quadriceps at the front of the thigh straightens the knee, so it is the extensor. The hamstrings at the back of the thigh bend the knee, so they are the flexor.