The human musculoskeletal system
The human musculoskeletal system consists of the skeleton (bones) and muscles. An adult skeleton has 206 bones. Bones give the body its shape, protect internal organs (the skull the brain, the rib cage the heart and lungs), give muscles something to pull on, make blood in the red marrow and store calcium and other minerals. Bones meet at joints: shoulder and hip are ball-and-socket joints, elbow and knee are hinge joints (bending in one plane). Ligaments hold bones together. Muscles attach to bones by tendons and can only contract, i.e. pull and never push; so they work in pairs (for example the biceps bends the elbow and the triceps straightens it). Bones are levers, joints are the fulcrums and muscles supply the force. The textbook calls the hand-forearm a “force lever”, but in the forearm the muscle attaches close to the elbow, so it must exert more force than the load and gains in speed and range of movement – that is why muscles tire quickly when holding a load. Muscles get the energy from food. Good posture, exercise, calcium-rich food and sunlight keep bones strong.
Arm model: join two cardboard strips (upper arm and forearm) with a paper fastener, tie a rubber band or string near the elbow on the forearm strip and fix it to the upper arm strip: when the string is shortened the forearm lifts. Safety: use blunt paper fasteners rather than sharp pins; cut cardboard with an adult’s permission.