Muscles pull bones: bend your elbow

A muscle can only pull, never push. Watch the biceps squeeze and swing the forearm up, and the triceps pull it back.

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Bones cannot move by themselves. Muscles move them. Here is your right arm: one long bone in the upper arm, two in the forearm, and lots of little ones in the wrist and hand. The big muscle on the front of the upper arm is the biceps, with a helper underneath called the brachialis. The one on the back is the triceps. Where the bones meet is the elbow, a hinge wrapped in a stretchy sleeve.

The elbow is a hinge joint between the humerus and the two forearm bones, the ulna (the hinge proper) and the radius (which also turns to flip the palm). The joint is wrapped in an articular capsule lined with synovial membrane. The skeletal muscles shown are the elbow flexors on the front (biceps brachii with its long and short heads, and the brachialis beneath it) and the extensor on the back (triceps brachii: long, lateral and medial heads). Skeletal muscle is about 40% of your body mass, some 600 muscles, and every one of them is wired to the nervous system: it contracts only when a motor neuron tells it to.

Squeeze: the biceps pulls

The biceps is tied to the shoulder at the top and to the forearm at the bottom. When your brain says "bend", the biceps squeezes: it gets shorter and fatter. Shorter means it pulls its two ends closer together. The shoulder end cannot move, so the forearm end has to: the whole forearm and hand swing up around the elbow. Here the muscle only fattens (a real one shortens too, and its lower end would follow the forearm).

A muscle contracts when its fibres shorten: inside each fibre, myosin heads walk along actin filaments and pull the sarcomeres shorter (the sliding filament model), burning ATP, after a signal from a motor neuron releases calcium inside the fibre. The biceps brachii runs from the scapula (long head from the supraglenoid tubercle, short head from the coracoid process) to the radial tuberosity, so shortening it flexes the elbow and, because it pulls on the radius, also supinates the forearm. The brachialis, from the front of the humerus to the ulna, is the stronger flexor of the two; brachioradialis helps. This model swings the forearm through 100 degrees; a real elbow flexes about 145.

Let go: the triceps pulls back

A muscle can only pull. It cannot push. So the biceps cannot straighten your arm: it can only let go. To straighten the arm, another muscle on the other side has to pull: the triceps, on the back of your upper arm. Watch it fatten as the arm goes straight. Muscles work in pairs like this all over your body, one on each side of a joint, taking turns.

Muscles come in antagonistic pairs: an agonist (prime mover) contracts while its antagonist on the other side of the joint relaxes. For the elbow the flexors are biceps and brachialis, the extensor is the triceps brachii, which runs from the scapula (long head, infraglenoid tubercle) and the back of the humerus (lateral and medial heads) to the olecranon, the tip of the ulna. The spinal cord coordinates the pair: the motor neurons of the working muscle also excite interneurons that inhibit the opposing muscle (reciprocal inhibition), so the two do not fight. Holding a heavy bag with the elbow still is both contracting at once (co-contraction), which stiffens the joint.

Tendons: where muscles hold on

A muscle does not grab the bone directly. At each end it turns into a tough rope called a tendon, and the tendon is glued to the bone. The biceps holds on to the shoulder blade at the top and to the radius at the bottom. The triceps holds on to the shoulder blade and the back of the upper arm bone, and pulls on the pointy tip of your elbow. Feel that tip: that is the ulna, and the triceps is tugging it right now.

Tendons are dense bundles of collagen fibres continuous with the connective tissue around the muscle; they spread the pull of thousands of fibres onto a small patch of bone. The end that stays still is the origin (usually nearer the trunk), the end that moves is the insertion. Biceps brachii: origins on the scapula, insertion on the radial tuberosity (plus the bicipital aponeurosis into the forearm fascia). Triceps brachii: origins on the scapula and the posterior humerus, insertion on the olecranon of the ulna. The distance from the joint to the insertion is the lever arm: the biceps inserts only a few centimetres from the elbow, so lifting a hand-held weight takes a pull several times heavier than the weight itself.