How Muscles Actually Contract

A muscle does not stretch to pull a bone toward it. It shortens. Every movement your body makes, from lifting a cup to standing upright, comes down to microscopic fibres inside muscle sliding past each other and pulling the whole muscle shorter.

Skeletal muscle is made of long, thin fibres bundled together, each one packed with smaller repeating units called sarcomeres. A sarcomere contains two types of filament, a thicker one and a thinner one, arranged so they can slide past each other. Movement happens when these filaments slide, not when the fibre itself stretches or coils. The signal that starts a contraction A muscle fibre contracts only when it receives a signal from a nerve. That signal triggers the release of calcium inside the fibre. Calcium is the switch: without it, the thick and thin filaments cannot grip each other, and without a nerve signal, calcium is never released in the first place. This is why damage to the nerve supplying a muscle can leave the muscle itself intact but unable to contract normally. How the filaments actually pull Once calcium is present, small projections on the thick filament attach to the thin filament and pull it inward, then release and reattach slightly further along, repeating this cycle rapidly. Each cycle pulls the thin filament a small distance further into the sarcomere. Multiplied across millions of sarcomeres lined up along the length of a fibre, and across thousands of fibres in a whole muscle, these tiny individual movements add up to a visible, forceful contraction. Why muscles need a constant energy supply Each cycle of attaching, pulling and releasing requires energy, supplied by a molecule the cell produces continuously from the food you eat and the oxygen you breathe. When a muscle works harder than its blood supply can keep up with, this energy system shifts to a faster but less efficient backup process. This is part of why intense effort cannot be sustained for long, and why heavily used muscles often feel a distinctive burning sensation. Why muscles only pull, never push A muscle can only actively pull, never push, which is why joints are almost always crossed by opposing pairs of muscles. One muscle bends a joint while an opposing muscle straightens it back out, each one relaxing while the other contracts. This arrangement is why damage to just one muscle in a pair can leave a joint able to move in one direction but not the other. What a doctor checks A doctor assessing muscle function is, in effect, checking whether the nerve signal, the muscle fibres themselves, or the energy supply to them is behind a problem. Strength testing. A doctor asks you to push or pull against resistance in specific directions, comparing both sides of the body to identify a pattern pointing toward a particular muscle, nerve or joint. Reflexes. Tapping a tendon triggers an automatic, brief muscle contraction, and an absent, reduced or exaggerated reflex can point toward where in the nerve to muscle pathway a problem lies. Blood tests. A blood test can measure a muscle enzyme that leaks into the blood when muscle fibres are damaged, helping distinguish a problem within the muscle itself from a problem with its nerve supply. When this system is affected When the connection between nerve and muscle, or the muscle fibres themselves, is disrupted by injury or illness, the result can be weakness, cramping, or pain that persists well beyond ordinary muscle fatigue. This is explored further in the lesson "Muscle Aches and Weakness".

A muscle produces movement by shortening, which happens when microscopic filaments inside its fibres slide past each other in response to a nerve signal and the presence of calcium.