Walking downstairs, your thigh muscles contract while getting longer
A contracting muscle need not shorten. Grip a heavy bag and your forearm works hard without moving at all; lower yourself down a staircase and your muscles lengthen while resisting gravity. That braking kind of contraction burns less energy than climbing, but it is also the kind that damages muscle most when overloaded.
Physiologists define contraction as switching on the tension-generating machinery inside muscle cells, whether or not the muscle changes length. That machinery is built from two filaments: thin ones made of two twisted strands of actin, and thick ones made mostly of the motor protein myosin, together forming myofibrils. Relaxation is the return to a low-tension state.
Skeletal muscle in vertebrates only fires when a motor neuron tells it to, and one neuron can trigger many fibres at once. The filaments then slide past one another, as described by the sliding filament theory, producing a twitch, summation or sustained tetanus depending on how rapidly signals arrive. Tension peaks at an intermediate stretch. Heart and smooth muscle work differently: their contractions start within the muscle cells themselves, though the autonomic nervous system can adjust them.
Contractions are classified by force and length. In an isometric one, tension rises while length holds steady, as in gripping. In an isotonic one, tension stays constant while length changes. If the muscle overcomes the load and shortens, as the biceps does during a curl, it is concentric; if the load wins and the fibres lengthen while still pulling, it is eccentric. In real movement, length and tension shift continuously.
Eccentric work is odd. The muscle absorbs work rather than doing it, yet still burns fuel, just less than a concentric effort of equal force, which is why climbing stairs costs more than descending them. Proteins such as titin and desmin are involved, but the mechanism is poorly understood. Eccentric contractions brake movements to protect joints and smooth motion, and baseball pitchers train to reduce that braking for extra power. Weightlifters call them negatives, and heavy eccentric loading causes more damage than concentric loading. Researchers are also studying high-load eccentric exercise for tendon problems such as Achilles tendinitis and jumper's knee, with reported benefits.
Source: Muscle contraction
