Muscle contraction begins with the brain setting off action potentials, which are waves in the electrical charges that extend along neurons.
The waves travel to a group of cells in a muscle, letting calcium ions out from the cells' sarcoplasmic reticula (SR), which are storage areas for calcium.
The released calcium lets myofibrils contract under the power of energy-carrying adenosine triphosphate (ATP) molecules.
Meanwhile, the calcium is quickly pumped back into the SR by fast calcium pumps.
Each muscle cell contracts fully; stronger contraction of the whole muscle requires more action potentials on more groups of cells in the muscle.
When the action potentials stop, the calcium stops flowing from the SR and the muscle relaxes.
The fast calcium pumps are powered by the sodium-potassium gradient.
The sodium-potassium gradient is maintained by the sodium-potassium pump and their associated ion channels.
A lack of potassium or sodium would prevent the sodium-potassium gradient from being strong enough to power the calcium pumps; the calcium ions would remain in the myofibrils, forcing the muscle to stay contracted and causing a cramp.
The cramp eventually eases as slow calcium pumps, powered by ATP instead of the sodium gradient, push the calcium back into storage.

Cramps can occur when muscles are unable to relax properly due to myosin proteins not fully detaching from actin filaments.
In skeletal muscle, ATP levels must be large enough to bind to the myosin heads for them to attach or detach from the actin and allow contraction or relaxation; the absence of enough levels of ATP means that the myosin heads remains attached to actin.
The muscle must be allowed to recover (resynthesize ATP), before the myosin proteins can detach and allow the muscle to relax.
Skeletal muscles work as antagonistic pairs.
Contracting one skeletal muscle requires the relaxation of the opposing muscle in the pair.
Muscle contraction begins with the brain setting off action potentials, which are waves in the electrical charges that extend along neurons. The waves travel to a group of cells in a muscle, letting calcium ions out from the cells' sarcoplasmic reticula (SR), which are storage areas for calcium. The released calcium lets myofibrils contract under the power of energy-carrying adenosine triphosphate (ATP) molecules. Meanwhile, the calcium is quickly pumped back into the SR by fast calcium pumps. Each muscle cell contracts fully; stronger contraction of the whole muscle requires more action potentials on more groups of cells in the muscle. When the action potentials stop, the calcium stops flowing from the SR and the muscle relaxes. The fast calcium pumps are powered by the sodium-potassium gradient. The sodium-potassium gradient is maintained by the sodium-potassium pump and their associated ion channels. A lack of potassium or sodium would prevent the sodium-potassium gradient from being strong enough to power the calcium pumps; the calcium ions would remain in the myofibrils, forcing the muscle to stay contracted and causing a cramp. The cramp eventually eases as slow calcium pumps, powered by ATP instead of the sodium gradient, push the calcium back into storage.
Cramps can occur when muscles are unable to relax properly due to myosin proteins not fully detaching from actin filaments. In skeletal muscle, ATP levels must be large enough to bind to the myosin heads for them to attach or detach from the actin and allow contraction or relaxation; the absence of enough levels of ATP means that the myosin heads remains attached to actin. The muscle must be allowed to recover (resynthesize ATP), before the myosin proteins can detach and allow the muscle to relax. Skeletal muscles work as antagonistic pairs. Contracting one skeletal muscle requires the relaxation of the opposing muscle in the pair.
Cramp