Sarcomere
The basic contractile unit of muscle, where actin and myosin slide past each other.
The sarcomere is the smallest contracting unit of muscle: the repeating segment between two Z-discs, containing interleaved thick myosin filaments and thin actin filaments. Chains of sarcomeres end to end make a myofibril, bundles of myofibrils make a fibre, and the regular banding of the whole arrangement is what makes skeletal muscle look striped.
Contraction is the filaments sliding past each other rather than shortening. Myosin heads bind to actin, pivot, release and rebind in a repeating cycle powered by ATP, dragging the Z-discs closer together. Multiply that across billions of sarcomeres and the muscle shortens.
Calcium is the switch. At rest, regulatory proteins block the binding sites on actin; a nerve impulse releases calcium inside the fibre, the blockade moves aside, and the cycling begins. When calcium is pumped back, the muscle relaxes, and that pumping costs energy too, which is part of why rigor sets in without ATP.
The arrangement explains the length tension relationship. There is an overlap between the filaments that produces maximum force, with less force available when the muscle is very short or stretched very long, which is why you are strongest in the middle of most ranges and weakest at the ends.
Muscles grow in two directions. Adding sarcomeres in parallel makes a fibre thicker, which is conventional hypertrophy; adding them in series makes it longer, which appears to happen with training through long muscle lengths and changes the range at which the muscle is strong.
That is the mechanical argument for training through full range rather than convenient partials, and part of why work at long muscle lengths has attracted so much recent interest.
In practice this is background, and the useful version is short: force is produced by overlap, so range of motion and control are not aesthetic preferences.