The muscles that move your arms and legs are skeletal muscles. They run from a few centimetres to tens of centimetres, large enough to see.
A sheet called the epimysium wraps the whole thing. Both ends become tendon and attach to bone, so when the muscle shortens the bone is pulled and the joint moves.
The fine lines drawn inside the belly are fascicles, the next level down. From here we go inward through six nested levels.
Inside the muscle, the tissue is divided into bundles called fascicles, 0.1 to 1 mm across.
The figure is a fascicle in cross section. The ring around the outside is the perimysium, which separates one bundle from the next.
Each cell packed inside is one muscle fibre seen end on. A single fascicle holds tens to hundreds of them.
Open a fascicle and you find muscle fibres, 10 to 100 µm across and sometimes long enough to run the whole length of the muscle.
This is one cell. A muscle fibre is also called a muscle cell.
It carries many nuclei, and they sit at the rim rather than in the middle. That is because a muscle fibre forms by the fusion of separate cells, and because the middle is already full of the next level down.
Running lengthwise through the fibre are the myofibrils, 1 to 2 µm across, packed in with no space to spare.
This is the contractile machinery itself. A fibre shortens because the myofibrils inside it shorten together.
Four of them are drawn side by side. Every one has its bands in the same places, and those places line up across the whole bundle. Because they line up, the microscope shows not one striped thread but a striped tissue.
One period of that stripe is a sarcomere, about 2 µm long.
The thick vertical lines at each end are Z lines, and one sarcomere runs from a Z line to the next. It is the smallest unit of contraction and the only part that shortens.
A single myofibril holds tens of thousands of them in series. Each one shortening by only 0.4 µm adds up, across tens of thousands, to centimetres of movement in the whole muscle.
Inside the sarcomere are two kinds of filament, 15 nm and 7 nm across.
The thick filament is a bundle of myosin molecules. Each molecule has two heads and a tail, and when the tails are bundled together only the heads stick out.
The thin filament is actin wound as a double strand, with a strand of tropomyosin lying in the groove and troponin attached to it in triplets.
These two sliding past each other is what contraction really is. Six levels down, all that is left is a protruding head gripping its partner and pulling.