A skeletal muscle is organized in bundles (also called fascicles) which are groups of cells. A cell is often referred to as a skeletal muscle cell or a skeletal muscle fiber or a skeletal myocyte or a skeletal myofiber.
Those skeletal muscle fibers are in turn organized in myofibrils, and the myofibrils are made of the repeated units of thick and thin myofilaments arranged in sarcomeres.
So muscle to fascicle or bundle to cell (skeletal muscle fiber) to myofibril to saromere.
You can think about the skeletal muscles being organized from the organ level to the ultrastructural level as Russian dolls (or stacking or nesting or Russian tea dolls) are. Each time you open a doll, you see a smaller doll. Well, you would make a cross section in a skeletal muscle and you would see the many bundles. In those bundles you have the cells. In the cells you have the many myofibrils. Now the myofibrils are made of the repeated pattern of thick and thin myofilaments called sarcomeres but those sarcomeres are attached one of the other one at the Z line to form a unique myofibril.
Because the sarcomeres are attached to each other and that the excitation-contraction coupling leads to the sliding of the thin actin filaments by the thick myosin filaments at each sarcomere, a single muscle fiber always contract to its maximum extent once it is stimulated.
The skeletal muscle fibers are organized in motor units, which means a group of cells is stimulated by the same neuron. So one neuronal stimulation leads to the contraction or twitch of an entire group of cells controlled by this motor unit.
Depending on the load, the brain is able to recruit more motor units, hence the muscle can develop more tension.
You can see the tension developed by recording a myogram and depending on the frequency of nervous stimulation, you may be able to observe the increase in tension due to the recruitment of an increasing number of motor units.
