Skeletal muscle cells are very large, multinucleated and highly ordered. Within a skeletal muscle cell the contractile proteins, actin and myosin, are arranged in precisely aligned sarcomeres, giving the cell a striated appearance. A skeletal muscle cell can be referred to as a muscle fiber. Within the muscle fiber, the sarcomeres are organized longitudinally into myofibrils. The Z lines mark the ends of each sarcomere.
Skeletal muscle may be categorized as red or white. Red muscle contains large amounts of myoglobin and many capillaries, so red muscle is able to replace ATP at the slow, steady rate of utilization typical with this type of muscle. White muscle has few capillaries, no myoglobin, and little nutrient storage in the form of fat or glycogen. ATP hydrolysis occurs faster than the rate of replacement in white muscle, so white muscle is prone to fatigue due to lactic acid build up.
Generally, the ATP present in muscle can only power contraction for a very short time. Muscle cells contain a reservoir of high energy phosphate groups in the form of creatine phosphate, which possesses an even higher phosphoryl transfer potential than ATP.