Muscles generate heat and force for movement, help us breathe, and keep our bodies upright. Skeletal muscle tissue is composed of two fibers, actin(thin fibers)and myosin (thick fibers). These two fibers give the muscle a striated appearance. In order for muscle to contract it must first be stimulated by nerves called motor neurons. A single motor neuron and the muscle fibers stimulated by it is called a motor unit. The recruiting motor units play a large part in the force of the muscle during contraction. The more motor units (muscle fibers) recruited, the stronger the force of contraction.

Muscle fibers are classified as Type I, Type IIa and Type IIb fibers. "Fast" and "slow" twitch are also two other classifications for muscle fibers. Type I fibers (slow twitch) fibers are more resistant to fatigue than Type IIa or IIb fibers and have a high capacity for aerobic metabolism, fatigue faster and are mainly anaerobic.

Slow twitch fibers are mainly for endurance while fast twitch are for speed and performance. A muscle will generally have an equal amount of both fast and slow twitch muscle fibers. In regards to hypertrophy (muscle growth), fast twitch fibers grow faster and larger than slow twitch. Within the fast twitch muscle fibers, type IIa fibers are considered intermediate between fast and slow twitch fibers in relation to speed and contraction. For example, Type IIa fibers can become more glycolytic or aerobic depending on the type of training an athlete performs. If an endurance runner were to stop running and start weight lifting, then his or her Type IIa fibers would become more glycolytic in order to handle the stress of the activity.

Muscle growth and endurance is an adaptation to stress. For example, a sprinter will develop large quadriceps and hamstrings in order to adapt to the stress, while an endurance runner will develop more endurance to efficiently handle the stress. Type I muscle fibers respond to stress by becoming more efficient and stronger with slight hypertrophy, rather than the extreme hypertrophy seen with Type IIa and IIb muscle fibers. This is the premise behind trainers recommending 6 reps for pure strength/muscle gain and why 10-15 reps are recommended to "tone" a muscle.

Finally, there are four different actions a muscle can perform; isometric, eccentric, concentric, and isotonic. An example of an isometric contraction would be pushing against a wall. Lifting a dumbbell during a bicep curl is considered the concentric portion while lowering of the weight is called the eccentric portion of the exercise. There are also called the positive and negative portions respectively. And finally, isotonic contractions are those that involve full body actions such as skating or running.