Muscle is a soft tissue, and its cellscontain proteinfilaments of actinand myosinthat slide past one another, producing a contractionthat changes both the length and the shape of the cell. Skeletal musclecontractions are under the voluntary nervous system control, which needs synaptic contributioncoming from motor neuronsto initiate muscle contractions. Muscle contraction produces force and movement, such as changing posture, locomotion, as well as myocardial contraction and peristalsisin the digestive system. The majority of body’s energy consumption is utilized by muscular activity. All muscle cells store chemical energy in the form of ATP molecules that are used to power the movement of myosinheads. Muscles have a short-term store of energy in the form of ATP, converted later for motion. Glycogenis stored in the muscles for immediate use by converting it to glucose. Glycogen energy is required as an energy source, for continual skeletal muscle contractions, through aerobic and anaerobic glycolysis. Fat drops are also present within the cells, utilized for energy during aerobic exercise. The myocardium can use aerobic fat, and glucose produces maximum ATPs from any molecule involved. The myocardium consumes lactate produced during exercise. At rest, skeletal muscleconsumes 13.0 kcal kg−1 day−1. This is greater than fat at 4.5 kcal·kg−1 bone at 2.3 kcal·kg−1 [Heymsfield (Am J Phys Endocrinol Metab 282:E132–8, 2002)]. Muscle contraction has a number of theories. The acceptable approach is the filament sliding, and accordingly, the actin (thin) muscles’ fiber filament slides over the myosin (thick) filament during muscle contraction. Muscle contraction produces tension, and sometimes, there are no changes in muscle length such as during isometric contraction. When muscle is shortened, this is a concentric contraction, while, when muscle length extends, it is eccentric contraction. In muscles, there are two kinds of muscle fibers: slow twitch fibers that are mainly power-driven by the oxidationof fatsand carbohydrates; nevertheless, anaerobicchemical reactions are also utilized, mainly by fast twitch fibers. During muscle contraction, there are three main features affecting skeletal muscle force potential: velocity, length, and time, when the velocity of shortening increases the force generation is decreases. Resistance exercise is the mode that saves for the long-rum muscle mass and strength, while aerobic exercise increases metabolic efficiency and VO2. The anatomic structure and function of the heart in endurance-trained athletes, strength-trained athletes, and combined endurance- and strength-trained athletes confirm the hypothesis of the existence of an endurance-trained and strength-trained heart.