The development and maintenance of musculoskeletal strength are essential to athletic performance and the overall fitness of any individual. In general terms, fitness is achieved through the combination of strength, speed and lateral quickness, power, endurance (both cardiovascular and muscular), and flexibility, represented by the optimal range of motion in the joints. Different sports may demand greater or lesser proportions of each of these fitness components.
The fundamental principles of strength training are tied to both the role that strength plays in the overall fitness and performance capabilities of an athlete, as well as the connection to the biology regarding muscle growth. Muscle fibers contract in response to a nerve impulse directed to a muscle through the network of the central and peripheral nervous systems. Each muscle fiber contraction is the means by which the muscle produces movement; the rate of contraction is dependent on the muscle type and the activity to which the muscle energy is being directed.
Strength training is intended to develop three separate but interrelated types of muscle strength: maximum muscle strength, which represents the greatest amount of force that a person is capable of generating in a single muscle contraction; elastic strength, which is the ability in the muscle to contract quickly in response to a event; and strength endurance, which is the ability to repeat an action or to sustain a force through a greater number of repetitions.
The development of each of these three types of muscle strength is achieved through the principle of overload training. Overload is the taking of a muscle past its present capacity without damage to the structure. In overload training, the muscle fibers will sustain minor injury, which the body naturally repairs by directing the formation of myoblast cells, which thicken and strengthen the damaged fiber when the body is at rest. The repeated breaking down and building up of muscle fiber will lead to larger and more powerful muscle fibers over time.
Overload training can be achieved in several ways. The first method is the increase in the number of repetitions of any strength exercise. The second aspect of overload training is the increase in the amount of weight lifted or resistance otherwise applied to the muscles. The third overload tool is the increase in the intensity of the work being done by the targeted muscles, through a reduction in the recovery interval between the repetitions.
Strength training that employs the overload principle will tend to achieve a number of musculoskeletal and neuromuscular results. The first are those known as myogenic changes; these are structural changes to the dimension and composition of the overloaded muscle. These changes induce a state of hypertrophy, meaning the muscle becomes larger and denser. The second results are neurogenic changes, when the rate of response by the nervous system is increased due to repetition of each muscular effort. Overload training will also create capillarisation in the vicinity of the muscle, where the increased workload imposed upon the muscle forces the body to increase blood flow to the region through the formation of new capillaries, which results in a greater ability in the muscle to store both adenosine triphosphate (ATP), the energy fuel in each cell, and glycogen, the cellular storage form of glucose.
There are a number of training methods that will develop each of the three different types of muscular strength. Weight training, using both resistance machines and free weights, is the type of strength training that has the broadest application, as it will aid in the development of maximum strength, elastic strength, and muscle endurance. Elastic strength is also enhanced through a combination of plyometrics, Swiss ball routines that stress core body strength, and flexibility exercises. Muscle endurance can also be achieved through circuit training, where the athlete does series of exercises that stress different muscle groups in sequence, as well as running that emphasizes resistance, such as hill workouts or running with a harness or parachute.
Strength training must also be tailored to the sport for which it is intended to be applied. All sports require a measure of strength, to put the body into balance; the strength requirements and resultant strength training for an American football lineman, who is responsible for the generation of explosive forces to be directed against opposing players, will be entirely different than the strength training of a marathon runner or a tennis player. Sports where maximum muscle strength is of primary importance include American football, rugby, wrestling, and ice hockey. Elastic strength is essential in baseball, volleyball, and cricket. Sports in which all three of the strength aspects will be relied on are those that require the athlete to master a number of differing techniques, such as the triathlon and cross-country skiing, when the entire body is engaged in the propulsion of the athlete.