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Resistance Training

Resistance training is one of the pillars of exercise that is essential for overall health and fitness as well as for athletic performance. Typically, resistance training is referred to as weight training or “lifting.” Resistance training is much more than just biceps curls and shoulder presses with dumbbells. This entry briefly discusses the body's adaptations to resistance training, as well as the sources of resistance that the body may use.

Resistance Training Adaptations

Resistance training adaptations are both acute and chronic. Acute responses to resistance training occur primarily in the neurological, muscular, and endocrine systems. Chronic responses to resistance training are seen in the muscular, skeletal, endocrine, cardiovascular, and neurological systems. Anthropometric (body composition) adaptations are also seen as chronic adaptations to resistance training.

Acute and Chronic Neurological Adaptations

When a force is applied to a muscle, a signal is transmitted that activates the muscle cells. When performing resistance training, the number and intensity of signals that are transmitted are increased until the muscle fatigues. The two neurological factors that govern muscle force are motor unit recruitment and rate coding. Motor unit recruitment is simply the size of the muscle force created by the muscle contraction for a given task. For example, fewer motor units in the biceps brachii muscle are recruited when performing a biceps curl with a 10-pound (lb; 1 lb = 0.45 kilogram [kg]) dumbbell than with a 50-pound dumbbell. According to the kinesiologist R. N. Enoka, motor unit recruitment is based on the size principle, which states that the motor units that recruit slow-twitch fibers recruit fewer fibers than the motor units that recruit fast-twitch fibers. Rate coding governs motor unit firing. During resistance training, the muscles fatigue with each repetition of a given movement pattern, and as a result, the rate coding becomes impaired and the firing sequence becomes less and less precise.

Chronic adaptations would result in a more efficient sequence of recruitment of motor units, making the muscle less apt to fatigue from neuromuscular factors. Other chronic adaptations to the neurological system include increased motor unit firing and decreased co-contraction of the antagonist muscles. Co-contraction takes place when both agonist and antagonist muscles fire at the same time. The decrease in the co-contraction of antagonist movement when the agonist muscles are being called on for work allows for greater movement efficiency.

Acute and Chronic Muscular Adaptations

One of the acute effects of muscle during resistance training is the depletion of metabolic substrates, such as creatine phosphate and glycogen. Due to the depletion of these two fuel sources during resistance training, muscle power production decreases. Another significant acute muscle adaptation during resistance training is the intramuscular elevation of hydrogen. This results in a “burning” sensation in the muscles on multiple repetitions. The elevation of hydrogen ions in the muscle results in decreased intramuscular pH. Chronic adaptations from resistance training include increased cross-sectional size of the muscle fibers, also known as muscle hypertrophy. Hypertrophy of muscle occurs in Type I and Type II muscle fibers; however, Type II muscle fibers have a greater response. Manipulation of volume and intensity of resistance training will cause more or less hypertrophy to these respective muscle fiber types. The chronic adaptation of increased cross-sectional size of the muscle fibers results in an increase of muscle strength and power. Another chronic adaptation to the muscles, which has been proven in animals but not yet in humans, is a phenomenon called hyperplasia. This is when the number of muscle fibers increases. The resulting hypertrophy and possible hyperplasia of muscle fibers cause a relative increase in protein synthesis. This is essential for the repair of muscle fibers in acute response to resistance training.

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