The use of resistance exercise in rehabilitation and conditioning programs has a substantial impact on all systems of the body. Resistance training is equally important for patients with impaired muscle performance and individuals who wish to improve or maintain their level of fitness, enhance performance, or reduce the risk of injury. When body systems are exposed to a greater than usual but appropriate level of resistance in an exercise program, they initially react with a number of acute physiological responses and then later adapt. That is, body systems accommodate over time to the newly imposed physical demands.

Adaptations to overload create changes in muscle performance and, in part, determine the effectiveness of a resistance training program. The time course for these adaptations to occur varies from one individual to another and is dependent on a person’s health status and previous level of participation in a resistance exercise program.

Neural Adaptations. It is well accepted that in a resistance training program the initial, rapid gain in the tension-generating capacity of skeletal muscle is largely attributed to neural responses, not adaptive changes in muscle itself. This is reflected by an increase in electromyographic (EMG) activity during the first 4 to 8 weeks of training with little to no evidence of muscle fiber hypertrophy. It is also possible that increased neural activity is the source of additional gains in strength late in a resistance training program even after muscle hypertrophy has reached a plateau.

Neural adaptations are attributed to motor learning and improved coordination and include increased recruitment in the number of motor units firing as well as an increased rate and synchronization of firing. It is speculated that these changes are caused by a decrease in the inhibitory function of the central nervous system (CNS), decreased sensitivity of the Golgi tendon organ (GTO), or changes at the myoneural junction of the motor unit.

Skeletal Muscle Adaptations. Hypertrophy. As noted previously, the tension-producing capacity of muscle is directly related to the physiological cross-sectional area of the individual muscle fibers. Hypertrophy is an increase in the size (bulk) of an individual muscle fiber caused by an increase in myofibrillar volume. After an extended period of moderate- to high-intensity resistance training, usually by 4 to 8 weeks but possibly as early as 2 to 3 weeks with very high-intensity resistance training, hypertrophy becomes an increasingly important adaptation that accounts for strength gains in muscle.

Although the mechanism of hypertrophy is complex and the stimulus for growth is not clearly understood, hypertrophy of skeletal muscle appears to be the result of an increase in protein (actin and myosin) synthesis and a decrease in protein degradation. Hypertrophy is also associated with biochemical changes that stimulate uptake of amino acids.

The greatest increases in protein synthesis and therefore hypertrophy are associated with high-volume, moderate-resistance exercise performed eccentrically.In addition, it is the type IIB muscle fibers that appear to increase in size most readily with resistance training.

Hyperplasia. Although the topic has been debated for many years and evidence of the phenomenon is sparse, there is some thought that a portion of the increase in muscle size that occurs with heavy resistance training is caused by hyperplasia, an increase in the number of muscle fibers. It has been suggested that this increase in fiber number, observed in laboratory animals, is the result of longitudinal splitting of fibers.It has been postulated that fiber splitting occurs when individual muscle fibers increase in size to a point where they are inefficient, then subsequently split to form two distinct fibers.

Critics of the concept of hyperplasia suggest that evidence of fiber splitting may actually be caused by inappropriate tissue preparation in the laboratory. The general opinion in the literature is that hyperplasia either does not occur; or if it does occur to a slight degree, its impact is insignificant. In a recent review article it was the authors’ opinion that if hyperplasia is a valid finding, it probably accounts for a very small proportion (less than 5%) of the increase in muscle size that occurs with resistance training.

Muscle Fiber Type Adaptation. As previously mentioned, type II (phasic) muscle fibers preferentially hypertrophy with heavy resistance training. In addition, a substantial degree of plasticity exists in muscle fibers with respect to contractile and metabolic properties. Transformation of type IIB to type IIA is common with endurance training, as well as during the early weeks of heavy resistance training, making the type II fibers more fatigue-resistant. There is some evidence that demonstrates type I to type II fiber type conversion in the denervated limbs of laboratory animals, in humans with spinal cord injury, and after an extended period of weightlessness associated with space flight. However, there is little to no evidence of type II to type I conversion under training conditions in rehabilitation or fitness programs.

Vascular and Metabolic Adaptations. Opposite to what occurs with endurance training, when muscles hypertrophy with high-intensity, low-volume training, capillary bed density actually decreases because of an increase in the number of myofilaments per fiber. Athletes who participate in heavy resistance training actually have fewer capillaries per muscle fiber than endurance athletes and even untrained individuals. Other changes associated with metabolism, such as a decrease in mitochondrial density, also occur with high-intensity resistance training. This is associated with reduced oxidative capacity of muscle.

Adaptations of Connective Tissues. Although the evidence is limited, it appears that the tensile strength of tendons and ligaments as well as bone increases with resistance training designed to improve the strength or power of muscles.

Tendons, Ligaments, and Connective Tissue in Muscle. Strength improvement in tendons probably occurs at the musculotendinous junction, whereas increased ligament strength may occur at the ligament-bone interface. It is believed that tendon and ligament tensile strength increases in response to resistance training to support the adaptive strength and size changes in muscle. The connective tissue in muscle (around muscle fibers) also thickens, giving more support to the enlarged fibers.Consequently, strong ligaments and tendons may be less prone to injury. It is also thought that noncontractile soft tissue strength may develop more rapidly with eccentric resistance training than with other types of resistance exercises.,

Bone. Numerous sources indicate there is a high correlation between muscle strength and the level of physical activity across the life span with bone mineral density. Consequently, physical activities and exercises, particularly those performed in weight-bearing positions, are typically recommended to minimize or prevent age-related bone loss.They are also prescribed to reduce the risk of fractures or improve bone density when osteopenia or osteoporosis is already present.

Although the evidence from prospective studies is limited and mixed, resistance exercises performed with adequate intensity and with site-specific loading through weight bearing of the bony area to be tested has been shown to increase or maintain bone mineral density.In contrast, a number of studies in young, healthy women and postmenopausal women have reported that there was no significant increase in bone mineral density with resistance training. However, the resistance exercises in these studies were not combined with site-specific weight bearing. In addition, the intensity of the weight training programs may not have been high enough to have an impact on bone density. The time course of the exercise program also may not have been long enough. It has been suggested that it may take as long as 9 months to a year of exercise for detectable and significant increases in bone mass to occur. In the spine, although studies to date have not shown that resistance training prevents spinal fractures, there is some evidence to suggest that the strength of the back extensors closely correlates with bone mineral density of the spine.

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