Physiological Adaptations for Resistance Training

Muscle fiber adaptations

The increase in the size of muscle is referred to as hypertrophy. The “pump” experienced following a resistance training session is a short-term effect, and is due to fluid accumulation in the muscle as a result of increased blood flow, swelling due to tissue breakdown and increased water content. In the long term, chronic hypertrophy will occur. This will result in an increase in the cross-sectional area of muscle fibers, ranging from 20% to 45%. Muscle fiber hypertrophy has been shown to require a minimum to produce significant effects.

The number of muscle fibers in any one individual is established at birth, and cannot increase with training. Therefore, increases in muscle mass will always be as a result of increases in the size of the fiber. This will be due to growth of actin and myosin filaments, as well as an increase in the size of the muscle cell (sarcoplasm).

Hyperplasia, which is an increase in the number of muscle fibers due to fibers splitting, has only been observed in animals, and although possible in humans, has not been proven to occur.

In addition, fast-twitch (Type II) muscle fibers develop greater increases in size as compared to slow-twitch (Type I) muscle fibers.

Strength adaptations

body building

Initial increases in strength, when starting a resistance training program, cannot be due to increases in the cross-sectional size of the muscle fibers as this takes at least 8 weeks to occur. Rather, if strength increases do occur prior to this 8 week period, it will be due to neural adaptations. This will include increased fiber recruitment, improved technique (motor learning) and the inhibition of the neural protective mechanisms (the muscle spindle and golgi tendon organ).

Long-term changes in strength are more likely attributable to hypertrophy of the muscle fibers or muscle group. The degree of strength gains has been found to be related to the speed at which the movement is performed. Therefore, slow-speed training will result in greater gains at slow movement speeds, while fast-speed training will realize the improvements in strength at faster movement speeds.

Bone tissue adaptations

Bone Anatomy and Biology

In response to loading of the bone, created by muscular contractions or other methods of mechanical forces, begins a process of bone modeling. This new bone formation occurs chiefly on the outer surface of the bone, or periosteum.

Activities that stimulate bone growth need to include progressive overload, variation of load, and specificity of loading (exercises that directly place a load on a certain region of the skeleton).

Programs to increase bone growth should be structural in nature, including exercises such as squats and lunges which direct the forces through the axial skeleton and allow greater loads to be utilized. The magnitude required to produce an effective stimulus for bone remodeling appears to be a 1 repetition maximum (RM) to 10 RM load.

Body composition adaptations

Resistance training programs can increase lean muscle mass and, as a consequence, decrease the percentage of body fat. Body composition is affected and controlled by resistance training programs using the larger muscle groups and greater total volume.

Total volume is determined by the total number of repetitions (repetitions x sets) performed times the weight of the load (total repetitions x weight). This total volume of training can be linked to duration and intensity. The greater the total volume, or workload, the greater the energy expenditure. A greater energy expenditure will aid in achieving weight loss goals, as long as the risk of over training is considered when designing exercise programs.

Since resistance training contributes to increasing lean muscle mass, it is one of the most effective means of training to accomplish weight loss goals. Larger muscle groups will utilise more energy during training as well as recovery. Exercises selected for a weight loss resistance training program must therefore be more compound in nature.

Heart rate adaptations

Heart rate has been shown to decrease in the long term as a result of resistance training. The extent of the decrease is, however, dependent on the type of resistance training implemented. The heart rates in certain test subjects has been shown to decrease by up to 11% following a resistance training program. This is believed to be as a result of increased stroke volume due to left ventricular hypertrophy.

Blood pressure adaptations to resistance training

During a resistance exercise session, both systolic and diastolic blood pressures may show dramatic increases, which suggest that caution should be observed in persons with cardiovascular disease or known cardiovascular risk factors. The extent of the increase in blood pressure is dependent on the time the contraction is held, the intensity of the contraction, and the amount of muscle mass involved in the contraction. In healthy individuals with no cardiovascular risk factors, blood pressure has been shown to decrease following long term resistance training programs. This is believed to be a result of increased blood vessel elasticity, as well as improved nervous control of blood vessel dilation and constriction. Circuit training, which involves moderate resistance and high repetitions with short rests are associated with the greatest reductions in blood pressure.

Blood vessels

Glucose metabolism adaptations

Initially, improvements in glucose metabolism were associated with decreases in percent body fat and increases in aerobic capacity, therefore aerobic training has always been thought to be the best form of training to improve glucose metabolism. However, improvements in glucose metabolism with strength training has been demonstrated even when aerobic training has not been included in the program. Studies have shown similar results in glucose metabolism improvements for both strength training and resistance training programs, with a program which includes both resistance training and aerobic training demonstrating the greatest improvements.