Skeletal Muscle Hypertrophy: How important is Exercise Intensity?

throughout a lifetime is essential for a favorable quality of life. Like most human tissue, skeletal muscle is highly plastic and is capable of responding to a stimulus through a network of anabolic signaling which results in growth and increases in strength. Skeletal muscle functions as the largest disposal site of ingested glucose 1, plays an important role in lipid oxidation 2, 3, and is one of the greatest modifiable contributors to the resting metabolic rate 4, demonstrating the importance of maintaining skeletal muscle quantity and quality. In contrast, the removal of that appropriate stimulus results in the loss of favorable muscle adaptations. Current recommendations for resistance training exercise intensities are commonly based on a percentage of the concentric one repetition maximum (1RM) for a particular exercise. By definition there is a direct relationship between the external load lifted and the exercise intensity (↑external load=↑exercise intensity and vice versa). The American College of Sports Medicine (ACSM) recommends that novice lifters lift at 70-85% 1RM and advanced lifters work with intensities ranging from 70-100% 1RM 5 in order to produce measurable changes in skeletal muscle hypertrophy. Recommendations from the National Strength and Conditioning Association (NSCA) in their current issue of the Essentials of Strength Training and Conditioning textbook mirror those of the ACSM 6. It is acknowledged that exercise intensity is only one of many variables (e.g. contraction velocity, contraction time, rest periods, volume, etc.) that can affect the chronic adaptation from resistance training. However, we emphasize exercise intensity in this review, because both the ACSM 5 and NSCA 6 emphasize this as an important variable for increasing skeletal muscle mass. Therefore, the purpose of this manuscript will be to discuss the skeletal muscle hypertrophy exercise intensity recommendations and discuss what these recommendations are largely based on. This manuscript will review recent acute and chronic data which suggests that exercise intensity may be playing less of a role with skeletal muscle hypertrophy than previously thought.