Nonuniform Muscle Hypertrophy: Its Relation to Muscle Activation in Training Session


Medicine & Science in Sports & Exercise:
doi: 10.1249/MSS.0b013e3182995349
Applied Sciences

Purpose: Muscle hypertrophy in response to resistance training has been reported to occur nonuniformly along the length of the muscle. The purpose of the present study was to examine whether the regional difference in muscle hypertrophy induced by a training intervention corresponds to the regional difference in muscle activation in the training session.

Methods: Twelve young men participated in a training intervention program for the elbow extensors with a multijoint resistance exercise for 12 wk (3 d·wk−1). Before and after the intervention, cross-sectional areas of the triceps brachii along its length were measured with magnetic resonance images. A series of transverse relaxation time (T2)-weighted magnetic resonance images was recorded before and immediately after the first session of training intervention. The T2 was calculated for each pixel within the triceps brachii. In the images recorded after the session, the number of pixels with a T2 greater than the threshold (mean + 1 SD of T2 before the session) was expressed as the ratio to the whole number of pixels within the muscle and used as an index of muscle activation (percent activated area).

Results: The percent activated area of the triceps brachii in the first session was significantly higher in the middle regions than that in the most proximal region. Similarly, the relative change in cross-sectional area induced by the training intervention was also significantly greater in the middle regions than the most proximal region.

Conclusion: The results suggest that nonuniform muscle hypertrophy after training intervention is due to the region-specific muscle activation during the training session.

Author Information

1Faculty of Sport Sciences, Waseda University, Tokorozawa, Saitama, JAPAN; 2Faculty of Health and Sports Science, Doshisha University, Kyotanabe, Kyoto, JAPAN; 3Graduate School of Sport Sciences, Waseda University, Saitama, JAPAN; 4Faculty of Sport and Health Science, Ritsumeikan University, Shiga, JAPAN; and 5Japan Society for the Promotion of Science, Tokyo, JAPAN

Address for correspondence: Taku Wakahara, Ph.D., Faculty of Health and Sports Science, Doshisha University, 1-3 Tatara Miyakodani, Kyotanabe, Kyoto, 610-0394, Japan; E-mail:

Submitted for publication December 2012.

Accepted for publication April 2013.

© 2013 American College of Sports Medicine