Kim, S., Kwon, O., Yi, C., Jeon, H., Oh, J., Cynn, H., & Weon, J. (2011). Comparison of abdominal muscle activity during a single-legged hold in the hook-lying position on the floor and on a round foam roll. Journal of athletic training, 46(4), S. 403-408. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3419152/pdf/i1062-6050-46-4-403.pdf

Kim, Su-Jung, Oh-Yun Kwon, Chung-Hwi Yi, Hye-Seon Jeon, Jae-Seop Oh, Heon-Seock Cynn, and Jong-Hyuck Weon. "Comparison of Abdominal Muscle Activity During a Single-legged Hold in the Hook-lying Position On the Floor and On a Round Foam Roll." Journal of Athletic Training 46, no. 4 (2011): S. 403-408.

Kim, Su-Jung, et al. "Comparison of Abdominal Muscle Activity During a Single-legged Hold in the Hook-lying Position On the Floor and On a Round Foam Roll." Journal of Athletic Training 46.4 (2011): S. 403-408.