Letter to the Editor: A Functional MRI Exploration of Hamstring Activation During the Supine Bridge Exercise

Dear editors of International Journal of Sports Medicine,

We were reading “A functional MRI exploration of hamstring activation during the supine bridge exercise” by Bourne et al. (2017b) with interest. The researchers continued efforts of investigating the behavior of hamstring muscle during a modified single leg supine bridge exercise (mSLB), first by surface electromyography (sEMG), and then using functional magnetic resonance imaging (fMRI) in this paper, is greatly appreciated. However, we were surprised by the following statements seen in the conclusion of the paper: “During the SLB exercise, the semitendinosus is most selectively targeted and the biceps femoris long head is preferentially activated over its short head.” This statement is misleading by missing the contribution of gluteus muscles and mischaracterizes the function of the biceps femoris short head. The biceps femoris short head is a mono-articular knee joint flexor muscle and the biceps femoris long head is a bi-articular hip joint extensor and knee joint flexor.

Single leg supine bridge is mainly an exercise for single joint hip flexors, i. e., gluteus medius and maximus during a hip joint extension movement against gravity. All bi-articular hamstring muscles of the supporting leg have been put in a disadvantaged position because of the flexed knee joint. As for biceps femoris short head, it is a mono-articular knee joint flexor and its contribution in this motion is a knee joint stabilizer at best. The biceps femoris short head is not directly involved in a hip joint extension motion. The mSLB exercise performed in this project, with the chosen leg placed up on a box 35 cm in height, enables the knee to be nearly full extended. This modification might increase the capacity of the bi-articular hamstring muscle to contribute to hip extension with the gluteus as the main contributors. An extra load, equivalent to participant’s approximated 12-RM, was placed on participant’s pelvis during the exercise. The extra load made it harder to raise the pelvis up from the floor by extending hip joint. fMRI scans of both thighs were acquired before and immediately after 5 sets of 10 repetitions of the mSLB exercise. Hamstring muscle belly (BF long and short heads, semitendinosus and semimembranosus) were scanned, recorded and analyzed in this project, but not the involvement of the gluteal muscles.

While it is important to study the heterogeneity of hamstring activation in different tasks, it is also important to include all main effectors of different movement in the study along with other muscles that are involved in the motion. By examining the relative change in transverse relaxation time (T2) of the hamstring muscles and using fMRI, the authors of this paper failed to present: (1). the comparison of the changes of hamstring muscles to gluteus muscles; and (2). the actual contraction intensity of hamstring muscles in the mSLB. To study the neuromuscular coordination of the mSLB, other researchers have focused on only core muscles, including the gluteus muscle but not the hamstrings (i.g., Ekstrom et al., 2007, Tan et al., 2013). Youda et al. (2015) included the gluteus muscles and the hamstring muscles in their study. They showed hamstrings, gluteus medius, and maximus all worked at a moderate intensity (30-40% MVIC) range during SLB.

With weak or injured gluteus muscles, hamstring muscles, except biceps femoris short head, could be a significant contributor to SLB or the mSLB, especially when knee joint is placed in a fully extended position. We suggest that a comprehensive project to study the neuromuscular contribution to the SLB or mSLB should include all of the mono- and bi-articular hip joint extensors.

Sincerely yours,
Li Li, PhD.
Matt Syno, MS. Ed.
Department of Health Sciences and Kinesiology
Georgia Southern University

• References

• 1 Bourne M, Williams M, Pizzari T, Shield A. A functional MRI exploration of hamstring activation during the supine bridge exercise. Int J Sports Med 2018; 39: 104-109
• 2 Bourne MN, Williams MD, Opar DA, Al Najjar A, Kerr GK, Shield AJ. Impact of exercise selection on hamstring muscle activation. Br J Sports Med 2017; 51: 1021-1028
• 3 Ekstrom RA, Donatelli RA, Carp KC. Electromyographic analysis of core trunk, hip, and thigh muscles during 9 rehabilitation exercises. J Orthop Sports Phys Ther 2007; 37: 754-762
• 4 Tan S, Cao L, Schoenfisch W, Wang J. Investigation of core muscle function through electromyography activities in healthy young men. J Exerc Physiol Online 2013; 16: 45
• 5 Youdas JW, Hartman JP, Murphy BS, Rundle AM, Ugorowski JM, Hollman JH. Magnitudes of muscle activation of spine stabilizers, gluteals, and hamstrings during supine bridge to neutral position. Physiother Theory Pract 2015; 31: 418-427