Author(s): TAKAHASHI, K., TOZAWA, H., KAWAMA, R., WAKAHARA, T., Institution: DOSHISHA UNIVERSITY, Country: JAPAN, Abstract-ID: 1219

INTRODUCTION:
Muscle mechanical action underpinning our movements is mostly determined by its architecture (1). The muscle architecture has been extensively examined in vivo by ultrasonography (2), but its use is limited to certain small muscles because of a limited field of view (3). We still poorly understand how giant muscles in the deep part of the body form and act in living humans. By a novel 3D architectural analysis based on MRI, we examined in-vivo architecture and the potential action of the human adductor magnus (AM), one of the giant leg muscles classified as a hip adductor. We hypothesized that the primary action of AM is hip “extension” rather than adduction, considering its configuration similar to hamstrings.
METHODS:
For 14 healthy young adults (9 males), thousands of muscle fascicles were automatically reconstructed over the whole AM at the hip neutral position with diffusion tensor MRI and tractography (4,5). The muscle physiological cross-sectional area, and each fascicle’s line of action and moment arm length about the hip joint were determined for the medial/proximal/lateral region divided based on insertion positions. Then, the product of these parameters was calculated as an estimate of the maximal isometric torque-generating capacity (potential torque: PT). The peak torque during maximal voluntary contraction for the isometric hip extension/adduction was measured at the hip neutral and 45° flexed positions on a dynamometer. The paired t-test and linear regression analyses were performed.
RESULTS:
Most AM fascicles ran parallel to the femur, showing hamstring-like orientations in 3D. The proximal and lateral regions showed comparable PTs between the hip extension and adduction. In the medial region, however, the hip extension PT was 6-fold greater than the adduction PT (p<0.01). Consequently, the whole AM (sum of all regions) had a greater PT for the hip extension (42±15 Nm) than adduction (29±15 Nm, p=0.02). The hip extension PT of the whole AM was strongly correlated with dynamometer-measured hip extension peak torque at both hip neutral and flexed positions (r=0.71–0.87, p≤0.01), but that was not the case for the hip adduction (r=0.42–0.59, p≥0.05).
CONCLUSION:
The results clearly indicate that AM primarily acts for hip extension rather than adduction, although it has been assumed a major adductor. The AM may thus play as a strong motor rather than a stabilizer in human locomotion such as sprint running demanding forceful hip extension. The present study redefining muscular action through the in-vivo 3D architectural analysis would motivate us to rename human muscles (e.g., “extensor” magnus) and rethink their functional role, providing novel insights into proper exercise selection for effectively improving motor performance in sports and rehabilitation.
REFERENCES
1) Lieber & Friden, Muscle Nerve, 2000
2) Narici et al., J Appl Physiol, 2003
3) Franchi et al., Ultrasound Med Biol, 2018
4) Takahashi et al., J Anat, 2022
5) Takahashi et al., Med Sci Sports Exerc, 2023