Paper Cadera

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clinical commentary
dONaLd a. NEUmaNN, PT, PhD, FAPTA1

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Kinesiology of the Hip: A Focus on Muscular Actions

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he hip joint serves as a central pivot point for the body as a whole. This large ball-and-socket joint allows simultaneous, triplanar movements of the femur relative to the pelvis, as well as the trunk and pelvis relative to the femur. Lifting the foot off the ground,reaching towards the floor, or rapidly rotating the trunk and pelvis while supporting the body over one limb typically demands strong and specific activation of the hips’ surrounding musculature.
Pathology that affects the strength, control, or extensibility of the hip muscles can significantly disrupt the fluidity, comfort, and metabolic efficiency of many routine movements involving bothfunctional and recreational activities. Furthermore, abnormal performance of the muscles of the hip may alter the distribution of forces across the joint’s articular surfaces, potentially causing, or at least predisposing, degenerative changes in the articular cartilage, bone, and surrounding connective tissues. Physical therapy diagnosis related to
t SYNOPSIS: The 21 muscles that cross the hip

thehip and adjacent regions often requires a solid understanding of the actions of the surrounding muscles. This knowledge is instrumental in identifying when a specific muscle or muscle group is weak, painful, dominant, or tight (ie, lacks the extensibility to permit normal range of motion). Depending on the particular muscle, any one of these conditions can significantly affect the alignment acrossthe lumbar spine, pelvis, and femur, ultimately affecting the alignment throughout the entire lower limb. Furthermore, understanding the actions of the hip
ness, force, and torque of a given muscle action. The role of certain muscles in generating compression force at the hip is also presented. Throughout the commentary, the kinesiology of the muscles of the hip are considered primarily fromnormal but also pathological perspectives, supplemented with several clinically relevant scenarios. This overview should serve as a foundation for understanding the assessment and treatment of musculoskeletal impairments that involve not only the hip, but also the adjacent low back and knee regions. J Orthop Sports Phys Ther 2010;40(2):82-94. doi:10.2519/ jospt.2010.3025 gluteus maximus, gluteusmedius, hip

muscles is fundamental to interventions used to specifically activate, strengthen, or stretch certain muscles. The primary purpose of this paper is to review and analyze the actions of the muscles of the hip. The discussion will include several topics associated with muscular kinesiology, including a muscle’s torque (strength) potential, moment arm (ie, leverage), cross-sectional area,overall fiber direction, and line of force relative to an axis of rotation. When available, data from the research literature will be cited. As will be pointed out, some actions of muscles are strongly supported by rigorous research, while others are not.

Line of Force
The discussion of muscle action will be organized according to the 3 cardinal planes of motion of the hip: sagittal,horizontal, and frontal. For each plane of motion, a muscle’s action is based primarily on the orientation of its line of force relative to the joint’s axis of rotation. FIGURE 1 illustrates this orientation for several muscles acting within the sagittal plane. This figure, based on a straight-line model of muscle action, stems from the work of Dostal and others.16,17 Using a male cadaver, the proximal anddistal attachments of the muscles were carefully dissected and then digitized. A straight line between the attachment points was used to represent the muscle’s line of force. Observe in FIGURE 1, for instance, that a muscle’s line of force that passes anterior to the joint’s medial-lateral axis of rota-

provide both triplanar movement and stability between the femur and acetabulum. The...