Kinesiology of the hip: a focus on muscular actions

The 21 muscles that cross the hip provide both triplanar movement and stability between the femur and acetabulum. The primary intent of this clinical commentary is to review and discuss the current understanding of the specific actions of the hip muscles. Analysis of their actions is based primarily on the spatial orientation of the muscles relative to the axes of rotation at the hip. The discussion of muscle actions is organized according to the 3 cardinal planes of motion. Actions are considered from both femoral-on-pelvic and pelvic-on-femoral perspectives, with particular attention to the role of coactivation of trunk muscles. Additional attention is paid to the biomechanical variables that alter the effectiveness, 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 from normal 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.