Neumann, D. A. (2010). Kinesiology of the hip: A focus on muscular actions. Journal of Orthopaedic & Sports Physical Therapy, 40(2), 82-94.
Background and Objective:
The review is an attempt to analyze the various actions of the hip muscles and its possible effects on adjacent regions of the body such as the knee and lower back, as well as its functional implications. The study uses the three cardinal planes to describe the muscle actions in the hip and utilizes the line of force, muscle action and muscle torque to analyze potential contributions to efficiency in functional daily tasks such as walking and standing, as well as potential dysfunctions from pathology. The author stated a few limitations including the use of data from one single cadaver, the lack of true muscular force potential measurement, as well as the listed muscular actions in an anatomical position when utilizing data for line of force and muscle action.
What the author did:
The paper provides much insights on biomechanical responsibilities of the hip muscles in all three cardinal planes by the utilization of moment arm and line of force. The author also included information on average maximal-effort torque in an attempt to further analyze muscular roles. The author sectioned each cardinal plane (flexion/extension, internal rotation/external rotation, abduction/adduction), and provided insights on functional responsibilities and potential dysfunctions in each section.
Muscle actions and responsibilities are well documented in literature when in an ANATOMICAL position. More research is needed in order to shed light on their responsibilities outside of neutral positions.
Summary of results:
Co-contraction of the core system is critical (with the TVA demonstrating a feed-forward mechanism) in supporting proper hip flexor action. This relationship applies to ALL hip flexors, with a lack of this force couple resulting in potential lower back dysfunctions.
Co-contraction of the core system applies to hip extensors with the exception of the transverse abdominis. With main hip extensors being the gluteus maximus as well as the posterior fibers of the adductor magnus. Hip extensor moment and torque potential increases as hip flexion angle increases, in addition, pelvic-on-femur extension provides a significant stretch on anterior hip capsule as well as hip flexors. Well centrated hip position allows for minimal metabolic demand and facilitates standing endurance and hence, bodily posture maintenance and energy sparing.
Hip external rotators:
The six deep external rotators of the hip with the exclusion of piriformis has contributions to femoacetabular stability. The anterior fibers of the gluteus maximus, posterior fibers of the gluteus minimus and piriformis change its rotary action as the hip is flexed beyond 90 degrees, making them internal rotators instead. This has implications in how the muscles can be stretched.
Hip internal rotators:
Relatively insignificant in terms of muscular action and torque potential in a neutral position due to its fiber orientation being vertical in relation to the greater trochanter. However, hip flexion up to 25 degrees DOUBLES the gluteus medius internal rotation torque, and up to 8-folds when flexed to 90 degrees. The progressive increase in internal rotation torque produced by internal rotators along with the role-reversals of the external rotators up to 90 degree hip flexion makes external rotator torque necessity even more important for people in a flexion dominant posture, or athletes engaging in constant force absorption activities such as jumping and running.
The adductor group has a high moment arm (six cm) that can facilitate adduction of the hip. With the exception of adductor magnus (especially the posterior fibers), all adductor group aid in extension of the hip in neutral hip position, and aid in extension of the hip beyond 40-70 degree hip flexion. Due to its versatility, and its constant concentric and eccentric demands, the adductor group has a relatively high susceptibility to strain.
Hip abductors has critical pelvic-on-femur stability requirement with single leg stance forces amounting to about 2.5 to 3 times the body weight, or up to 6 times the body weight when running. Its force-length curve in femur-on-pelvic hip abduction has important implication during manual testing as data shows a near vertical decline in torque as hip abduction angle move from -10 to 40 degree hip abduction (as shown to the right).