Maintaining an appropriate femoral head position within the joint capsule and labral complex is paramount to normal hip function and failure in this mechanism can lead to debilitating labral and cartilage compression in active individuals. Thus, hip congruency, although affected by, is not solely dependent upon the femoral head-acetabular bony and labral constituents for complete hip stabilization. The ligaments described above and the muscles that cross the hip joint contribute and provide for articular congruency (ie, proper joint rotation of the femoral head within the acetabular-labral complex) and maintain articular stabilization (ie, limit translations of the femoral head within the acetabular-labral complex). To accomplish this, muscles that cross the hip must act as force regulators across a very wide range of motions by regulating their stiffness. Muscular stiffness is determined by a complex neural feedback control system. A highly regulated hierarchy of neuromuscular control strategies begins with the activation of the single fiber and progresses to the mechanical properties of the whole muscle. Discussing the exact mechanisms that are involved in this neuro-mechanical hierarchy is beyond the scope of this article, but a few of the more pertinent aspects are listed briefly below:
1. Muscle stiffness is regulated by muscle activation frequency (ie, temporal summation) .
2. Muscle stiffness is regulated by muscle fiber recruitment (ie, spatial summation) .
3. Muscle stiffness is regulated by the sarcomere length-tension relationship .
4. Muscle stiffness is regulated by sarcomere force-velocity relationship .
5. Muscle stiffness is regulated by passive sarcomere length tension relationships .
6. Intrafusal and extrafusal (muscle spindle) fibers feedback mechanisms .
7. Muscle force and moment regulation by skeletal muscle architecture [29,30].
The first six points relate a specific muscle's function primarily to its intrinsic properties and are standard across all skeletal muscles. However, point 7, muscle stiffness regulation by skeletal muscle architecture (ie, the physical arrangement of the muscle fibers within a specific muscle) is of substantial importance at the hip given the large, "irregular" shaped muscles that cross this joint, and much work has been recently constructed in this area [31,32]. Functionally, the force generated by a muscle is proportional to its physiologic cross-sectional area (PCSA). The total excursion of a muscle is determined by its fiber length. Traditionally, fiber length were determined by dissection methods and histologic analysis; but recently, newer MRI-based technologies have been used with great success and detail [31,33,34]. Thus, from a muscle design perspective, muscle architecture results in muscle function based on unique fiber arrangements. Mechanical properties of many of the larger muscles surrounding the hip have been characterized and are presented in Table 1. Although detailed studies of muscles architecture have been conducted for the lower extremity , these studies often omit many of the smaller muscles (eg, pirifirmis, superior and inferior gemullus and obturator internus and externus) that cross the hip.
Because many of the hip muscles involve very complex geometric architectures, determining their exact mechanical influence on hip function is difficult. Computer modeling techniques enhanced by computer tomography (CT) and MRI are some of the newer techniques of estimating the complex hip muscular actions. These methods have allowed researchers to reconstruct the hip muscle geometry with "lumped parameter muscle models," where each muscles is represented by a single line of action estimated from a centroid of the muscles taken from the a 3D reconstruction via an MRI image [31,33,34]. These "lumped parameter muscle models," however, only allow for a one length of muscle fiber and moment arm to be estimated for each muscle path [31,33,34].
Muscle-tendon parameters for the hip muscles
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