Functional Analysis Of Hip Biomechanics

In vivo estimates of hip mechanics for dynamic activities have been attempted using optical capture, accelerometer, or goniometric methods. Optical methods employ high speed cameras to capture the 3D motion of reflective markers that are placed on pertinent and relative boney landmarks of the subjects. These systems produce 3D trajectories of the markers, which used to estimate internal joint centers and determine segment motions, velocities, and accelerations. These kinematic parameters are then combined with subject's anthropometric inertial data and external forces to yield external reaction forces and moments. These external forces and moments are then used to estimate internal joint reaction forces and internal "muscle" moments. The internal muscles moments must generate equal and opposite forces to the externally measured moments, and are composed of the muscle contraction, passive soft tissues, and joint reaction forces. However, using the inverse dynamics solution only yields net muscle moments, and these cannot be decomposed into individual muscle contributions to the motion without appropriate assumptions to obtain an equal number of unknowns and equations; or by employing an optimization scheme. Optimization methods assume that the force distribution among the muscles is made by applying an objective function (usually based on a physical property of a muscle). Early hip models [35] were limited in that they assumed muscles

were single bundles represented by straight muscle paths that possess similar fibers lengths with the same moment arms over the cross-section of a large muscle. Today, more sophisticated models [38] have employed more precise muscle paths with better defined "wrapping functions" to deflect muscles path around pertinent anatomic structures and more specific fiber length parameters for individual muscle bundles within the complex geometry of a whole muscle. These advancements have contributed to the understanding of the functional roles for the individual muscles surrounding the hip, as they more closely represent the true functional geometry of those muscles in vivo.

Anderson and Pandy [38] developed a muscle model that included select hip musculature to analyze a complete gait cycle. This model contained 54 independent muscles, and the results estimated each muscle's contribution to the support phase of gait. A muscle's potential for generating support was described by its contribution to the vertical ground reaction force per unit of muscle force. Of the hip muscles, the gluteus medius, maximus, and minimus provided the majority of the support in first 0% to 30% of stance (Fig. 1A) . From foot flat to just after contralateral toe-off (eg, 10-50% of stance), the gluteus maximus and posterior medius/minimus contributed significantly to the vertical ground reaction force. With assistance from joints and bones to gravity, the anterior and posterior gluteus medius/minimus generated nearly all the support evident in midstance. Posterior gluteus medius/minumus provided support throughout midstance, while the anterior gluteus medius/minimus contributed only toward the end of midstance (Fig. 1B). Interestingly, the iliopsoas developed substantial forces during late stance, but this muscle did not make substantial contributions to support [38].

The study of Anderson and Pandy [38] has shown that the muscular actions of the gluteus medius and minimus depend strongly on body positions. Anterior gluteus medius/minimus developed forces as large as the posterior gluteus

Fig. 1. (A-C) Individual muscles contributions to support during gait from heel strike (HS) to toe-off (TO). Here, support is represented by the shaded gray area, which is the vertical ground reaction force. Symbols used to represent muscles in the figure are: DF, ankle dorsiflexors; GAS, gastrocnemius; GMEDP, posterior gluteus medius/minimus; GMAX, medial and lateral portions of the gluteus maxumus; GMEDA, anterior gluteus medius/minimus; SOL, soleus; VAS, vasti. In this figure the gluteus maximus contributes the most muscle force to supprt in early stance; The posterior gluteus medius/minimus contributes notable force throughout the stance phase. In later stance, the anterior gluteus medius/minimus is most effective at maintaining support during gait. The passive resistance of the skeleton to the force of gravity was less then 50% of body weight through out stance, suggesting that muscles are the most important parameter to support the body during gait. Of these muscles, the hip gluteus maximus contributed the most force to support, followed by the vasti, gluteus medius/minimus, and soleus/gastrocnenius of the body compared with all other muscles during gait. Unfortunately, the mechanical roles of the smaller hip muscles such as the pectineus, pirifirmis, superior and inferior gemullus, and obturator internus and externus were not included in this model. (From Anderson FC, Pandy MG. Individual muscle contributions to support in normal walking. Gait Posture 2003;17(2):159-69; with permission.)

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