A Biomechanical Approach for Dynamic Hip Joint Analysis - 3D Multiscale Physiological Human - page 232

10.2.3.1 Hip Loads Estimation

Hip forces or loads weremeasured in the literature experimentally by using telemetric
implants [ 3 , 50 ]. Unfortunately this in vivo method cannot be used in a non operated
hip. Moreover, the resulting data cannot be considered as subject-specific. In fact,
measured forces concern aged persons who underwent hip replacement surgery (car-
tilage and labrum removed). Finally, the studied movements are limited to routine
activities which are not useful in our case study.
To overcome this problem, a neuromuscular simulation [ 51 - 53 ]isexploitedasan
alternative. This kind of simulation offers the possibility to estimate internal parame-
ters (e.g., muscle activations, forces) by analyzing the subject kinematics and kinetics
during the performance of activities. Such simulations were used in different appli-
cations like gait analysis [ 54 ], simulation of neuromuscular abnormalities [ 55 ], or
design of ergonomic furniture [ 56 ]. In neuromuscular simulations, several sets of data
measured experimentally (e.g., motion capture, force plates and Electromyography
(EMG)) are exploited into a specific process [ 57 ].
In this study, a neuromuscular model is adopted [ 58 ] to analyze the dynamic sim-
ulations of movements. A specific pipeline is required to estimate forces acting on
the hip joint. The first step consists of scaling the generic model to match the anthro-
pometry of the subject-specific anatomical model. The achieved scaling is based on
a hybrid method using measured data resulting from different approaches (3D body
scan model [ 48 ], 3D anatomical models, MRI data and initial marker positions).
These sets of data are combined and processed to realize an anisotropic scaling by
calculating the scaling factors for each part of the body. From the resulting model and
motion capture data, the joint coordinate values (e.g., joint angles) that reproduce the
subject movement (markers positions) are calculated by using an inverse kinematic
(IK) approach (see Fig. 10.5 ).
To complete the process, the ground reaction force (GRF) is required. In our
case, a computational method based on Newtonian analysis is used to replace the
unavailability of the force plate's data. A dynamic 3Dmodel using the motion capture
data and the scalingmodel data (body segment weights) is used to estimate GRF. This
approach is conceivable due the nature of studied movements. Indeed, the studied
movements are characterized by the static foot position of one leg and the air position
of the other leg (e.g., arabesque movement), or static position of both feet (e.g.,
bending movement). Estimated GRF F LG of the leg acting on the ground is expressed
as:

F i = F LG + m . g = m i . a i (10.1)

i i

where m is the subject mass, g denotes the gravity vector, m i and a i are the masses
and the accelerations of the body segments, respectively.
Based on kinematical data, the body segments' velocity v i and acceleration a i are
computed from their mass center positions p i :

v i = ( p i + 1 − p i )/ t (10.2)