ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Optimal acceleration adjustment to improve data computed using inverse
dynamics approach
1. ABSTRACT
F. Leboeuf 1 , F. Colloud 2
Estimating accurately the acceleration of joint segments is crucial in inverse dynamics.
The double differentiation, usually used, amplifies noise measurement, leading to
inconsistencies between measured external force and the rate of change of linear
momentum. The aim of this paper was to propose an optimal joint acceleration
adjustment based on a dynamic optimization problem. The resolution was made through
a parametric problem, embedded the measured joint angles which prevents numerical
constraints. An application has been carried out on a flexion-extension movement. The
acceleration adjustment led to a superposition of the measured and calculated contact
forces. Optimal joint torques were smoothed and revealed new intrinsic information.
2. INTRODUCTION
Inverse dynamics procedure leads to the knowledge of the internal force coordination,
especially important for neuro-physiological or clinical investigations. Inverse dynamics
is very sensitive to the propagation of uncertainties because the mathematical model is
based on a set of iterative equations using noisy experimental data as guest. The
drawback of this iterative method is to be error prone (Riemer et al., 2008) whatever the
formalism implemented, e.g. Lagrange or Newton-Euler formalisms. As a result,
computed rate of change of linear momentum and measured ground reaction force
patterns usually show large inconsistencies. A residual force is always computed at the
last segment of the kinematic chain whatever the inverse dynamic strategy (bottom-up
or up-bottom) implemented.
Recently, some researchers addressed the residual force minimization by adjusting body
segment parameters. Delp et al. (2007) optimized the mass distribution to improve
human movement simulations. Riemer and Hsiao-Wecksler (2009) proposed a double
step optimization procedure. They showed that subject-specific body segment
parameters reduced joint torques errors by 77%. Their method consists in short
calibrations (first step) offering initial guests to a refinement optimization (second step).
Similar to functional methods used to locate joint centers (Ehrig et al., 2006), calibration
phases could be difficult to achieve with disabled subjects.
Another way to minimize residual force is to tackle the problem of joint accelerations.
1 Research Engineer, Laboratoire d’Analyse du Mouvement, Pôle Médecine Physique et Réadaptation,
Hôpital Saint Jacques, CHU de Nantes, 86 boulevard Saint Jacques, 44093 Nantes, France
2 Associate Professor, Institut Pprime UPR 3346 (CNRS - Université de Poitiers - ENSMA), Département
Génie Mécanique & Systèmes Complexes, 11 boulevard Marie et Pierre Curie, BP 30179, 86962
Futuroscope, France