Human movement analysis using stereophotogrammetry

Abstract

When using optoelectronic stereophotogrammetry, skin deformation and displacement causes marker movement with respect to the underlying bone. This movement represents an artifact, which affects the estimation of the skeletal system kinematics, and is regarded as the most critical source of error in human movement analysis. A comprehensive review of the state-of-the-art for assessment, minimization and compensation of the soft tissue artifact (STA) is provided. It has been shown that STA is greater than the instrumental error associated with stereophotogrammetry, has a frequency content similar to the actual bone movement, is task dependent and not reproducible among subjects and, of lower limb segments, is greatest at the thigh. It has been shown that in in vivo experiments only motion about the flexion/extension axis of the hip, knees and ankles can be determined reliably. Motion about other axes at those joints should be regarded with much more caution as this artifact produces spurious effects with magnitudes comparable to the amount of motion actually occurring in those joints. Techniques designed to minimize the contribution of and compensate for the effects of this artifact can be divided up into those which model the skin surface and those which include joint motion constraints. Despite the numerous solutions proposed, the objective of reliable estimation of 3D skeletal system kinematics using skin markers has not yet been satisfactorily achieved and greatly limits the contribution of human movement analysis to clinical practice and biomechanical research. For STA to be compensated for effectively, it is here suggested that either its subject-specific pattern is assessed by ad hoc exercises or it is characterized from a large series of measurements on different subject populations. Alternatively, inclusion of joint constraints into a more general STA minimization approach may provide an acceptable solution.

Introduction

The fundamental role of human movement analysis in the advancement of the understanding of musculo-skeletal system physiopathology is well established [1], and the utilization of this technique continues to flourish. However, there are limitations due to limited awareness of the methodological fundamentals and experimental inaccuracies associated with the instrumentation examining a biological system. The present paper is the third in a series of articles addressing the major issues concerning the reconstruction of human skeletal system 3D kinematics when analyzed using optoelectronic stereophotogrammetry, by far the most widespread technique used. The series is aimed at enhancing the comprehension of the fundamentals of human movement analysis techniques and its concomitant problems.

The first article in this series [2] provided the necessary theoretical bases for the description of human movement, and suggestions for appropriate terminology. The second [3] reported on the instrumental errors associated with any stereophotogrammetric system and on the analytical and technical procedures necessary to cope with this source of inaccuracy. In this article, where rigidity of the body segments was assumed and only the stereophotogrammetric error was dealt with, it was shown that the reliability with which joint degrees of freedom (DOF) with limited range of motion are reconstructed is very low. It is an unfortunate reality that, apart from a few special in vivo tests [4], routine in vivo movement analysis experiments must deal with deformable tissues. This introduces methodological problems that are recognized to be the primary limitation to further advancements of human movement analysis [1].

Two different sources of error originate at the interface between the stereophotogrammetric system and the bony segment under analysis: anatomical landmark (AL) misplacement [5] and soft tissue artifact (STA). The present paper addresses the latter source of error, the nature of which resides in the relative movement between the markers and the underlying bone. This is associated with the specific marker set and experimental protocol adopted. Inertial effects, skin deformation and sliding, which occur mainly in areas closer to the joints [6], and deformation caused by muscle contractions, contribute independently to STA. Because of its nature, the artifact has a frequency content similar to the actual bone movement and it is therefore very difficult to distinguish between the two by means of any filtering technique.

A comprehensive review of the studies aimed at assessing STA and at devising methods for the minimization of its effects on the description of the musculo-skeletal function is presented first. Pelvic and lower limb segments are dealt with, as these are of great interest in human movement analysis. Proposed techniques designed to minimize these effects are also reported, as divided into those analyzing skin surface motion and deformation and those including joint motion constraints. The main purpose is to facilitate the rapid identification of the most critical issues and the retrieval from the literature of the most salient solutions.