Metrics for describing soft-tissue artefact and its effect on pose, size, and shape of marker clusters.

In human movement analysis based on stereophotogrammetry, bone pose is reconstructed by observing a cluster of skin markers. Each marker undergoes a displacement relative to the underlying bone that is regarded as an artefact (soft-tissue artefact, STA) since it affects accuracy in bone pose estimation. This paper proposes a set of metrics for the statistical description of the STA and its effects on cluster pose, size, and shape, with the intent of contributing to a clearer knowledge of its characteristics, and consequently of setting the bases for the development of more accurate bone pose estimators than presently available. Skin marker clusters behave as deformable bodies in motion relative to the underlying bone. Their motion can be described, based on Procrustes analysis, as the composition of four independent transformations: translation and rotation (rigid motion, RM), and change in size and shape (nonrigid motion, NRM). Statistical parameters describing the time histories of both the individual marker STA and the cluster transformations listed earlier were defined. For demonstration purposes, data collected ex vivo were used. The lower limbs of three cadavers were made to undergo movements with prevailing flexion-extension components. Femur pose was accurately measured using pin markers and the movement of twelve thigh skin markers observed relative to it. The STAs of all possible clusters of four skin markers were analysed. RM and NRM exhibited similar magnitudes and therefore impact on bone pose estimation. Thus bone pose estimators should not account for NRM only, as is normally the case, but also for RM.

Adjustments to Zatsiorsky-Seluyanov's segment inertia parameters.

Zatsiorsky et al. (in Contemporary Problems in Biomechanics, pp. 272-291, CRC Press, Massachusetts, 1990a) obtained, by means of a gamma-ray scanning technique, the relative body segment masses, center of mass (CM) positions, and radii of gyration for samples of college-aged Caucasian males and females. Although these data are the only available and comprehensive set of inertial parameters regarding young adult Caucasians, they have been rarely utilized for biomechanical analyses of subjects belonging to the same or a similar population. The main reason is probably that Zatsiorsky et al. used bony landmarks as reference points for locating segment CMs and defining segment lengths. Some of these landmarks were markedly distant from the joint centers currently used by most researchers as reference points. The purpose of this study was to adjust the mean relative CM positions and radii of gyration reported by Zatsiorsky et al., in order to reference them to the joint centers or other commonly used landmarks, rather than the original landmarks. The adjustments were based on a number of carefully selected sources of anthropometric data.

Joint center longitudinal positions computed from a selected subset of Chandler's data.

Mathematical models of the human body are indispensable tools for studying the biomechanics of human movement. The geometrical centers of the 12 main joints (shoulders, elbows, wrists, hips, knees ankles), modeled as simple mechanical joints, are widely used as reference points for building mathematical models of the human body. These reference points, typically defined as "joint centers", are assumed to maintain a fixed 3D position relative to both the segments forming the joint, throughout the range of joint motion. No single point in a human joint perfectly meets this assumption, and no simple method is available for locating the points that are closest to meet it. Researchers often have recourse to subjective methods, based on their knowledge of anatomy. Objective estimates are easily attainable if the positions of a few bony landmarks can be measured on the subject, and the longitudinal distances of the joint centers from these landmarks are known. A subset of the anthropometric measurements performed by Chandler et al. (NTIS No. AD 710-622, 1975) on six cadavers was critically selected and utilized to compute the percent longitudinal distances of the 12 main joint centers from neighboring bony landmarks, relative to the lengths of the respective proximal and/or distal segments. Three-dimensional positions are attainable as well, by assuming joint centers lay on the respective segment longitudinal axes. The use of a method for accurately locating joint centers is recommended, particularly when they are used as reference points for defining a personalized geometrical model of a subject's body.