Dumas Raphaël, Camomilla Valentina, Bonci Tecla, Chèze Laurence, Cappozzo Aurelio
J Biomech Eng. 2015 Jun;137(6):064502. doi: 10.1115/1.4030363. Epub 2015 Apr 24.
When joint kinematics is analyzed using noninvasive stereophotogrammetry, movements of the skin markers relative to the underlying bone are regarded as artefacts (soft tissue artefact (STA)). Recent literature suggests that an appropriate estimation of joint kinematics may be obtained by compensating for only a portion of the STA, but no evidence for this case has been reported, and which portion of the STA should be selected remains an issue. The aim of this study was to fill this gap. A modal approach was used to represent the STA. This resulted in a series of additive components (modes) and in the possibility to select a subset of them. The following STA definitions were used: individual skin marker displacement (MD), marker-cluster geometrical transformation (GT), and skin envelope shape variation (SV). An STA approximation for each of the three definitions was obtained by ordering modes on the basis of their contribution to the total STA energy and truncating the relevant series at 90% of it. A fourth approximation was obtained when the GT definition was used, by selecting the modes that represented the marker-cluster rigid transformation (i.e., three translation and three rotation modes). The different STA approximations were compared using data obtained during the stance phase of running of three volunteers carrying both pin and skin markers. The STA was measured and knee joint kinematics estimated using four skin marker datasets compensated for the above-mentioned STA approximations. Accuracy was assessed by comparing results to the reference kinematics obtained using pin markers. The different approximations resulted in different numbers of modes. For joint angles, the compensation efficiency across the STA approximations based on an energy threshold was almost equivalent. The median root mean square errors (RMSEs) were below 1 deg for flexion/extension and 2 deg for both abduction/adduction and internal/external rotation. For the joint displacements, the compensation efficiency depended on the STA approximation. Median RMSEs for anterior/posterior displacement ranged from 1 to 4 mm using either MD, GT, or SV truncated series. The RMSEs were virtually null when the STA was approximated using only the GT rigid modes. This result, together with the limited number of modes used by this approximation (i.e., three translations and three rotations of the marker-cluster), makes the STA rigid component and a good candidate for designing an STA model to be incorporated in an enhanced bone pose estimator.
当使用非侵入性立体摄影测量法分析关节运动学时，皮肤标记相对于其下方骨骼的运动被视为伪影（软组织伪影，STA）。最近的文献表明，通过仅补偿一部分STA可能会获得关节运动学的适当估计，但尚未有关于这种情况的证据报道，并且应该选择STA的哪一部分仍然是一个问题。本研究的目的是填补这一空白。采用模态方法来表示STA。这产生了一系列相加的分量（模态），并有可能选择其中的一个子集。使用了以下STA定义：单个皮肤标记位移（MD）、标记簇几何变换（GT）和皮肤包络形状变化（SV）。通过根据各模态对总STA能量的贡献对模态进行排序，并将相关序列截断为其90%，得到了这三种定义中每一种的STA近似值。当使用GT定义时，通过选择表示标记簇刚体变换的模态（即三个平移和三个旋转模态），得到了第四个近似值。使用三名同时佩戴针式标记和皮肤标记的志愿者在跑步站立阶段获得的数据，对不同的STA近似值进行了比较。测量了STA，并使用针对上述STA近似值进行补偿的四个皮肤标记数据集估计了膝关节运动学。通过将结果与使用针式标记获得的参考运动学进行比较来评估准确性。不同的近似值产生了不同数量的模态。对于关节角度，基于能量阈值的STA近似值的补偿效率几乎相同。屈伸的均方根误差（RMSE）中位数低于1°，外展/内收和内旋/外旋的RMSE中位数低于2°。对于关节位移，补偿效率取决于STA近似值。使用MD、GT或SV截断序列时，前后位移 的RMSE中位数范围为1至4毫米。当仅使用GT刚体模态近似STA时，RMSE实际上为零。这一结果，连同该近似值使用的有限数量的模态（即标记簇的三个平移和三个旋转），使得STA刚体分量成为设计一个要纳入增强型骨位姿估计器的STA模型的良好候选者。
