Figure 11.

Loadings of the 1st PC, the Grasp Mode, when considering the 3D force analysis case (normal plus two tangential force components, empty box plots) for each digit vs. when considering only its normal force (normal force analysis, gray box plots). Box plots show loadings from all subjects and trials. Note that the loadings of all tangential forces (Fx and Fy) straddle the zero line, demonstrating that they are not relevant to the correlation structure of the 1st PC. The normal force components (Fz) of all digits have positive and non-zero loadings, indicating that the structure of this PC using normal forces is equivalent to that of the full 3D force analysis. The dispersion or exact median values in the box plots are not the means to establish the task-relevance or task-irrelevance of the PC. That dispersion is a consequence of natural variability and inaccuracies in motor performance, and unavoidable sensor noise. It is the goals of the task and mechanical analysis that determine how to identify the task-relevant and task-irrelevant Modes.