(A) Perimeter ratio. The curved hand movement trajectory was projected on the straight line and a fine partition at equally spaced points obtained. The maximum normal distance from the curve to the straight line (marked with a red star) was obtained. The partial perimeter enclosing the yellow area up to the point of max bending (max normal distance) was computed

*P *^{Partial} =

*X *^{Partial}+

*Y *^{Partial} +

*Z* where

*X *^{Partial} is the length of the curve from beginning to max bending,

*Y *^{Partial} is the length of the straight line from beginning to max bending and

*Z* is the max normal distance (max bending value). The perimeter ratio is the partial perimeter divided by the total perimeter (length) of the curve and the straight line divided. The area ratio was similarly computed but using the area enclosed between the line and the curve and the partial area (in yellow) up to the point of max bending. (B) Schematics of the inclination of the plane of the arm obtained from the unit vectors running along the red vectors spanning a plane defined by the positions of the upper arm and the forearm. Vectors run from shoulder to elbow and from shoulder to wrist. The angle

*θ* between the unit vector

normal to this plane and the unit vector

perpendicular to the horizontal plane define the inclination of the plane of the arm.

## PubMed Commons