Moment arms of the human digital flexors

Abstract

For the extrinsic hand flexors (flexor digitorum profundus, FDP; flexor digitorum superficialis, FDS; flexor pollicis longus, FPL), moment arm corresponds to the tendon’s distance from the center of the metacarpalphalangeal (MP), proximal interphalangeal (PIP), or distal interphalangeal (DIP) joint. The clinical value of establishing accurate moment arms has been highlighted for biomechanical modeling, the development of robotic hands, designing rehabilitation protocols, and repairing flexor tendon pulleys (Brand et al., 1975; An et al., 1983; Thompson and Giurintano, 1989; Deshpande et al., 2010; Wu et al., 2010). In this study, we define the moment arms for all of the extrinsic flexor tendons of the hand across all digital joints for all digits in cadaveric hands.

On computing task-oriented grasps

“…While the subjects performed the circles drawing and cutting easily, they were not experts in performing screw driving tasks and as explained previously, they were clumsy in the way they directed the tool to fit within the screw resulting in this big dispersion along the screw driving torque task ellipsoid. The range of force/torque involved in these experiments, could easily be checked from the columns µ f and µ t in tables (6,7)showing that the average forces involved in the circles drawing, cutting and screw driving tasks along the major axis are respectively 1.67N, 8.3N and 11.49N while the corresponding torques are of 3.52N.cm, 61.75N.cm and 9.64N.cm.…”

“…The choice of this task forces the subject to move the pen in the different directions that are involved in the motion of writing. The subjects were instructed to perform the circles drawing task using 3 different grasps of the sensorized tool, a tripod grasp, a cylindrical grasp and a power grasp, see figure (6). For each grasp type, each of the seven subjects performed the task three times for a total of 21 trials per grasp type.…”

“…Again, seven subjects performed successfully each of the previous tasks 3 times, for a total of 21 trials per task. Tables (6,6) show the force/torque ellipsoid shapes of the circles drawing, cutting and screw driving tasks, tables (8,9) show the corresponding normalized force/torque ellipsoid shapes, and table (10) shows the normalized average of their projections along the x, y, z axes of the object.…”

“…More recent values of the human hand moment arms can be found in [6]. However, the authors do not mention in their paper the maximum force per tendon of their model.…”

Extrapolatable Analytical Functions for Tendon Excursions and Moment Arms From Sparse Datasets

“…Therefore, most studies in biomechanics use the second approach: empirical models. These almost always consist of polynomial expressions (including splines, which are piecewise polynomials stitched together) mapping joint angles to tendon excursions, and are regressed from experimental measurements, such as using cadaveric specimens [6], [13], [14], [8], [18], [19]. But these polynomial regressions have several inherent mathematical pitfalls; they can fail to extrapolate, may require large datasets to train, are not robust to noise, and often have numerous free parameters [20].…”