Radial force distribution changes associated with tangential force production in cylindrical grasping, and the importance of anatomical registration

Radial force (F _r) distributions describe grip force coordination about a cylindrical object. Recent studies have employed only explicit F _r tasks, and have not normalized for anatomical variance when considering F _r distributions. The goals of the present study were (i) to explore F _r during tangential force production tasks, and (ii) to examine the extent to which anatomical registration (i.e. spatial normalization of anatomically analogous structures) could improve signal detectability in F _r data. Twelve subjects grasped a vertically oriented cylindrical handle (diameter=6cm) and matched target upward tangential forces of 10, 20, and 30N. F _r data were measured using a flexible pressure mat with an angular resolution of 4.8°, and were registered using piecewise-linear interpolation between five manually identified points-of-interest. Results indicate that F _r was primarily limited to three contact regions: the distal thumb, the distal fingers, and the fingers' metatacarpal heads, and that, while increases in tangential force caused significant increases in F _r for these regions, they did not significantly affect the F _r distribution across the hand. Registration was found to substantially reduce between-subject variability, as indicated by both accentuated F _r trends, and amplification of the test statistic. These results imply that, while subjects focus F _r primarily on three anatomical regions during cylindrical grasp, inter-subject anatomical differences introduce a variability that, if not corrected for via registration, may compromise one's ability to draw anatomically relevant conclusions from grasping force data.