Final answer:
A non-circular handgrip with a perimeter greater than 15 inches should have a graspable finger recess on both sides.
Step-by-step explanation:
In order for a non-circular handgrip to have a graspable finger recess on both sides, the perimeter must be greater than 15 inches.
Research suggests that a non-circular handgrip should have a graspable finger recess on both sides if its perimeter is as much as 8 diameters side-to-side and 15 diameters deep. This improves the firmness of the grip.
Therefore, to ensure a secure grip, a non-circular handgrip with a perimeter greater than 15 inches should have a graspable finger recess on both sides.