In water sports, should fingers be pressed together to increase drag?

My water aerobics instructor says to press my fingers together to increase resistance. The total surface area is the same with fingers open or closed, so why does it make a difference?

Stephen Johnson
Eugene, Oregon, US

The question of whether or not fingers should be pressed together to increase resistance, or drag, in water sports has been debated for decades. In a lifetime of competitive swimming, I first encountered this discussion in the early 1970s, although at that time there wasn’t any real science on the subject.

Since 1979, there have been numerous experiments and simulations studying this topic. …