Twisted said..
A non optimal diameter for a certain hand length would require more muscle force and strain therefore make injury more likely. Ideally you want maximum grip with minimal force.
That makes sense. I'd be careful in interpreting the results of the study, though. A lot of tool studies focus on things like exerting torque with a screw driver. In such tasks, a bigger diameter handle will give you more leverage, so the optimum diameter is larger than when just holding on. Plenty of studies give different optimum diameters depending on the task. For example, a 40 mm diameter may be optimal for "power grips", and 16 mm for "precision grips".
For windsurfing, the more relevant studies may focus on "breakaway strength", which measures just holding on. Breakaway strength can be significantly higher than grip force, mostly due to friction. I did not find any decent studies that looked at diameters similar to windsurf booms, though. So we're stuck with using what feels best (or what we have).
