I once used a toothbrush shaped like this:
Let’s call this toothbrush the “backbend”. It’s sort of bent backwards (relative to the direction the bristles are facing). Perhaps* the motive behind designing a toothbrush with this shape is that that’s the direction your hand is pushing it anyway. But what your hand and wrist actually want is for the object they’re using to be shaped in opposition to directions/angles they’re exerting force in. Your hand exerts force on the toothbrush such that your thumb pushing on the toothbrush in the middle is what allows you get the bristles to exert force on your teeth. So if the toothbrush is shaped like the above, the shape of the handle is giving way just where you want to be exerting the most force on it, which in turn requires you to exert even more force. In other words, you want an object’s (handle’s) surface to curve in a direction opposite of where you’re exerting force, so that you get the most leverage.
The ironic thing is that many objects today (pencil grips, Microsoft mouses, etc.) that claim to be friendly to the human grip fail to accomplish this simple task of providing leverage because they instead make it their goal to conform to the hand’s grip, which defeats their goal of being more comfortable to use because it’s this leverage, this opposition that you want, which you get from things that oppose the hand’s grip, like round, bulbous handles as in these pens and this paintbrush and this bottle opener. And this toothbrush:
(Bonus points: it plays Black Eyes Peas.)
* Perhaps the backbend toothbrush isn’t really designed to be easier to hold. Maybe the purpose is to fit in your mouth easier? If so, I think it’s not worth it for the annoyance of having to hold it.
In my travels through Google images during the making of this post, I found another interesting sort of toothbrush: