When it comes to touchscreen devices, we’re not making the best use of our fingers.
Our fingers (except for the thumbs) are made up of three hinge joints. (Thumbs have a unique (to humans) joint at their base—the saddle joint—and only one hinge joint.) These hinge joints allow us to bend our fingers into several shapes. Doing this is a form of flexion. (Extension straightens the finger.)
Barring disability and age, most adult fingers have at least two comfortable positions: fully extended or curled. The index finger, generally being the most flexible, can have several (at least two) comfortable curled positions. As proof of this, watch yourself type and see how many finger positions particularly your fingers move into. (This isn’t true for hunt-and-peck typists, if there are any of those still around.)
Kinesiology lesson over, flash forward to touchscreens. Most of these use one finger position—fully extended—for taps, swipes, etc. Often, buttons are placed at the very top of the screen, forcing users to reach using a fully extended finger to access them. This is likely because in the desktop or application space, designers are used to putting menu items at the top, out of the way, where they can be just as easily accessed by a mouse and cursor as a bottom menu. But with touchscreens, items close by (at the bottom of the screen) are better for two reasons: they don’t require changing a hand position (often even removing a hand), and secondly, they are less likely to cause screen coverage, when the user’s own hand hides items on the screen.
Most reading apps on the iPad, for instance, put the Back or Home buttons at the top left of the screen, where you would usually find it on a browser. This is also because, I’m guessing the designers decided, the thumbs are already in use: to swipe turning pages. This is true, but the thumbs could have another use in the flexed (curled) position to access menu items at the bottom of the screen. (This isn’t to say that the menu items need to be visible all the time, I should mention. With many apps for reading, they are invisible until the screen is tapped.)
Granted, this is much less of a problem on touchscreen mobile devices like Android phones or the iPhone, since the thumbs can typically reach the top of the screen without much interference (although you’ll still get screen coverage, and it can be an overextension for people, particularly women, with smaller thumb lengths). But this use of a second “space” with a curled finger for touch targets could have many different uses, such as in gaming, typing, and different menu styles.
Fingers being less ideal than cursors for many tasks, this is one area where the finger has the cursor beat by having two natural “modes.” It’s an interaction and ergonomic pattern well worth putting to use.