Your hands are probably heavier than you think, a new study suggests. In a series of experiments, researchers showed that people consistently underestimated the weight of their hands. The findings might one day help us better design prosthetics that feel more comfortable for their users to wear, the authors say.
The study was led by psychologists from the University of London in the UK. The team was inspired by research findings that amputees will commonly complain about the weight of their new prosthetic limbs, despite these limbs often weighing less than the genuine article. Stroke patients who experience partial paralysis will also sometimes report that their limbs feel heavier afterward—a change that might be caused by higher levels of fatigue post-stroke.
To better understand this phenomenon, the team decided to recruit groups of healthy adults for two separate experiments.
In the first experiment, the team had 20 volunteers sit in a chair with armrests and fixed screens that blocked them from seeing their hands. Each volunteer’s left hand was first left to dangle off the edge of the chair while the rest of the arm was supported by a cushion. Then the volunteers were made to carry differently sized bags of rice hooked onto a wristband on their left arm while their left hand was supported by another cushion (in effect, replacing the weight of the hand with the rice bags). With each new weight attached, the volunteers were asked to judge whether the rice bag was heavier or lighter than their hand.
People were good at telling apart light and heavy weights, the researchers found. But the volunteers also systematically underestimated the weight of their own hands—by an average of 49%. Since the typical adult hand is around 400 grams (just under 0.9 pounds), this would amount to people thinking that their hands instead weighed about 200 grams.
In the second experiment, the team had another group try the same task with an added twist. This time, the volunteers were first asked to use a dynamometer (a device that measures hand grip strength) for ten minutes in order to tire them out. As before, the volunteers underestimated the weight of their hands, but only by 29% on average. The findings were published Monday in the journal Current Biology.
The results are based on a small sample size, so more research will be needed to confirm the team’s observations. But they do provide some of the first experimental support for the idea that we experience the weight of our body parts differently than we do for objects that aren’t attached to us.
The researchers aren’t quite sure why this happens, but they have their theories. Not sensing the weight of our limbs might allow us to be more precise in judging the weight of things we’re carrying in those limbs, for instance. Simply not feeling the burden of our body all the time might also make us more willing and able to stay active. On the flip side, if fatigue can literally make our hands and other body parts feel heavier than before, as the team’s findings suggest, then that sensation might provide us with an added incentive to take a break.
“By making actions feel effortless, weight underestimation may encourage activity. Conversely, the reappearance of body weight with fatigue may produce the opposite effect,” the researchers wrote.
There have long been efforts to create more real-seeming prosthetics, including those that try to incorporate some level of sensory feedback for their users. And some research has already suggested that these improved prosthetics will also get to feel lighter as a result as well, the authors note.