People think their hands are about half as heavy as they really are.
We don’t often think about how much our hands weigh, but people with prosthetics sometimes complain that their artificial limbs are too heavy, even when they weigh less than a real limb, says Denise Cadete at Birkbeck, University of London. Few studies have looked in detail at how we perceive the weight of our body parts, she says.
Cadete and her colleagues tested 20 adults who were each told to relax their left arm on a pillar resembling an armrest. Initially, each person let their left hand hang freely, but then it was given support and a weight was attached to their left wrist. Throughout the experiment, the participants couldn’t see their hands or the weights because they were covered by a screen.
The researchers asked each participant to judge whether their hand or the weight was heavier, and they did this repeatedly with different weights.
The average hand weighs about 400 grams, but the people in the study underestimated their hand weight by 49.4 per cent on average.
The team is unsure why this was the case, but Cadete suspects this underestimation makes movement less taxing. “Our hands may feel lighter [than they actually are] so that we find it easier to move more freely in the world,” she says.
Next, the team investigated whether fatigue would affect a person’s perception of their hand weight. They repeated the experiment on 20 people, then asked the participants to repeatedly squeeze a hand-held dynamometer, which is typically used to measure grip strength, for 10 minutes, before carrying out the test again.
After the exercise, the participants only underestimated their hand weight by around 29 per cent. The more fatigue people reported in their hand, the heavier they perceived their hand to be. This could be a mechanism to encourage us to rest after strenuous exercise, says Cadete.
According to Cadete, the findings could help us design better prosthetics. She speculates that, if scientists could better convince people wearing prosthetics that their artificial limbs are part of their body, they would perceive them as lighter, the same way we do with our hands.