People think their hands are 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, he says. Denise Cadet 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 his colleagues tested 20 adults who were told to relax their left arm on a pillar that looked like an armrest. Initially, each person allowed their left hand to hang freely, but was then given support and a weight attached to the left wrist. Throughout the experiment, the participants could not 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 so repeatedly with different weights.
The average hand weighs around 400 grams, but people in the study underestimated their hand weight by 49.4 percent on average.
The team isn’t sure why that was, but Cadete suspects that this underestimation makes the move less demanding. “Our hands can feel lighter [than they actually are] to make it easier for us to move more freely around the world,” he says.
The team then investigated whether fatigue would affect a person’s perception of the weight of their hand. They repeated the experiment with 20 people, then asked the participants to repeatedly squeeze a hand dynamometer, typically used to measure grip strength, for 10 minutes, before testing again.
After the exercise, the participants only underestimated the weight of their hands by 29 percent. The more fatigue people reported in their hand, the heavier they perceived it to be. This could be a mechanism to encourage us to rest after strenuous exercise, Cadete says.
According to Cadete, the findings could help us design better prostheses. She speculates that if scientists could better convince people who wear prosthetics that their artificial limbs are part of their body, they would perceive them as lighter, in the same way that we do with our hands.
