Astronauts’ brains misjudge body mass in space

In microgravity, muscles, bones and organs no longer press against one another as they do on Earth. This reduces feedback from proprioceptors: sensors in muscles and joints that tell the brain how the body is loaded and moving. That same feedback is how the brain estimates body mass. When the signal weakens, the brain’s estimate can drift.

In the study, published in eLife, twelve taikonauts aboard the China Space Station performed a standard hand-reaching task: touch a target as quickly as possible. Compared to their pre-flight performance and an age-matched ground control group, the taikonauts moved more slowly in space. They generated less force in the initial, automatic phase of the movement, then compensated through sensory-based corrections mid-movement.

Two hypotheses, one winner

Two competing explanations had been proposed for slower movement in space. The first: astronauts are deliberately cautious because moving quickly can disrupt posture and stability. The second: the brain underestimates body mass due to reduced proprioceptive input. The data from this experiment contradict the first explanation and support the second. Reduced initial force generation followed by corrective feedback precisely matches what you would expect from a faulty mass estimate, not from strategic caution.

Relevance for aging and movement control

Proprioceptive signals also decline with age. Aging is associated with muscle loss, slower nerve conduction, and less accurate body perception. Whether a similar mass-estimation problem contributes to fall risk and coordination loss in older adults has not been directly tested, but the parallel is worth exploring. The findings suggest that the brain can adopt an inaccurate body model without spontaneously correcting it through ordinary movement, a finding with potential implications for rehabilitation and movement training in aging populations.

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