Systematic Underestimation of Human Hand Weight

Ferre, Elisa and Joel, Jonathan and Cadete, Denise and Longo, Matthew (2023) Systematic Underestimation of Human Hand Weight. Current Biology 33 (14), R758-R759. ISSN 0960-9822.

Abstract

According to Newton’s laws, the weight of a body part is equal to its mass times gravitational acceleration. Our experience of body part weight, however, is constructed by the central nervous system. No sensory receptors directly specify the weight of body parts, and the factors influencing perceived weight remain unknown. The perceived weight of held objects has been linked to sensations of the magnitude of central motor commands sent to the muscles, what Helmholtz called the effort of will and has subsequently been called the sense of effort1. This is shown by manipulations of fatigue1, anaesthesia2, and following brain damage3. Similar drive to muscles is required to counteract the force of gravity on the limbs themselves, though few studies have investigated the perceived weight of body parts4. Stroke patients with hemiplegia frequently comment that their limbs feel heavy5, an effect linked to fatigue in the affected limb6. Similarly, amputees commonly complain of the weight of prosthetic limbs7, despite these typically weighing less than actual limbs. Here we show that healthy adult humans systematically underestimate hand weight. We used a psychophysical matching task to measure experience of hand weight, which was underestimated on average by 49.4%. We further show that experimental induction of hand fatigue causes a systematic increase in perceived hand weight. Our results demonstrate that humans fail to experience the full weight of their body.

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