Precise limb-speed afference reveals rhythmogenic drive modality and declines with age

Human locomotion requires precise coordination of bilateral limb movements that degrades with age increasing frailty and loss of independence. Here we tested whether the nervous system senses small differences in limb speed and whether this signal is independent of loading and changes across the lifespan. Using a two-alternative forced-choice task on a split-belt treadmill, healthy young adults detected inter-limb speed differences as small as 1.3% (mean 1.3%, standard deviation 0.6%). Manipulating whole-body load by ±10% did not alter thresholds, consistent with computation of whole-limb speed rather than raw somatosensory drive. Across ages 6-80 years, discrimination followed a U-shaped trajectory: thresholds were higher in children, lowest in young adults, and increased in middle-aged and older adults. Here, we show that limb-speed afference is measured with high acuity in humans and declines with age, supporting its role as a control variable for rhythmogenic circuits and highlighting a potential target for assessment and rehabilitation in age-related mobility decline.