Investigating energy and stability trade-offs in human walking using vestibular sensory disruption

Walking involves multiple, sometimes competing, objectives, including minimizing energy expenditure and maintaining stability. When stability is compromised, such as through sensory disruption, individuals often adopt wider step widths to maintain balance. However, it remains unclear whether these wider widths increase energetic expenditure or remain energy optimal. Our purpose was to investigate trade-offs between energy and stability during walking using vestibular disruption. Our primary hypothesis was that when experiencing vestibular disruption, participants would adopt wider widths at the expense of increased cost. Participants (n=16) walked on a treadmill at preferred step widths under natural conditions and with electrical vestibular stimulation (EVS). Step width was computed using motion capture, metabolic energy expenditure was measured using indirect calorimetry, and residual head variance (Vres) was calculated from an inertial measurement unit to assess stability. We also used visual feedback to command participants to walk at widths narrower (-3 s.d., -5 s.d.) and wider (+3 s.d., +5 s.d.) than preferred, allowing us to assess the relationship between energy and stability across width. As expected, EVS increased Vres, indicating decreased stability, while wider widths decreased Vres, suggesting improved stability. Under EVS, participants preferred wider widths compared with natural walking (natural: 0; EVS: 0.88 s.d.±1.17 s.d., P=0.01), and the energy-optimal width also shifted wider by a similar magnitude; however, we were underpowered to detect this change (natural: 0.04 s.d.±3.35 s.d.; EVS: 1.28 s.d.±2.92 s.d., P=0.16). These findings suggest that wider step widths adopted under sensory disruption to improve stability may remain energetically optimal, rather than reflect an often-assumed stability-energy trade-off.