The energy cost of split-belt walking for a variety of belt speed combinations

Walking on a split-belt treadmill is often compared to walking on tied belts at the average speed, but the relationship between the metabolic energy costs of split- and tied-belt walking remains largely unexplored. Recent simulation work has suggested that people could take advantage of a belt speed difference and lower their energy costs, but this effect has not yet been observed experimentally. To relate metabolic energy costs across a range of belt speed combinations, we had 10 participants each complete 14 tied-belt and 39 split-belt walking trials, with early split-belt trials incorporating additional time for training. The average speeds ranged from 0.6 to 1.8 m/s with belt speed differences up to 1.4 m/s. We used ANOVA to determine differences in energy cost due to average speed and belt speed difference. We fit a linear model to estimate the tied-belt speed with the same energy cost as a given pair of split belt speeds. The cost of split-belt walking was on average just 0.13 ± 0.32 W/kg more expensive than the cost of tied-belt walking at the average speed. Contrary to predictions from simple dynamical models, increased belt speed difference resulted in increased energy cost, and the energetically equivalent tied-belt speed could be estimated as v_eq=v_avg+0.065⋅Δv. Clinicians designing rehabilitation protocols can balance the therapeutic benefits of higher belt speed difference with increased energetic demands. Open questions remain about why people cannot fully take advantage of mechanical work performed by a split-belt treadmill.