Overground vs. treadmill walking on biomechanical energy harvesting: An energetics and EMG study

A biomechanical energy harvester is a wearable device that generates electricity from human motion. Walking on a treadmill has been used almost exclusively by researchers for studying the energetic effects of biomechanical energy harvesters and wearable robotics. A treadmill provides the advantage of having long duration trials within a stationary motion capture volume. However, no consensus exists on whether the results from treadmill walking accurately represent overground walking. We aim to investigate how a biomechanical energy harvester performs overground compared to on a treadmill by measuring energy expenditure and muscle activity. Participants (n = 15) walked both overground and on a treadmill with and without a lower limb-driven biomechanical energy harvester. Energy expenditure was measured using indirect calorimetry and muscle activity was collected with surface electromyograms on seven superficial lower limb muscles. We observed a similar increase in metabolic cost of transport (Δoverground: 0.28 ± 0.24 J/kgm, Δtreadmill: 0.30 ± 0.24 J/kgm) from normal walking (overground: 2.56 ± 0.33 J/kgm, treadmill: 3.39 ± 0.31 J/kgm) to harvester walking (overground: 2.83 ± 0.35 J/kgm, treadmill: 3.69 ± 0.32 J/kgm) in both walking modes (p > 0.05). This was accompanied a significant increase in muscle activity of select muscle groups (p < 0.05). There was also a significant increase observed during walking on a treadmill compared to overground walking (p < 0.05). In conclusion, our results suggest that although the metabolic cost of transport and muscle activation for treadmill walking is higher than that of overground, when studying the effects of a biomechanical energy harvester, treadmill will give similar net increases when compared to a controlled walking condition, such as normal walking, on the same walking surface.