An in-depth look at shallow-water walking: the mechanical determinants of the energy metabolic cost of shallow water walking in humans

Human locomotion in water involves unique forces (buoyancy, drag) influencing metabolic cost. However, a validated model integrating these forces to predict the cost of transport (COT) during shallow water walking (SWW) is lacking, and energetic optimization strategies remain unclear compared to terrestrial gaits. We measured the COT in nine healthy men during SWW across four immersion depths (knee to xiphoid) and four walking speeds (0.2-0.8 m/s). We developed and validated a physiomechanical model based on the mechanical work done against buoyancy-affected body weight and hydrodynamic drag. Using this model, we compared the energetics of SWW with swimming and dry land walking (including hypogravity conditions) and analyzed self-selected walking speeds. The minimum COT occurred at hip immersion depth (0.2 m/s), rather than at intermediate speeds, with the J-shaped relationship observed only at knee immersion depth. Metabolic power, in contrast, remained relatively constant during self-selected walking across immersion depths. An immersion depth threshold near the center of mass emerged, above which swimming becomes more economical than SWW. Our physiomechanical model accurately predicted the measured COT. The interplay between buoyancy and drag dictates SWW energetics, shifting optimization away from intermediate speeds common on dry land. From a physiological perspective, these findings quantify the energetic consequences of human locomotor adaptation to the unique mechanical challenges posed by aquatic environments. Furthermore, identifying an immersion depth threshold influencing the economical choice between walking and swimming provides new insights into human aquatic locomotor adaptations.