Gait-Specific Pulmonary Oxygen Uptake Kinetics During Gait Transitions Reflect the Energetics of Walking and Running in Humans

PURPOSE: Human terrestrial locomotion involves a transition between walking and running. While the energetics of steady-state walking and running have been well characterized, cardiac, ventilatory, and gas exchange kinetics during gait transitions remain poorly understood. METHODS: In this study, we used a sinusoidally changing speed of locomotion to investigate cardiac, ventilatory and gas exchange kinetics (beat-by-beat heart rate [HR], breath-by-breath pulmonary ventilation [VE], CO2 output [VCO2], and O2 uptake [VO2]) as well as step frequency adjustment across three gait conditions: walking (Walk), running (Run), and the walk-run transition (WRtrans). In each condition, the treadmill speed was varied sinusoidally in a range of mid-speed ± 1.5 km•h -1 with two different frequencies: the periods of 2 and 5 min. RESULTS: The VO2 in the WRtrans condition was intermediate between walking and running, whereas the amplitude of the VO2 in the WRtrans was significantly greater than in the other two condition (both p < 0.001). The VO2 and HR response during running was significantly delayed, as showing larger phase shifts, compared to walking. Notably, the observed VO2 response in the WRtrans closely matched the estimated VO2 response, reconstructed from the amplitude and phase shift values individually obtained in the Walk and Run conditions. Additionally, the average step frequency of all participants abruptly increased at gait transition and remained constant during the running phase of the WRtrans condition. CONCLUSIONS: These results suggest that: (i) the VO2 kinetics during gait transitions reflect the integration of walking- and running-specific VO2 energetic responses, and (ii) gait transitions are accompanied by adjustments in step frequency and length that are consistent with energetically optimal locomotor patterns.