Contribution of blood oxygen and carbon dioxide sensing to the energetic optimization of human walking

People can adapt their gait to minimize energetic cost, indicating that walking9s neural control has access to ongoing measurements of the body9s energy use. Here we test the hypothesis that an important source of energetic cost measurements arises from blood gas receptors that are sensitive to oxygen and carbon dioxide concentrations. These receptors are known to play a role in regulating other physiological processes related to energy consumption, such as ventilation rate. And due to the role of oxygen and carbon dioxide in oxidative metabolism, sensing their levels can provide an accurate estimate of the body9s total energy use. To test our hypothesis, we simulated an added energetic cost for blood gas receptors that depended upon a subject9s step frequency and determined if subjects changed their behaviour in response to this simulated cost. These energetic costs were simulated by controlling inspired gas concentrations to decrease the circulating levels of oxygen and increase carbon dioxide. We found this blood gas control to be effective at shifting the step frequency that minimized the ventilation rate and perceived exertion away from the normally preferred frequency, indicating that these receptors provide the nervous system with strong physiological and psychological signals. However, rather than adapt their preferred step frequency towards these lower simulated costs, subjects persevered at their normally preferred frequency even after extensive experience with the new simulated costs. These results suggest that blood gas receptors play a negligible role in sensing energetic cost for the purpose of optimizing gait.