The aim was to develop and to investigate the technical feasibility of a novel smartphone-based mobile system for feedback control of heart rate during outdoor running. Accurate control is important because heart rate can be used for prescription of exercise intensity for development and maintenance of cardiorespiratory fitness. An Android smartphone was employed together with wearable, wireless sensors for heart rate and running speed. A simple feedback design algorithm appropriate for embedded mobile applications was developed. Controller synthesis uses a low-order, physiologically-validated plant model and requires a single bandwidth-related tuning parameter. Twenty real time controller tests demonstrated highly accurate tracking of target heart rate with a mean root-mean-square tracking error (RMSE) of less than 2 beats per minute (bpm); a sufficient level of robustness was demonstrated within the range of conditions tested. Adjustment of the tuning parameter towards lower closed-loop bandwidth gave markedly lower control signal power (0.0008 vs. 0.0030m2/s2, p <0.0001, low vs. high bandwidth), but at the cost of a significantly lower heart rate tracking accuracy (RMSE 1.99 vs. 1.67bpm, p <0.01). The precision achieved suggests that the system might be applicable for accurate achievement of prescribed exercise intensity for development and maintenance of cardiorespiratory fitness. High-accuracy feedback control of heart rate during outdoor running using smartphone technology is deemed feasible. 
Feedback control of heart rate during outdoor running: A smartphone implementation