Coupling Between Cardiac and Locomotor Rhythms: The Phase Lag Between Heart Beats and Pedal Thrusts

During some rhythmic exercises, the heart and exercise rates may become coupled (be within 1% of each other). If the intraarterial and skeletal intramus cular pressure cycles were reciprocal, blood flow to exercising muscle should be maximized and cardiac load minimized. In this study the authors tested the hypothesis that, while coupling is present, the phase lag between the pedaling and cardiac contraction cycles is consistent and appropriate. Twenty-seven sub jects pedaled, at a frequency natural to them, on an electronically braked bicy cle ergometer that held the power output constant regardless of pedaling rate. To assess the phase lag between pedal thrust (two per revolution) and heart beat, pedal-gated plots of the electrocardiography signal were generated throughout the most coupled five-minute work load for each of the 9 subjects in whom the rates were within 1% of each other for at least two consecutive four- second samples taken every fifteen seconds. During this interval of thirty-sec onds in which the rates were within 1 % of each other, the phase lag of most subjects gradually lengthened and shortened and there was considerable varia tion among subjects, refuting the authors' hypothesis. The results of this study illustrate the importance of beat-by-beat analysis when studying coupling phe nomena. The preliminary assumption, that the coupling between cardiac and locomotor rhythms during cycling was on the basis of a single ischemic muscle group, has apparently been disproven.