Comparison of cardio-locomotor synchronization during running and cycling.

By comparing the characteristics of cardiac-locomotor synchronization (CLS) in running and cycling individuals, we tested whether the characteristics of CLS occurring during rhythmic exercise adhere to the central origin hypothesis, which postulates a direct interaction between cardiovascular centers in the brain and the pattern generator in the spinal cord. Ten healthy subjects performed both exercises at the same intensity (150 beats.min(-1)) and cadence (150 steps.min(-1) during running and 75 rpm during cycling), while electrocardiograms and electromyograms from the right vastus lateralis muscle were monitored continuously. An examination of the occurrence of heart beats with respect to the locomotor phase revealed that, in running subjects, CLS exists for relatively prolonged periods at specific phases, whereas, in cycling subjects, it occurs intermittently and is not phase-specific [maximum duration of CLS: 113.6 (66.5) and 58.0 (29.3) s ( P<0.05), respectively]. Determining the probability of CLS by chance as a function of its duration, we also found that, during running, CLS likely results from entrainment, whereas, during cycling, it results from chance, occurring when the cardiac rhythm approached the locomotor rhythm. Our result indicated that the duration of muscle contraction during cycling [317.0 (18.1) ms] was significantly longer than during running [205.6 (20.2) ms]. These results indicated that the difference in the CLS characteristics between running and cycling might be influenced by differences in peripheral inputs between exercise modes.