Impact of phase difference between cardiac systole and skeletal muscle contraction on hemodynamic response during electrically-induced muscle contractions

Percutaneous low-frequency electrical muscle stimulation (LF-ES) is a new alternative exercise prescription for individuals who cannot adequately perform voluntary exercise. However, substantial undesirable elevation of both systolic blood pressure (SBP) and cardiac afterload occurs during LF-ES and must be resolved. Therefore, this study examined whether or not the synchrony between cardiac systole and skeletal muscle contraction affects instantaneous blood pressure and cardiac afterload during intermittent evoked muscle contractions. In eight subjects, the quadriceps and biceps femoris muscles of each limb were simultaneously stimulated at 20 Hz with a duty cycle of 0.3 s stimulation and 0.7 s pause for 15 min. The phase difference between the ECG R-peak and the onset of muscle contraction (τc–s) was measured for all heartbeats. Then, instantaneous SBP, tension-time index (TTI), and peripheral vascular resistance (PVR) associated with each heartbeat were plotted as functions of τc–s. The results showed that SBP, TTI, and PVR were significantly lowered at positive τc–s (i.e., the moment at which a muscle contraction started during the cardiac recovery phase). These results suggest that a well-designed stimulator, one that induces muscle contractions coupled with heartbeats with appropriate phase difference, would effectively attenuate the elevation of SBP and cardiac afterload during LF-ES.