Effects of low frequency functional electrical stimulation with 15 and 50 Hz on muscle strength in heart failure patients

Purpose. To compare the acute effects of functional electrical stimulation (FES) with 15 and 50 Hertz (Hz) frequencies on muscle strength in patients with heart failure with healthy individuals. Methods. Twenty-two 61.6 +/- 1.0 y-old male volunteers were studied: 10 patients with heart failure (functional class II-III) and 12 healthy controls. The isometric muscle peak torque (IMPT) of the quadriceps femoral muscle was measured through a Biodex dynamometer in maximum voluntary contraction (MVC), and under FES of 50 Hz and 15 Hz, which was applied with a 0.4 ms pulse width, 10-s contraction time, 50-s resting time and maximum tolerable intensity. Results. The IMPT differed in MVC, 50 Hz and 15 Hz FES both in patients (201.9 +/- 14, 55.6 +/- 13 and 42.1 +/- 12 Newton-metre, respectively; p < 0.001) and in controls (179.3 +/- 9, 62.4 +/- 8 and 52.3 +/- 7 Newton-metre, respectively; p < 0.001). There were no differences between the groups. In patients and controls, respectively, the 50 Hz FES corresponded to 27% versus 35% and the 15 Hz to 21% versus 29% of the IMPT generated at the MVC (p < 0.001). Discussion. This result can be attributed to the fact that muscle strength is proportional to the stimulation frequency and to the number of recruited motor units. Thus, the higher the frequency, the greater the motor recruiting, producing increased muscle strength. Conclusion. The IMPT generated by acute 50 Hz application of FES is higher than the one generated by 15 Hz, but it is lower than MVC in controls and patients with heart failure.