Whole body oxygen uptake and evoked knee torque in response to low frequency electrical stimulation of the quadriceps muscles: V•O2 frequency response to NMES

Abstract

Background: There is emerging evidence that isometric Neuromuscular Electrical Stimulation (NMES) may offer a way to elicit therapeutically significant increases in whole-body oxygen uptake in order to deliver aerobic exercise to patients unable to exercise volitionally, with consequent gains in cardiovascular health. The optimal stimulation frequency to elicit a significant and sustained pulmonary oxygen uptake has not been determined. The aim of this study was to examine the frequency response of the oxygen uptake and evoked torque due to NMES of the quadriceps muscles across a range of low frequencies spanning the twitch to tetanus transition.

Methods: Ten healthy male subjects underwent bilateral NMES of the quadriceps muscles comprising eight 4 minute bouts of intermittent stimulation at selected frequencies in the range 1 to 12 Hz, interspersed with 4 minutes rest periods. Respiratory gases and knee extensor torque were simultaneously monitored throughout. Multiple linear regression was used to fit the resulting data to an energetic model which expressed the energy rate in terms of the pulse frequency, the torque time integral and a factor representing the accumulated force developed per unit time.

Results: Additional oxygen uptake increased over the frequency range to a maximum of 564 (SD 114) ml min-1 at 12 Hz, and the respiratory exchange ratio was close to unity from 4 to 12 Hz. While the highest induced torque occurred at 12 Hz, the peak of the force development factor occurred at 6 Hz. The regression model accounted for 88% of the variability in the observed energetic response.

Conclusions: Taking into account the requirement to avoid prolonged tetanic contractions and to minimize evoked torque, the results suggest that the ideal frequency for sustainable aerobic exercise is 4 to 5 Hz, which coincided in this study with the frequency above which significant twitch force summation occurred.

Figure 1

Sample torque and oxygen uptake record. (a) Measured joint torque and (b) rate of oxygen consumption $\stackrel{.}{V}{O}_2$ for a representative subject over the 60 minute test period showing 4 minute bouts of NMES at selected frequencies. Inserts, (c through e), show torque responses on an enlarged time-scale so that partial twitch fusion can be seen.

Figure 2

Sample time-averaged oxygen uptake and torque functions. Average $\stackrel{.}{V}{O}_2$_,TTI, and dTTI for the last 60 seconds of each bout, as a function of stimulation frequency, for a typical subject.

Figure 3

Group mean pulmonary responses and torque functions. Group mean (SEM) for the following responses as a function of stimulation frequency: (a) Additional $\stackrel{.}{V}{O}_2$ over resting levels for the last 60 seconds of each bout, (b) peak torque T, (c) integral of torque, TTI, (d) integral of torque development, dTTI, (e) breathing frequency, Rf ,(f) ventilation, ${\stackrel{.}{V}}_E$(g) ventilation equivalent for oxygen, ${\stackrel{.}{V}}_E/\stackrel{.}{V}{O}_2.$

Figure 4

Respiratory exchange ratio. Group mean (SEM) for the Respiratory Exchange Ratio (RER), averaged over the final 60 seconds of each bout, as a function of stimulation frequency.

Figure 5

Energetic model performance (a). Comparison between the experimental group mean (SEM) energy rate and the group mean (SEM) energy rate predicted by the regression model of Equation 7. (b). The contributions of the f, TTI and dTTI terms of the model.