Comparison of Cardiorespiratory and Metabolic Responses Between Tetanic and Sub-Tetanic Neuromuscular Electrical Stimulation

The purpose of this study was to compare the cardiorespiratory and metabolic responses between tNMES and sNMES.

Thirteen healthy adult males (20.7 ± 0.6 years old) participated in the study. Each subject underwent three trials: 30-min bed rest, 30-min tNMES, and 30-min sNMES. NMES was applied to the bilateral skeletal muscles distal to the waist using belt electrodes, with stimulation intensity set to the maximum tolerable level. During each trial, respiratory gas exchange (VO₂, oxygen consumption; VE, minute vantilation; RQ, respiratory quotient) and electrocardiography (HR, heart rate) were measured, and blood examinations (blood glucose, lactate, and BDNF, brain-derived neurotrophic factor concentrations) were conducted before and after each trial. Statistical analyses for these parameters were performed using repeated measures ANOVA or Friedman test, with a significance level set at 5%.

While there was no significant difference in the stimulation electrical current applied to the thighs between tNMES and sNMES, the current applied to the lower limbs during tNMES was significantly lower than that during sNMES due to pain and discomfort. sNMES resulted in significantly higher VO₂ (15.9 ± 3.6 vs. 12.0 ± 3.5 ml/min/kg, p 0.05, 95% CI [-6.35, -1.59]) and HR (100.2 ± 8.8 vs. 89.6 ± 9.4 bpm, p 0.05, 95% CI [-17.12, -4.08]) compared to tNMES. sNMES increased RQ more than tNMES (p 0.05, 95% CI [-0.083, -0.020]) and exhibited a stronger blood glucose-lowering effect (p 0.05, 95% CI [3.6, 13.32]). Although lactate and BDNF concentrations significantly increased with both NMES modalities (both p 0.05), no significant differences were observed between the two (both p > 0.05).

sNMES, which induces lower torque than tNMES, elicits significantly higher cardiorespiratory and metabolic responses, requiring VO₂ equivalent to walking activity. Furthermore, sNMES appears to enhance glycolytic activity due to NMES-specific disorderly recruitment more strongly than tNMES.

sNMES may complement reduced physical activity during hospitalization and, when combined with tNMES, could further improve physical function in individuals with reduced phisical activity.