Changes in Heart Rate Variability During Low-Frequency Transcutaneous Electrical Nerve Stimulation in Athletes with Acute Musculoskeletal Injury

Abstract

Background: Pain is a relevant clinical symptom to ask for medical management. Since pain is a subjective sensation, its intensity is difficult to precisely assess. Objectives: The present study aimed to determine whether heart rate variability (HRV) can be a potential parameter to evaluate the pain-relieving effect of low-frequency transcutaneous electrical nerve stimulation (TENS). Methods: Athletes with lower limb musculoskeletal injuries were voluntarily included in the study. Pressure pain thresholds (PPT) was measured immediately after the intervention, and 24 hours after the treatment. Heart rate variability was also determined from electrocardiography (ECG) monitoring throughout the intervention. Results: The HRV of 25 participants revealed that 30 minutes of low-frequency electrical stimulation immediately increased PPT (before = 2.67 ± 0.79; after = 2.92 ± 0.90, P = 0.011). Thirteen participants had more than a 10% increase in PPT after the intervention; nevertheless, three participants had a decrease in PPT. The PPT was returned to the baseline 24 hours after the intervention. Electrocardiography demonstrated that only the frequency domain of low frequency-high frequency (LF/HF) ratio was significantly increased after the intervention (before = 1.454 ± 0.739; after = 1.922 ± 1.378; P = 0.035) but not time-domain HRV. Time-domain HRV significantly decreased during the intervention before returning to the baseline. The correlation between the change in PPT is mostly correlated with LF/HF. Conclusions: Heart rate variability revealed the potential change in sympathovagal balance toward the sympathetic domain during low-frequency TENS intervention. The frequency-domain LF/HF ratio was the most sensitive parameter to be affected by pain.