Comparison of the effects of kilohertz- and low-frequency electric stimulations: A systematic review with meta-analysis

Abstract

Objective: This study aimed to determine whether kilohertz-frequency alternating current (KFAC) is superior to low-frequency pulsed current (PC) in increasing muscle-evoked torque and lessening discomfort.

Data sources: The electronic databases PubMed, PEDro, CINAHL, and CENTRAL were searched for related articles, published before August 2017. Furthermore, citation search was performed on the original record using Web of Science.

Review methods: Randomized controlled trials, quasi-experimental studies, and within-subject repeated studies evaluating and comparing KFAC and PC treatments were included. The pooled standardized mean differences (SMDs) of KFAC and PC treatments, with 95% confidence intervals (CIs), were calculated using the random effects model.

Results: In total, 1148 potentially relevant articles were selected, of which 14 articles with within-subject repeated designs (271 participants, mean age: 26.4 years) met the inclusion criteria. KFAC did not significantly increase muscle-evoked torque, compared to PC (pooled SMD: -0.25; 95% CI: -0.53, 0.06; P = 0.120). KFAC had comparable discomfort compared to that experienced using PC (pooled SMD: -0.06; 95% CI: -0.50, 0.38; P = 0.800). These estimates of the effects had a high risk of bias, as assessed using the Downs and Black scale, and were highly heterogeneous studies.

Conclusions: This meta-analysis does not establish that KFAC is superior to PC in increasing muscle-evoked torque and lessening discomfort level. However, no strong conclusion could be drawn because of a high risk of bias and a large amount of heterogeneity. High quality studies comparing the efficacy between PC and KFAC treatments with consideration of potential confounders is warranted to facilitate the development of effective treatment.

Fig 1. Review flow diagram.

Fig 2. SMD and 95% CI for the muscle performance between PC and KFAC stimulations.

A. %MVIC. B. Muscle evoked torque that is non-normalized data by MVIC. The diamond represents the pooled effect size using the DerSimonian-Laird method. The vertical solid line at 0 represents no difference. The vertical dotted line represents pooled SMD. SMD: standardized mean difference; CI: confidence interval; MVIC: maximum voluntary isometric contraction; PC: pulsed current; KFAC: kilohertz frequency alternating current.

Fig 3. SMD on %MVIC in the included articles, according to BMI, together with a summary of random effects meta-regression analysis.

The size of each circle is inversely proportional to weight to correspond to a random effects analysis. The transverse dotted line at 0 represents no difference. Reference numbers are shown. Note that articles by Laufer (2001) and Snyder-Mackler (1989) are not shown because of lack of BMI data. BMI: body mass index; SMD: standardized mean difference; MVIC: maximum voluntary isometric contraction.

Fig 4. Funnel plot representing publication bias shows a comparison of the effects of PC and KFAC stimulations on %MVIC (A) and muscle torque (B).

Egger’s regression test was negative for %MVIC (P = 0.578) and muscle torque (P = 0.973). Two diagonal lines represent pseudo 95% confidence limits around the summary effect for each standard error on the vertical axis. Reference numbers are shown. CI: confidence interval; KFAC: kilohertz frequency alternating current; SMD: standardized mean difference; MVIC: maximum voluntary isometric contraction.

Fig 5. SMD and 95% CI for the discomfort level between PC and KFAC stimulations.

The diamond represents the pooled effect size using the DerSimonian-Laird method. The vertical solid line at 0 represents no difference. The vertical dotted line represents pooled SMD. SMD: standardized mean difference; CI: confidence interval; MVIC: maximum voluntary isometric contraction; PC: pulsed current; KFAC: kilohertz frequency alternating current.

Fig 6. Funnel plot representing publication bias shows a comparison of the effects of PC and KFAC stimulations on discomfort level.

Egger’s regression test was negative (P = 0.788). Two diagonal lines represent pseudo 95% confidence limits around the summary effect for each standard error on the vertical axis. Reference numbers are shown. CI: confidence interval; KFAC: kilohertz frequency alternating current; SMD: standardized mean difference.