Sarah Tenberg, Jonas Weinig, Daniel Niederer, Lutz Vogt, Markus Leisse, Steffen Müller

• Background: Electrical stimulation is an effective treatment method for improving motor function after stroke, but the optimal current type for patients with stroke and arm paresis remains unclear. Objective: To compare the effects of kilohertz frequency with low-frequency current on stimulation efficiency, electrically induced force, discomfort, and muscle fatigue in patients with stroke. Design: A randomized crossover study. Setting: Neurological inpatient rehabilitation clinic in Germany. Participants: A total of 23 patients with arm paresis after stroke within the last 6 months were recruited, 21 were enrolled, and 20 completed the study (7 females; mean ± SD: 66 ± 12 years; 176 ± 11 cm; 90 ± 19 kg; 57 ± 34 days since stroke). Intervention: All patients underwent both kilohertz and low-frequency stimulation in a randomized order on 2 days (48-hour washout). Each day included a step protocol with a gradual increase in stimulation intensity, starting at the first measurable force (up to 12 steps, 1 mA increments, 8 seconds stimulation, 60 second rest) and a fatigue protocol (30 repetitions, 8 second stimulation, 3 second rest). Main Outcome Measure: Primary outcome was stimulation efficiency (electrically induced force/stimulation intensity) [N/mA], measured during each step of the stepwise increase in current intensity protocol. Results: Linear-mixed-effects models showed significantly higher stimulation efficiency for low-frequency stimulation (mean difference 0.14 [95% confidence interval, 0.01–0.27 N/mA], p = .031). However, current type did not significantly affect electrically induced force, level of discomfort, or muscle fatigue (p > .05). Conclusion: The findings suggest that low-frequency stimulation is more efficient than kilohertz-frequency stimulation. However, both current types yield similar effects on force, discomfort, and fatigue, making them both viable options for wrist extensor stimulation in patients after stroke. Considering the variability among individuals, customizing the current type based on electrically induced force and perceived discomfort may enhance therapeutic outcomes. Further research on the long-term treatment effects of both current types is warranted.