Frequency-specific neuromuscular adaptations: comparative effects of high- and low-frequency neuromuscular electrical stimulation on muscle function and quality

Abstract

This study investigated the frequency-specific effects of high- and low-frequency neuromuscular electrical stimulation (NMES) on neuromuscular function and muscle quality. Sixteen healthy young males were randomly assigned to either a high-frequency stimulation group (HFES, 100 Hz; n=8) or a low-frequency stimulation group (LFES, 50 Hz; n=8) for 6 weeks of NMES intervention. Neuromuscular function was assessed using maximum voluntary isometric contraction (MVIC), while muscle quality of the rectus femoris (RF) and vastus lateralis (VL) was evaluated via ultrasound echo intensity (EI). Assessments were taken at baseline, midintervention (week 3), and postintervention (weeks 6, 8 and 10). HFES significantly increased MVIC during the intervention, indicating an immediate neuromuscular improvement. In contrast, LFES showed delayed effects, with significant EI improvements in RF and VL emerging only after the intervention ended. Statistical analysis revealed a significant interaction effect between time and intervention for MVIC and EI, highlighting the frequency-dependent nature of NMES adaptations. These results suggest that NMES induces neuromuscular adaptations in healthy adult males, with HFES promoting immediate gains and LFES leading to delayed benefits. Thus, frequency selection is critical in designing effective NMES protocols.

Keywords: Echo intensity; Muscle quality; Neuromuscular electrical stimulation; Neuromuscular function; Stimulation frequency.