Effects of electrical stimulation on neuromuscular efficiency, hypertrophy, and electromyographic activity in strength training practitioners

Abstract

Background: Neuromuscular Electrical Stimulation (NMES) has been widely used as a complementary strategy in rehabilitation and strength training; however, the segmented neuromuscular and morphological responses of individual quadriceps muscles when NMES is combined with resistance training remain insufficiently explored. Objective: To investigate the effects of 12 weeks of NMES combined with strength training on maximal isometric force, neuromuscular efficiency, segmented electromyographic activity, and muscle thickness of the quadriceps in resistance-trained individuals. Methods: Twenty resistance-trained adults (18–35 years) were randomly allocated into two groups: NMES combined with strength training (EES) and strength training only (CONT). Both groups performed leg extension training twice weekly for 12 weeks. The EES group additionally performed 40 maximal bilateral isometric contractions (7 s on/7 s off) using the Compex® 8.0 “Strength” program. Maximal voluntary isometric contraction (MVIC) was assessed via dynamometry. Surface electromyography (EMG) was recorded from Vastus Medialis (VM), Vastus Lateralis (VL), and Rectus Femoris (RF), analyzing Root Mean Square (RMS), Median Frequency, and Central Frequency. Muscle thickness was measured using B-mode ultrasonography. Results: The EES group demonstrated a significant increase in MVIC (66.29 ± 13.61 kg to 72.48 ± 13.37 kg; p = 0.012), while no significant change was observed in CONT. Significant reductions in mean RMS were found in VM (p = 0.048) and RF (p = 0.0397) in EES, despite increased force output, indicating improved neuromuscular efficiency. Spectral variables remained stable (p > 0.05), suggesting absence of neuromuscular fatigue. Ultrasonography revealed increased quadriceps muscle thickness in EES. Conclusion: NMES combined with resistance training promoted significant improvements in maximal strength, neuromuscular efficiency, and muscle hypertrophy, indicating synergistic neural and morphological adaptations in trained individuals.