Author(s): SHINYA, M., SOICHI, A., TAKANOBU, O., AI, H., HIROYASU, M., AYUMU, S., DAICHI, T., RINA, S., MASAYOSHI, H., TATSUKI, F., TAKEAKI, M., SHIGEKI, S., Institution: KYORIN UNIVERSITY, Country: JAPAN, Abstract-ID: 1226

INTRODUCTION:
Neuromuscular electrical stimulation (NMES) has been reported to improve maximal oxygen uptake (VO₂max) in both clinical populations and healthy individuals. Previous studies have proposed different physiological mechanisms for this improvement, including central cardiovascular adaptations as well as peripheral morphological and metabolic changes. However, most evidence supporting peripheral mechanisms has been derived from sedentary or clinical populations, and it remains unclear whether NMES-induced improvements in aerobic capacity among healthy young adults are accompanied by changes in skeletal muscle mass. Therefore, this study aimed to investigate the effects of NMES on VO₂max in healthy young adults and to determine whether improvements in aerobic capacity are accompanied by morphological muscle adaptations.
METHODS:
A randomized crossover trial was conducted in sixteen healthy young adults (15 males). Participants completed 12-week NMES and control conditions separated by a 12-week washout period. NMES was delivered daily for 60 minutes using a belt electrode–skeletal muscle electrical stimulation system applied to the lower limbs. VO₂max was assessed using a cardiopulmonary exercise test. Skeletal muscle morphology was assessed using magnetic resonance imaging . Axial images perpendicular to the longitudinal axis of the femur were obtained at 50% of femoral length, and cross-sectional area (CSA) of the quadriceps femoris and its individual muscle components was quantified. Changes in VO₂max and CSA were analyzed using a two-way analysis of variance with factors of condition (NMES vs. control) and time (pre vs. post).
RESULTS:
VO₂max showed a significant condition × time interaction (p < 0.05). Specifically, VO₂max increased after NMES from 32.8 ± 5.6 to 34.9 ± 6.0 mL·kg⁻¹·min⁻¹, whereas no improvement was observed in the control condition, changing from 35.8 ± 9.2 to 34.7 ± 6.9 mL·kg⁻¹·min⁻¹. Total quadriceps femoris muscle CSA increased after NMES from 61.5 ± 9.5 to 63.7 ± 9.1 cm², while no change was observed in the control condition, changing from 59.8 ± 19.8 to 59.7 ± 19.7 cm². The condition × time interaction for total quadriceps femoris CSA was not significant. In contrast, vastus lateralis CSA showed a significant condition × time interaction (p < 0.05). Vastus lateralis CSA increased after NMES from 23.0 ± 4.2 to 24.2 ± 3.4 cm², whereas no change was observed in the control condition, changing from 24.7 ± 3.7 to 24.5 ± 3.6 cm².
CONCLUSION:
NMES improved VO₂max in healthy young adults and induced localized hypertrophy within the quadriceps femoris, particularly in the vastus lateralis. These findings suggest that peripheral muscle adaptations are likely contributors to NMES-induced improvements in aerobic capacity.