Therapeutic Effects of Capacitive and Resistive Monopolar Radiofrequency on Muscle Stiffness and Local Hemodynamics of the Rectus Femoris: A Randomized Crossover Study of Female University Soccer Players

Background: Capacitive resistive monopolar radiofrequency (CRMF) improves muscle extensibility and local tissue oxygen saturation in the trunk region. However, its effects on the lower extremities, particularly the rectus femoris, remain unclear. Purpose: To examine the immediate effects of 448-kHz CRMF on muscle stiffness and hemodynamics of the rectus femoris in female collegiate soccer players. Study Design: Randomized crossover study. Methods: Healthy female collegiate soccer players from a university team were recruited. Those with a history of thigh muscle injury or orthopedic disorders such as fractures within the previous year, any history of neurological or cardiac disease, and presence of a cardiac pacemaker were excluded. Each participant received three interventions (CRMF, hot pack, and sham) in randomized order with ≥2-day washout periods. Muscle stiffness (kPa) was assessed using shear wave elastography; hemodynamics (oxygenated hemoglobin [Oxy-Hb], deoxygenated hemoglobin [deOxy-Hb], total-Hb, and tissue oxygen saturation [StO₂]) were measured using near-infrared spectroscopy at depths of 1.5 and 3.0 cm. Measurements were performed at pre-intervention, immediately post-intervention, and 30 min post-intervention. Results: 18 female soccer players (mean age ± standard deviation, 20.2 ± 1.4 years) participated in and completed the study protocol. CRMF significantly reduced rectus femoris stiffness immediately post-intervention in the stretch position compared with hot pack (p < 0.001) and sham (p = 0.006). In the resting position, stiffness did not differ significantly across conditions. At 3 cm depth, CRMF and hot pack significantly increased Oxy-Hb immediately compared with sham (p = 0.008 and 0.025, respectively). CRMF also exhibited higher total-Hb at 3 cm immediately (p = 0.028). StO₂ at 3 cm was higher with CRMF than with the hot pack immediately (p = 0.002) and sham at 30 min (p = 0.030). Conclusions: CRMF reduced rectus femoris muscle stiffness in the stretched position and increased deep-tissue Oxy-Hb, total-Hb, and StO₂ in female collegiate soccer players. Effects were immediate, and some hemodynamic responses persisted up to 30 min, suggesting that CRMF may be an effective thermotherapy for improving deep-tissue oxygenation and reducing muscle stiffness in sports and rehabilitation settings. Level of Evidence: 2.