The Effect of Vagus Nerve Stimulation on Lower Limb Muscle Strength and Balance in Nonelite Athletes

The vagus nerve (VN), the tenth cranial nerve, is a key component of the neuroendocrine-immune axis and plays a critical role in regulating autonomic functions such as cardiovascular, endocrine, respiratory, and digestive system activity through its extensive central and peripheral connections (1,2). Non-invasive vagus nerve stimulation (VNS), particularly through auricular stimulation, has been shown to modulate autonomic balance by reducing sympathetic activity, enhancing cardiac baroreflex sensitivity, and improving parasympathetic tone (3,4).

In the context of athletic performance, the autonomic nervous system's role in adaptation to training and recovery is well recognized (5). Post-exercise parasympathetic reactivation is increasingly used as a biomarker for recovery and performance enhancement. Auricular VNS (aVNS), by stimulating afferent auricular branches of the VN, may facilitate recovery by reducing exercise-induced fatigue, pain, and lactate accumulation, while improving parasympathetic activity (3). Importantly, these physiological effects have been observed without significant alterations in heart rate or blood pressure.

Balance training is a known contributor to improved functional performance in athletes, and neuromuscular control is essential for injury prevention and overall athletic capacity (6). Considering the limited research on the effects of aVNS on neuromuscular parameters such as muscle strength and balance response in non-elite athletic populations, this study aims to provide evidence on the efficacy of aVNS in this context.

This study is designed as a non-randomized controlled trial. Thirty non-elite athletes aged 18-35 years, who have been physically active for at least three months and currently exercise 2-5 days per week (averaging 3.2 hours per week), will be recruited. Eligible participants must have a BMI between 18.5 and 25 kg/m² and provide informed consent. Individuals with conditions affecting the inner or outer ear or BMI values outside the specified range will be excluded.

The intervention group will receive a single session of bilateral aVNS using a biphasic, asymmetric waveform at a frequency of 25 Hz, pulse width of 300 microseconds, applied continuously for 20 minutes. The post-intervention follow-up period will last 30 minutes (3).

Assessment Parameters

Demographic and Baseline Characteristics:

Collected via a structured interview, including age, gender, height, weight, weekly exercise volume, and auditory health status.

Balance Assessment:

Balance will be evaluated using the Biodex Balance System, which quantifies postural sway and center of pressure (COP) metrics. Tests will be performed under both eyes-open and eyes-closed conditions, with three trials conducted for each. Lower sway index values indicate better balance performance (7).

Muscle Strength Assessment:

Maximal isometric strength of the quadriceps femoris and hamstring muscles will be measured using a Hand-Held Dynamometer (HHD). For quadriceps testing, participants will be seated with knees flexed to 90°, and the dynamometer placed 1-2 cm above the malleoli. For hamstring assessment, participants will lie prone with knees flexed to 90°. The "make test" method will be used, where the participant applies maximal voluntary contraction against a stationary dynamometer held by the examiner. Each muscle group will be tested three times with 60-second rest intervals, and the average value will be used for analysis (8).