Author(s): CHEN, C., YU, T.A., CHEN, L.Y., WANG, C.K., HE, C.S., Institution: NATIONAL CHUNG CHENG UNIVERSITY, Country: TAIWAN, Abstract-ID: 1256

INTRODUCTION:
Isometric wall squat exercise effectively reduces blood pressure and promotes autonomic adaptations, combining minimal equipment requirements with high time efficiency, enabling wide applicability across various settings. (1). Ultraviolet A (UVA) from natural sunlight can enhance skin-derived nitric oxide (NO) release via non-enzymatic mechanisms, increasing its systemic bioavailability, which may influence neuromuscular function due to the role of NO in neurotransmission and synaptic plasticity. (2, 3). This study examined the effects of a four-week isometric wall squat intervention with natural sunlight exposure on muscle strength and NO production.
METHODS:
Sixty healthy adults (30 men, 30 women; age: 20.92 ± 2.3 years; BMI: 22.24 ± 2.8 kg/m²) were randomly assigned to an outdoor, indoor, or control group. Pre- and post-intervention assessments included body composition, blood NO, an incremental isometric wall squat test, and an isometric mid-thigh pull (IMTP) test for peak force and rate of force development (RFD) at 50 and 200 ms. The wall squat test consisted of five 2-min stages with a 10° reduction in knee angle per stage until task failure, during which peak heart rate (HR_peak) and wall squat duration were recorded. The 4-week training program (three sessions per week) comprised 4 × 2-min isometric wall squats performed at 95% HR_peak. The outdoor group trained outdoors with ~25% of their body surface area exposed to sunlight at one-quarter of the minimal erythema dose (MED) without sunscreen, the indoor group trained indoors without sunlight, and the control group received no intervention. Statistical analysis was conducted using a two-way mixed-design analysis of variance.
RESULTS:
Significant group × time interactions were observed for IMTP peak force, 200 ms rate of RFD, and blood NO concentration. Following the intervention, the outdoor group demonstrated significant increases in IMTP 200 ms RFD and blood NO concentration (p < .05). IMTP peak force increased significantly in both the outdoor and indoor groups (p < .05), with greater improvements observed in the outdoor group. Heart rate responses during the wall squat test were significantly reduced (p < .01), and wall squat duration was significantly longer in both the outdoor and indoor groups compared with the control group (p < .01). The average daily ultraviolet exposure was 11.25 ± 1.83, 2.70 ± 2.47, and 2.95 ± 3.08 mJ/cm² for the outdoor, indoor, and control groups, respectively.
CONCLUSION:
Four weeks of isometric wall squat exercises effectively improved lower-limb muscle strength and wall squat endurance performance. When combined with natural sunlight exposure, ultraviolet radiation may promote skin-derived NO release and enhance its systemic bioavailability, thereby modulating neuromuscular adaptations. Moderate sunlight exposure may confer physiological benefits and merits consideration in future exercise and health promotion strategies.