Topical Methyl Salicylate Effects on Skin

Participants were asked not to take any food or drinks (other than water) and to avoid moderate to vigorous physical activity until 2 hours before to the measurements to minimize potential confounding effects on the investigated outcomes. It was advised not to shower or use any body lotion on their back in the morning before the measurement to avoid interference with other topically applied agents. Body height (GPM Stadiometer, Zurich, Switzerland) and body weight were assessed using Tanita body fat scale (TBF-611, Tokyo, Japan), and body mass index (kg/m²) was calculated. Lower body fat percentage was estimated using Tanita body fat scale (TBF-611, Tokyo, Japan). Participants were asked to lie in prone position in their underwear. This position should be as comfortable as possible and should not be changed. The skin area of the thoracic spine was rinsed with distilled water (room temperature), using a soft cotton cloth. Two skin areas (5 cm × 10 cm) were marked on each side of the thoracic spine (Th4-Th7) with elastic strips (Leukotape K BSN medical, Hamburg, Germany). Subcutaneous adipose tissue layer at the investigated paravertebral skin areas was determined using ultrasound (MyLab Class C, Esaote, Genoa, Italy) and analyzed with OsiriX Lite software (Pixmeo SARL, Osirix V.8.0.2. Bernex, Switzerland). The allocation of the methyl salicylate and placebo gel applications to the left or right side of the thoracic spine was randomized by the researcher drawing lots (single-blinded). The participants were asked to remain still and to limit their speech to only what was necessary. To avoid any cooling effects through air circulation, all windows and doors were closed and investigators minimized their movements. The environmental conditions in the laboratory were controlled (Voltcraft MT52 digital multimeter, Hirschau, Germany) and kept constant during all experiments (room temperature: 23.3 ± 0.9°C; relative humidity: 39.2 ± 0.8%). During an acclimatization period of 20 min, the participants adapted to laboratory environment, to achieve stable and accurate outcome measurements. To ensure consistent experimental conditions, the methyl salicylate and placebo gel were applied simultaneously. Methyl salicylate and placebo gel were weighed (Kern 770 precision scale, Balingen, Germany), 1 g each, and applied simultaneously paravertebrally by two investigators wearing sterile surgical gloves with a circular motion of the index finger. Consistent application conditions on both sides were maintained by asking the participants to subjectively rate the pressure applied on each side. In addition, the two investigators visually monitored the execution of similar circular motions with the index fingers. Topical methyl salicylate application was expected to have mainly local effects to the site of application (Green and Flammer, 1989 (link)). T0 was considered to be the time when both gels had been completely rubbed into the skin. Subsequent measurements were taken at 5-min intervals over a period of 45 min (T5 to T45). The same sequence of assessments was performed at each measurement time point: (1) MC_skin, (2) T_skin, (3) thermosensation and (4) SmO₂. Due to technical issues during data collection three participants had to be excluded from the analysis. During analysis of SmO₂ data, one outlier was identified with consistently lower values (>1.5 x IQR) in both placebo and methyl salicylate and was excluded from the SmO₂ dataset. The experimental protocol was otherwise carried out without any deviation.