Bioceramic arm garment does not increase diameter of brachial artery and blood flow—Compliance and biceps microcirculation in male recreational sports practitioners at rest: A single blinded randomized cross-over study

Highlights

• •Use of a far-infrared (FIR) reflective arm garment for 40-min at rest in physically active men did not induce significant effects on brachial artery diameter, carotid–radial pulse wave velocity, tHb, HHb, or O₂Hb and skin temperature.
• •Under the conditions of our study (i.e., at rest in physically active men), the FIR arm garment are likely ineffective in improving micro- and macrovascular function.
• •From a practical standpoint, this technology is intended to be used during exercise or recovery. Based on our experimental design, it appears that the FIR arm garment would be ineffective in inducing any vascular benefit at rest if worn for 40 min before exercise and during the recovery period, when hemodynamic is stabilized.

Abstract

Background

Interest in clothing that reflects far infrared (FIR) radiation naturally emitted by the body during and after exercise has increased based on the assumption that this can increase arterial and venous blood flow. Indeed, in vitro and animal model research seems to report promising effects of FIR on the nitric oxide pathway and microcirculation. However, to date, there are no well controlled studies investigating the effect of FIR garments on resting microvascular and macrovascular function in humans. Thus, the aim of this study was to examine the acute effects of wearing FIR arm garments on vascular function.

Methods

Thirty-one male recreational sport practitioners (4.9 ± 3.3 h of sport/week; 32.1 ± 9.5 years; 178.6 ± 7.9 cm; 74.1 ± 11.2 kg; 23.2 ± 2.5 kg/m² (mean ± SD)) completed 4 visits: repeatability measurements (2 sessions), placebo, and FIR conditions in random order. Measurements (i.e., without arm garment) of brachial artery diameter and blood flow, carotid-radial pulse wave velocity (CR-PWV), as well as total-, oxy-, and deoxy-hemoglobin were completed after 15 min of rest and repeated when wearing FIR or placebo arm garments for 25 min, 40 min, and 50 min. Two skin sensors were positioned on the upper arm and the forearm to continuously record skin temperature and pressure under the garment.

Results

The main results were that at all time points, compared to placebo, FIR did not significantly affect brachial artery diameter and blood flow, CR-PWV, or total hemoglobin (condition × time interaction: p = 0.22, 0.54, 0.51, 0.96, respectively). Moreover, no significant condition × time × sensor position interaction effect was found in skin temperature (p = 0.99). However, pressure under garment was significantly higher under the FIR condition compared to placebo (+53%, p = 0.01, 95% confidence interval: FIR: 1.64–2.92 mmHg; placebo: 0.88–2.14 mmHg) while microvascular parameters were unchanged.

Conclusion

Contrary to our hypothesis, wearing an FIR arm garment at rest does not lead to an improvement of either macro or microvascular function. These results suggest limited benefits in the sports context, notably during recovery.

Graphical abstract

1. Introduction

Enhancing vascular and microvascular functions is important for athletes, given its impact on hemodynamics and venous oxygen content.¹ Ergogenic strategies (e.g., compression garment) to increase arterial and venous blood flow during and after exercise have recently gained interest.^2,3 This is also the case in relation to far infrared (FIR) technologies,⁴ which are products that use polymer coatings or nanoparticles of mineral oxides (sodium oxide, silica oxide, and aluminum oxide) imbedded directly within clothes to reflect FIR electromagnetic radiation naturally emitted by the body.

In vitro or in animal models, studies using FIR technologies reported promising vascular effects (e.g., increase in endothelial cell function) at rest.⁵ A typical example involved rats kept on a sericite flooring (FIR re-emitting technology) for 7 days, which demonstrated an eight-fold increase in blood nitric oxide (NO) concentration and a two-fold increase in endothelial nitric oxide synthase (eNOS) expression.⁶ Unfortunately, potential confounders, such as environmental and skin rat temperatures, were not controlled in this study. Specifically, it is known that rat skin temperature increases during 30 min of FIR electric generator exposure.⁷ Similar to the results in animal models, an increase in ·NO after FIR exposure (30 min of electric FIR generator) was also reported in cultured bovine aortic endothelial cells⁸ and human umbilical vein endothelial cells.⁹ This increase of ·NO in human umbilical vein endothelial cells lasted 24 h, 48 h, and 72 h after FIR exposure.⁶ In vitro and animal-based model research seems to report promising effects of FIR on the nitric oxide pathway and microcirculation.

To date, the effect of FIR garments on resting vasculature in humans has been shown to enhance peripheral blood flow or skin and muscle tissue oxygenation.⁴ Indeed, FIR re-emission induced by the garment has been shown to increase systemic arterial oxygen saturation and transcutaneous oxygen pressure.⁹ In 9 participants (5 women and 4 men) who wore 9 ceramic disks, as opposed to plastic ones, an increase in skin forearm blood-flow was reported.¹⁰ Moreover, a likely increase compared to control (qualitative magnitude-based inference method, p = 0.269) in microvascular blood flow 62 h after wearing FIR socks has been reported in elderly people.¹¹ Although these results illustrate a possible effect of FIR on microvascular function under real-life conditions, experimental limits were apparent, including no skin temperature control, no control of pressure under clothes, report of the typical error of measurement, and small sample size. Moreover, none of these studies have investigated the effect of FIR garments on macrovascular function.

Before recommendations can be made regarding their effectiveness in the sport and rehabilitation environments, more rigorous evaluations of these garments is needed, specifically in relation to their impact on macrovascular and microvascular functions. Therefore, the aim of this study was to examine the acute (i.e., 40 min) effect of wearing a bioceramic arm cuff (Stimcare-eNOsynthex™, Tortel Industries, Dieulefit, France) on macrovascular and microvascular function, hemodynamics, and arterial stiffness during supine rest in healthy men. A placebo-controlled, randomized, cross-over study was completed. We hypothesized that FIR garments would induce brachial artery vasodilation and a decrease in arterial stiffness due to reductions in vascular tone. In addition, we further hypothesized that FIR would increase microcirculatory blood flow of the biceps muscle tissue.

2. Methods

2.1. Participants

This study was approved by an ethical committee (Comité de protection des personnes Est III, 2023-A01567-38) and conducted according to the Declaration of Helsinki. The inclusion criteria were: male between 18 years and 45 years of age and practicing sport at least 1 h/week. The non-inclusion criteria were: individuals with cardiovascular disease or cardiovascular disease risk factors, individuals with chronic kidney disease, individuals using any medication that could influence the cardiovascular and autonomic nervous system. Data were removed from the dataset when participants did not follow the procedures outlined in the instructions provided to them. This included arriving at the laboratory in a fed state, having consumed caffeine during the preceding 24 h, having taken vitamins or consumed alcohol in the preceding 12 h, or having smoked during the preceding 6 h.¹² No female participants were included because of the confounding influence of monthly physiological and hormonal changes, specifically on macrovascular function.¹³ Participants were informed of the benefits and risks of the investigation prior to signing an institutionally approved informed consent document to participate.

2.2. Overview

Participants completed 4 sessions separated by 1–7 days (Fig. 1): 2 sessions were dedicated to assessing between-session variability and reliability (see Section 2.7.) and 2 sessions were dedicated to the evaluation of bioceramic arm garment FIR vs. placebo. Nine participants completed a within-session reliability evaluation consisting of 3 baseline measurements (Fig. 1; Baseline, R₁, and R₂). The sessions were randomized, while the pressure under arm cuff and skin room temperatures were monitored. All measurements were performed under standardized conditions in an air-conditioned room (22.9°C ± 0.5°C; 61.1% ± 4.2% humidity (mean ± SD)) at the same time of the day.

Fig. 1.

Representative diagram of the study protocol. Each blue arrow represents brachial artery diameter, carotid-brachial pulse wave velocity, tissue oxygenation, and skin temperature measurements (±5 min). BP = blood pressure measurement; FIR = far-infrared; HR = heart rate; M₁ = 1st measurement; M₂ = 2nd measurement; M₃ = 3rd measurement; Placebo = placebo arm cuff; R₁ = 1st repeated measurement; R₂ = 2nd repeated measurement; T° = room temperature measurement.

All sessions were performed with participants in a resting supine position on a physiotherapist’s folding table (SISSEL-Basic, Coueron, France) with the non-dominant arm extended at an angle of ∼80° abduction. All measurements except blood pressure (Fig. 2B) were made on the non-dominant arm. The non-dominant arm was defined as the arm not used to write. All participants were right-handed.

Fig. 2.

(A) Photo of arm garment; (B) Schematic representation of experimental set-up. a = position of skin temperature sensor; b = position of pressure measurement under garment; c = position of skin temperature sensor and pressure measurement under garment; FIR = far-infrared; NIRS = near infrared spectroscopy.

Participants were instrumented during an initial 15-min rest period (i.e., Baseline period, Fig. 1). All sensors and skin probe locations were marked with black permanent marker, and participants were encouraged to retrace the marks when they faded between the 2 testing sessions (Fig. 2). Then, the pressure exerted upon the skin by the placebo or FIR arm garment was measured before quickly removing them (<1 min) (Fig. 1). Afterward, blood pressure (BP) and heart rate (HR) were recorded twice (after 10 min and 13 min of rest; Fig. 1) using an automatic sphygmomanometer (M6 AC; OMRON Healthcare, Kyoto, Japan). The average of the 2 measurements was used to determine the brachial systolic blood pressure (SBP) and diastolic blood pressure (DBP) as well as HR. Room temperature was measured concomitantly to these measures (Kestrel 5500; Kestrel Instruments, St. Louis, MO, USA). Following this 15-min period, baseline measurements (Baseline; Fig. 1) without garments were completed: (a) 60-s brachial artery diameter and blood velocity was obtained using Doppler ultrasonography; (b) continuous recording of absolute and relative tissue oxygenation was obtained using near-infrared spectroscopy (NIRS); (c) carotid-brachial pulse wave velocity (CR-PWV) was obtained using piezoelectrical sensors. Next, either the FIR or placebo garment was donned, and the above-described measurements were repeated at 25 min (M₁), 40 min (M₂), and 50 min (M₃), with measurements of SBP, DBP, HR, and room temperature obtained before each time point (Fig. 1).

2.3. Arm garment: characteristics and pressure and temperature measurements

The FIR technology consisted of an arm cuff incorporating a mixture of natural minerals (eNOsynthex™: titanium aluminum, zirconium; 30 g/m²) below a synthetic membrane. The characteristics (emissivity: 156 w/m², reflected 95% infrared radiation between 2.5 µm and 19.0 µm) of the FIR re-emission were tested by an independent institute of measurement (Sigvaris, Saint-Just-Saint-Rambert, France). An incision (large = 5 cm; height = 8 cm) was made one-third of the distance from the upper edge of the sleeve to create a window to enable Doppler ultrasound measures at the brachial artery. The size of the FIR and placebo garments was selected according to biceps and forearm circumferences, following manufacturer guidelines.

Pressure under the FIR garment was measured in the supine position (Millar Mikro-Cath®; Millar, Houston, TX, USA) in the middle of the upper arm, just under the belly of the biceps and in the middle of the lateral face of the forearm (Fig. 2B). The pressure sensor was calibrated before each measurement according to manufacturer recommendations.

The skin temperature underneath the garment was measured with temperature sensors (eFlex; BodyCAP, Hérouville-Saint-Clair, France) placed just distal to the insertion of the deltoid and at the same position where pressure was measured (Fig. 2B). Sensors were secured with 5-mm adhesive tape (Strappal®; BSN Medical, Vibraye, France) to prevent movement and minimize the increase in temperature. Skin temperature was averaged over 5 min at Baseline, M₁, M₂, and M₃.

2.4. Doppler ultrasonography

Non-dominant brachial artery diameter and blood velocity signals were obtained (Vinno 5; Vinno, Suzhou, China) by the same trained investigator following recommendations.^12,14 Recordings of all vascular variables were analyzed offline using specialized edge-detection software (Cardiovascular Suite; Quipu, Pisa, Italy). Mean blood velocity was calculated by subtracting mean negative velocity from the mean positive velocity.¹⁵ Blood flow was calculated according to Jenderka and Delorme¹⁶ Mean diameter and blood flow of the brachial artery were calculated over the 30 s with the smallest standard deviation within the 1-min recording.

2.5. NIRS

A NIRS probe (PortaLite; Artinis Medical System, Einsteinweg, the Netherlands) was placed on the belly of the biceps muscle 3 cm proximal to the elbow axis rotation and was secured with a black elastic strap lightly tightened to prevent movement and minimize intrusion of extraneous light. Tissue oxyhemoglobin (O₂Hb) and deoxyhemoglobin (HHb) and total hemoglobin (tHb = O₂Hb + HHb) of the biceps muscle was monitored over 240 s, at a sampling rate of 10 Hz, with a differential pathlength factor of 4.0.¹⁷ Before data analysis, data from 5 s to 125 s was extracted from the recording, and a 10th-order Butterworth low-pass filter (cutoff frequency = 0.1 Hz) was used to remove possible artifacts.¹⁸ Considering that participants were in a quiet resting position, a SD of tHb >2% was defined as abnormal and data were removed. The average of the 120 s was reported for relative O₂Hb, HHb, tHb, and tissue saturation index (TSI) at each measurement time point (i.e., Baseline, M₁, M₂, and M₃).

2.6. Pulse wave velocity using tonometry

For all measures of pulse wave velocity, piezoelectric sensors were placed on the right side of the neck superficial to the common carotid artery and at the level of the radial artery near to the wrist by the same trained investigator. The distance between carotid and radial sensors was measured using measuring tape, and CR-PWV was recorded and calculated using an automated device (Complior; ALAM Medical, Saint-Quentin-Fallavier, France) using an intersecting tangent algorithmic method, as described elsewhere.¹⁹

A difference between 2 datasets >2.0 m/s could not be due to a normal physiological variation or the intervention (i.e., arm cuff), so the dataset furthest from the mean was removed (due to technical error).²⁰ If there was a difference >2.0 m/s between 2 datasets but there was no difference compared to another dataset, we averaged the two closest datasets (without difference >2.0 m/s). This was considered to be the reference and was compared to the other values.

2.7. Statistical analysis

All statistical analyses were performed using R 4.3.1 software and the Rstudio interface (open source, https://posit.co/downloads/). Brachial artery diameter, CR-PWV, and NIRS between-session variability and reliability were assessed using coefficient of variation and intraclass correlation coefficient (ICC), respectively, from the 4 baseline measures: Baseline session 1 = Baseline₁; Baseline session 2 = Baseline₂; Baseline session 3 = Baseline₃; Baseline session 4 = Baseline₄. Within-session variability and reliability were calculated using the same parameters from three baseline measures on the same day (i.e., Baseline, R₁, and R₂). Two-way mixed-effects models (type = agreement) were used to calculate ICCs (package: psych) for within and between-session reliability.²¹ Typical error of measurement from each variable was also calculated²² (Supplementary Material 1) to take into account the variability due to machine calibration, human error, or day-to-day biological variability.²³

For all variables, a linear mixed model (LMM) was used as it considers that all participants are not the same.²⁴ Moreover, LMMs are more robust for small sample size datasets and the obligation to respect all statistical assumptions.²⁵

An LMM (package: lme4) was used to assess all dependent variables. The fixed effects were conditions (FIR, placebo, time (Baseline, M₁, M₂, and M₃)), position of skin temperature sensor (Fig. 2B, Positions a and c) and pressure measurement (Fig. 2B, Positions b and c), while participant was a random effect. The level of significance of interactions and main effects were calculated using analysis of variance with Type 3 Wald Chi-square tests with Kenward-Roger degrees of freedom approximation (package: stats) on the LMM output. p value, partial eta-squared (${\eta }_{\text{p}}^2$) for analysis of variance, and F ratios were reported (packages: effectsize). If there was a significant interaction or main effect, post hoc analyses were performed using multiple pairwise comparisons with a Holm correction (package: emmeans). The significance level was p < 0.05. Data are presented as mean ± SD of absolute values. The level of significance (p value) and effect size (dz)²⁶ are reported for each comparison.

3. Results

Thirty-one male recreational sport practitioners (4.9 ± 3.3 h of sport per week; 32.1 ± 9.5 years; 178.6 ± 7.9 cm; 74.1 ± 11.2 kg; 23.2 ± 2.5 kg/m²) from the local population completed the study. Fourteen practiced sports involving uniquely lower limbs, 13 practiced sports involving lower and upper limbs, and four practiced sports involving uniquely upper limbs. Eight were regular smokers (7.4 ± 2.3 cigarettes/day).

For ultrasound Doppler, NIRS, and PWV recordings, the total number of measurements was 124 for both placebo and FIR conditions. Six placebo and 7 FIR Doppler scans did not meet the methodological criteria (see Section 2.4) and were hence removed. Also, 6 placebo and 5 FIR PWV measurements were removed because of too much variability (see Section 2.6) compared to other values. Two placebo and 4 FIR NIRS recordings and one skin sensor measurement were removed due to technical issues. Variability and reliability results are presented in Supplementary Material 2.

3.1. Doppler ultrasonography

The results for brachial artery diameter are presented in Fig. 3A. There was no condition × time interaction (F(3, 165.1) = 1.47, p = 0.22, ${\eta }_{\text{p}}^2$ = 0.03), nor condition (F(1, 29.9) = 0.85, p = 0.36, ${\eta }_{\text{p}}^2$ = 0.03) or time effect (F(3, 165.2) = 0.61, p = 0.60, ${\eta }_{\text{p}}^2$ = 0.01).

Fig. 3.

Violin plots representing distribution and kernel probability density of (A) brachial artery diameter, (B) tHb, and (C) CR-PWV when wearing placebo or FIR arm cuff. Point and vertical range line represent mean and standard deviation, respectively. CR-PWV = carotid-radial pulse wave velocity; FIR = far-infrared; M₁ = 1st measurement; M₂ = 2nd measurement; M₃ = 3rd measurement; tHb = total hemoglobin.

The results for blood flow are presented in Table 1. There was also no condition × time interaction (F(3, 165.4) = 0.73, p = 0.54, ${\eta }_{\text{p}}^2$ = 0.01) or condition effect (F(1, 29.9) = 1.87, p = 0.18, ${\eta }_{\text{p}}^2$ = 0.06). However, there was a time effect (F(3, 165.5) = 19.72, p < 0.001, ${\eta }_{\text{p}}^2$ = 0.26). Post hoc analyses (Table 2) showed a significant decrease between Baseline and M₁ (p < 0.05, dz = 0.40), Baseline and M₂ (p < 0.05, dz = 0.70), Baseline and M₃ (p < 0.05, dz = 0.87), M₁ and M₃ (p < 0.01, dz = 0.64), and M₂ and M₃ (p < 0.05, dz = 0.43), without significant difference between M₁ and M₂ (p = 0.10, dz = 0.35).

Table 1.

Mean and standard deviation of absolute value for all variables at Baseline, 1st (M₁), 2nd (M₂), and 3rd (M₃) measurement, either wearing placebo or far-infrared (FIR) arm garment.

Variable	Condition	Baseline	M1	M2	M3
Room temperature (°C)	Placebo	22.9 ± 0.6	22.8 ± 0.6	22.9 ± 0.5	22.9 ± 0.5
	FIR	22.9 ± 0.5	22.9 ± 0.5	22.8 ± 0.5	22.9 ± 0.6
Room air humidity (%)	Placebo	60.3 ± 3.3	61.0 ± 4.1	61.5 ± 4.4	61.4 ± 4.5
	FIR	60.8 ± 4.5	61.1 ± 3.9	61.5 ± 4.2	61.5 ± 3.9
SBP (mmHg)	Placebo	123 ± 12	123 ± 12	123 ± 11	124 ± 11
	FIR	122 ± 9	123 ± 10	123 ± 10	125 ± 10
DBP (mmHg)	Placebo	72 ± 9	74 ± 9	73 ± 9	74 ± 9
	FIR	72 ± 9	74 ± 8	74 ± 9	75 ± 9
HR (beats/min)	Placebo	56 ± 8	54 ± 8	53 ± 8	53 ± 8
	FIR	54 ± 7	54 ± 7	53 ± 7	53 ± 7
Blood flow (mL/min)	Placebo	170.3 ± 96.9	152.5 ± 73.1	144.6 ± 94.6	124.6 ± 86.5
	FIR	164.3 ± 87.6	142.9 ± 81.9	120.4 ± 58.5	122.9 ± 91.3
TSI (%)	Placebo	63.9 ± 2.2	64.1 ± 2.1	64.9 ± 2.1	65.2 ± 2.2
	FIR	63.8 ± 2.7	64.0 ± 2.7	64.6 ± 2.7	64.9 ± 3.0
O2Hb (ΜM)	Placebo	0.07 ± 0.65	−0.03 ± 0.54	0.17 ± 0.46	0.16 ± 0.42
	FIR	−0.02 ± 0.53	0.01 ± 0.43	0.12 ± 0.62	0.03 ± 0.52
HHb (ΜM)	Placebo	0.09 ± 0.61	0.07 ± 0.35	−0.11 ± 0.36	−0.08 ± 0.36
	FIR	0.16 ± 0.49	−0.07 ± 0.41	0.09 ± 0.53	0.11 ± 0.42

Abbreviations: DPB = diastolic blood pressure; HHb = deoxyhemoglobin; HR = heart rate; O₂Hb = oxyhemoglobin; SBP = systolic blood pressure; TSI = tissue saturation index.

Table 2.

Time variation of brachial artery blood flow, TSI, and HR.

Variable	Baseline	M1	M2	M3	p	${\eta }_{\text{p}}^2$
Blood flow (mL/min)	167.4 ± 91.7	147.5 ± 77.2*	133.5 ± 80.5*	123.7 ± 88.1*,#,†	<0.001	0.26
TSI (%)	63.9 ± 2.4	64.1 ± 2.4	64.8 ± 2.4*	65.0 ± 2.6*,#	<0.001	0.26
HR (beats/min)	55 ± 8	54 ± 8	53 ± 7*	53 ± 8*	<0.05	0.05

Note: Post hoc analysis with holm correction.

^⁎p < 0.05, significantly different from Baseline.

^#p < 0.01, significantly different from M₁.

^†p < 0.05, significantly different from M₂.

Abbreviations: HR = heart rate; M₁ = 1st measurement; M₂ = 2nd measurement; M₃ = 3rd measurement; ${\eta }_{\text{p}}^2$ = partial eta-squared, which corresponds to effect size for the main effect of time; TSI = tissue saturation index.

3.2. Tonometry

The results for CR-PWV are presented in Fig. 3C. There was no condition × time interaction (F(3, 164.5) = 0.77, p = 0.51, ${\eta }_{\text{p}}^2$ = 0.01), nor condition (F(1, 28) = 4.12, p = 0.13, ${\eta }_{\text{p}}^2$ = 0.13) or time (F(3, 164.5) = 1.75, p = 0.16, ${\eta }_{\text{p}}^2$ = 0.03) effect.

3.3. NIRS

The results for TSI, O₂Hb, and HHb are presented in Table 1. The results for tHb are presented in Fig. 3B. For TSI, there was no condition × time interaction (F(3, 204.1) = 0.16, p = 0.92, ${\eta }_{\text{p}}^2$ < 0.01) or condition effect (F(1, 204.2) = 1.07, p = 0.30, ${\eta }_{\text{p}}^2$ < 0.01), but there was a time effect (F(3, 204.1) = 6.38, p < 0.001, ${\eta }_{\text{p}}^2$ = 0.09). Post hoc analyses (Table 2) showed a significant increase between Baseline and M₂ (p < 0.05, dz = 0.54), Baseline and M₃ (p < 0.05, dz = 0.67), and M₁ and M₃ (p < 0.01, dz = 0.71), without a significant difference between Baseline and M₁ (p = 0.79, dz = 0.14), M₁ and M₂ (p = 0.07, dz = 0.69), and M₂ and M₃ (p = 0.78, dz = 0.26).

For tHb, there was no condition × time interaction (F(3, 205.3) = 0.88, p = 0.45, ${\eta }_{\text{p}}^2$ = 0.01) or time effect (F(3, 205.3) = 1.48, p = 0.22, ${\eta }_{\text{p}}^2$ = 0.02), but there was a condition effect (F(1, 205.9) = 0.10, p = 0.75, ${\eta }_{\text{p}}^2$ < 0.001).

For O₂Hb, there was no condition × time interaction (F(3, 204.8) = 0.26, p = 0.85, ${\eta }_{\text{p}}^2$ < 0.005), nor time (F(3, 204.8) = 0.34, p = 0.42, ${\eta }_{\text{p}}^2$ = 0.02) or condition (F(1, 205.3) = 0.83, p = 0.36, ${\eta }_{\text{p}}^2$ < 0.005) effect.

For HHb, there was no condition × time interaction (F(3, 204.7) = 2.00, p = 0.11, ${\eta }_{\text{p}}^2$ = 0.03), nor time (F(3, 204.7) = 1.31, p = 0.27, ${\eta }_{\text{p}}^2$ = 0.02) or condition (F(1, 205.1) = 2.12, p = 0.15, ${\eta }_{\text{p}}^2$ = 0.01) effect.

3.4. Room and skin temperature

Room temperature and air humidity data are presented in Table 1. There was no condition × time interaction (F(3, 210) = 0.32, p = 0.81, ${\eta }_{\text{p}}^2$ < 0.005), nor time (F(3, 210) = 0.14, p = 0.94, ${\eta }_{\text{p}}^2$ < 0.005) or condition effect (F(1, 210) = 0.09, p = 0.77, ${\eta }_{\text{p}}^2$ < 0.001) for room temperature. Similarly, for air humidity, there was no condition × time interaction (F(3, 210) = 0.09, p = 0.97, ${\eta }_{\text{p}}^2$ < 0.005), nor time (F(3, 210) = 1.22, p = 0.30, ${\eta }_{\text{p}}^2$ = 0.02) or condition effect (F(1, 210) = 0.11, p = 0.74, ${\eta }_{\text{p}}^2$ < 0.001).

The results for skin temperature are presented in Fig. 4. There was no condition × time × sensor position interaction effect (F(3, 383) = 0.02, p = 0.99, ${\eta }_{\text{p}}^2$ < 0.001), condition × time interaction (F(3, 383) = 0.14, p = 0.94, ${\eta }_{\text{p}}^2$ < 0.005), condition × sensor position interaction (F(1, 384.2) = 1.67, p = 0.20, ${\eta }_{\text{p}}^2$ < 0.005), and no main effect for condition (F(1, 29.3) < 0.001, p = 0.98, ${\eta }_{\text{p}}^2$ < 0.001). However, there was a significant time × sensor position interaction effect (F(3, 383) = 21.65, p < 0.001, ${\eta }_{\text{p}}^2$ = 0.14). Moreover, there was a significant main effect for time (F(3, 383) = 179.96, p < 0.001, ${\eta }_{\text{p}}^2$ = 0.58) and sensor position (F(1, 30) = 59.12, p < 0.001, ${\eta }_{\text{p}}^2$ = 0.66). At the forearm level, skin temperature was significantly higher during M₁, M₂, and M₃ compared to Baseline (p < 0.001, dz = 1.45, 1.17, and 0.84, respectively; Fig. 4) and significantly lower during M₃ compared to M₁ (p < 0.005, dz = 0.76; Fig. 4). At the upper arm level, skin temperature was significantly higher during M₁, M₂, and M₃ compared to Baseline (p < 0.001, dz = 1.78, 1.84, and 1.70, respectively; Fig. 4). Skin temperature was significantly higher at the forearm vs. upper arm position at each timepoint of measurement (p < 0.001, dz = 1.65, 1.18, 1.02, 0.90, respectively; Fig. 4).

Fig. 4.

Whisker plots representing from 1st to 3rd quartile of the forearm (C) and the insertion of deltoid (A) skin temperature under placebo or FIR arm cuff. Mean skin temperature and standard deviation are represented by a crossbar and an error bar respectively. Significant difference was observed between sensor Position c and Position a at each time (p < 0.001). Post hoc analysis with holm correction: * p < 0.001 significantly different from Baseline for sensor Position c; ^# p < 0.005 significantly different from M1 for sensor Position c; ^† p < 0.001 significantly different from Baseline for sensor Position A. FIR = far-infrared; M₁ = 1st measurement; M₂ = 2nd measurement; M₃ = 3rd measurement.

3.5. BP and HR

The results for SBP, DBP, and HR are presented in Table 1.

For SBP and DBP, there were no significant differences between placebo and FIR at any time of measurement. SBP exhibited no condition × time interaction (F(3, 210) = 0.3, p = 0.82, ${\eta }_{\text{p}}^2$ < 0.005) and no time (F(3, 210) = 0.68, p = 0.56, ${\eta }_{\text{p}}^2$ < 0.01) or condition (F(1, 210) = 0.02, p = 0.90, ${\eta }_{\text{p}}^2$ < 0.001) effects. DBP also exhibited no condition × time interaction (F(3, 210) = 0.14, p = 0.94, ${\eta }_{\text{p}}^2$ < 0.005) and no time (F(3, 210) = 2.37, p = 0.07, ${\eta }_{\text{p}}^2$ = 0.03) or condition (F(1, 210) = 0.44, p = 0.51, ${\eta }_{\text{p}}^2$ < 0.005) effects.

For HR, no condition × time interaction (F(3, 210) = 0.45, p = 0.71, ${\eta }_{\text{p}}^2$< 0.01) or condition effects (F(1, 210) = 2.83, p = 0.09, ${\eta }_{\text{p}}^2$ = 0.01) were observed, although there was a time effect (F(3, 210) = 3.39, p = 0.02, ${\eta }_{\text{p}}^2$ = 0.05). Post hoc analyses (Table 2) revealed that HR was significantly lower at M₂ compared to Baseline (p < 0.05, dz = 0.65) and at M₃ compared to Baseline (p < 0.05, dz = 0.67). No significant differences were observed between Baseline and M₁ (p = 0.43, dz = 0.36), M₁ and M₂ (p = 0.70, dz = 0.28), M₁ and M₃ (p = 0.70, dz = 0.31), and M₂ and M₃ (p = 0.99, dz = 0.07).

3.6. Pressure under garment

The pressure under garment at forearm and upper arm positions were as follows: placebo: 1.6 ± 1.5 mmHg and 1.5 ± 1.7 mmHg; FIR: 2.4 ± 2.0 mmHg and 2.2 ± 2.0 mmHg. There was no condition × pressure measurement position interaction (F(1, 61) = 0.40, p = 0.53, ${\eta }_{\text{p}}^2$ < 0.01) or position effect (F(1, 61) = 2.91, p = 0.09, ${\eta }_{\text{p}}^2$ = 0.05), but pressure under garment was significantly higher in the FIR condition compared to placebo (F(1, 30.3) = 7.45, p = 0.01, ${\eta }_{\text{p}}^2$ = 0.20; Table 3).

Table 3.

Condition variation of pressure under garment, O₂Hb, and HHb.

Variable	Placebo	FIR	p	${\eta }_{\text{p}}^2$
Pressure (mmHg)	1.5 ± 1.6	2.3 ± 2.0	0.01	0.20

Abbreviations: FIR = far-infrared arm cuff; HHb = deoxyhemoglobin; ${\eta }_{\text{p}}^2$ = partial eta-squared, which corresponds to effect size for the main effect of condition; O₂Hb = oxyhemoglobin; Placebo = placebo arm cuff.

4. Discussion

This study demonstrates that wearing an FIR arm cuff for 40 min does not significantly affect macrovascular (i.e., no significant brachial artery vasodilation or arterial stiffness reduction) or microcirculation (as assessed by NIRS) functions at rest in healthy men who practice recreational sports. Moreover, skin temperature was not significantly different with FIR compared to placebo, though pressure under the garment was higher with FIR than placebo. However, these potential confounding factors do not seem to influence vascular function.

To our knowledge, no previous study has assessed the effect of an FIR garment in healthy people using Doppler ultrasonography. Contrary to our hypothesis, no significant change of brachial artery diameter and blood flow, CR-PWV, blood pressure, or HR was observed while participants wore FIR garments compared to placebo. This means that at the macrovascular level, FIR has no effect. Our initial hypothesis was derived from previous reports showing that FIR increases ·NO concentration^6,8,27 and may induce vasodilatation^20,28 and decrease PWV.^20,28,29 Our unchanged vascular responses are nevertheless in accordance with previous studies on healthy subjects²⁷ and hemodialysis patients with peripheral artery occlusive disease³⁰ that reported no FIR effect on ankle-brachial index. A possible explanation is that the increase of ·NO using FIR is too small to reduce vascular tone. Indeed, an increase in ·NO concentration of approximately 5 µmol/L (i.e., ≈0.17 µg/L) following FIR exposure did not significantly change brachial-ankle index.²⁷ This is consistent with other findings of no change in brachial PWV after a small dose (i.e., 2.5 µg/min for 4 min) of nitroglycerin.³¹ This explanation remains nevertheless to be confirmed, as a brachial PWV decrease with as little as 0.03 µg/min of nitroglycerin has been reported²⁸ and since in vivo ·NO assessments remain highly challenging.³²

Another point regarding the absence of change in macrovascular function is that FIR likely exerts its effects only on superficial tissues.⁵ Indeed, an increase in forearm skin blood flow when wearing a ceramic disk was reported,¹⁰ as was a significantly larger increase in transcutaneous oxygen pressure when wearing an FIR T-shirt at rest.⁹ Also, a possible increase in calf microvascular blood flow at rest when wearing an FIR garment in elderly people (67.9 ± 4.2 years) has been reported.¹¹ In contrast, we did not observe any significant difference in TSI, tHb, HHb, and O₂Hb between placebo and FIR. Due to limited evidence and methodological differences,¹¹ the discrepancies between our and previous results require further study. It is known that tcPO₂ measurement primarily reflects the surface skin microcirculation, while NIRS provides microcirculation information from both superficial and deeper tissues.³³ Hence, we can hypothesize that FIR radiation probably does not penetrate beyond the skin and is probably too weak to induce significant effects on macrocirculation and microcirculation assessed by NIRS. Thus, further in situ investigations are required, with stronger ·NO dosages alongside macro- and microcirculatory measurements, as are comparisons of skin and muscle blood flow (e.g., laser doppler flowmetry vs. NIRS).

Previous studies examining FIR effects are derived from in vitro^8,34 or animal models^6,7 and/or present some limitations, including no control condition, no recording of pressure and temperature under the garment, and no reporting of the typical error of measurements.^9,11,27 For example, an increase in ·NO concentration in endothelial cells up to 72 h after exposure to a sericite reemitting board was reported⁶ but without control or recording of rat skin temperature. Also, Washington et al.⁹ did not record temperature under the T-shirt in their study. In studies conducted in vitro (i.e., bovine aortic endothelial cells) reporting an increase in relative ·NO release 30 min after using an FIR electric emitter,⁸ temperature was also increased; however, temperature is not always linked to increase in ·NO concentration.³⁴ Conversely, another study²⁷ reported in vivo increases in ·NO 14 days after participants had worn an FIR blanket; in this study, skin temperature was significantly increased 30 min after using the FIR or placebo blanket, but especially in the upper body (from thymus to the top of the head) with the FIR blanket. An increase in temperature could explain the increase in microcirculatory blood flow and the decrease in the affinity of oxygen for hemoglobin,³⁵ which could ultimately lead to an increase of tcPO₂.³⁶

In our study, we observed an increase of skin temperature under the garment but without significant differences between placebo and FIR conditions. This further supports our hypothesis that the FIR effect on vascular function is likely linked to temperature. It is also important to consider that the emittance and radiance of FIR radiation is proportional to the temperature of the tissue emitting this energy, and in this study it is the arm.^37,38 The characteristics of the arm garment used in our study (see Methods section) have been tested at 37°C. However, the mean temperature under the FIR garment was around 31°C–32°C (Fig. 3). Hence, in our in vivo situation, the FIR radiation power and hence the potential vascular effect of FIR reemitting garments would have been lower than expected. Further in vivo research is needed to understand how temperature influences FIR-induced vascular effects, what the timeline is for these effects, and any potential dose-response (i.e., total irradiated energy) effects. This is especially important in relation to the many different technologies and protocols being used (i.e., FIR electric emitter, FIR remitting garments, short- or long-term exposure).

4.1. Limitation

First, independent of the condition, we observed a decrease in blood flow and HR throughout the protocol, which could be explained by a slight whole-body cooling effect,³⁹ likely due to the difference between the outside (protocol was conducted in summer in the South of France) and air-conditioned room temperature, despite a prolonged rest period before starting the protocol. However, it seems that this did not significantly influence cardiovascular hemodynamics (i.e., HR, SBP, and DBP were not significantly affected); and, more importantly, no significant differences between FIR and placebo were observed for brachial artery diameter, blood flow, CR-PWV, and tHb measurement under arm cuff. Also, skin temperature under arm cuff displayed a similar increase at all times and under both conditions. Another point is that while pressure under the FIR garment was higher compared to placebo (Table 3), we did not observe any change in indices of microvascular function (i.e., NIRS parameters) between Baseline and M₁ and/or FIR and placebo conditions, which suggests that a pressure effect can be discounted. Finally, we observed an increase in TSI in both conditions, which could likely be explained by the arm position, which reduced venous return. However, this variation was small and close to typical error measurement (Table 1A and 1B in Supplementary Material 2) and, therefore, probably did not influence the results.

4.2. Practical applications

This FIR technology is intended to be used in sport practice or recovery. Considering our experimental design, it seems that FIR arm cuffs would be ineffective at inducing any vascular benefits if they were worn for 40 min before sport practice and during the recovery period when hemodynamics are stable. However, it is important to note that our study was conducted in the supine resting position, which is unlike sport situations or the acute sport recovery period. Thus, it may be important to confirm the lack of differences under more ecologically relevant recovery and sport routines in the field.

5. Conclusion

This study found that FIR arm garments do not alter brachial artery diameter, blood flow, CR-PWV, tHb, HHb, and O₂Hb when applied for 40 min, despite an increased temperature under garment, which was not different between conditions. Therefore, under the conditions of our study (i.e., resting condition, young male recreational sport practitioners), the FIR arm cuff is not likely effective at enhancing micro and macrovascular function. Further investigations are required to assess the FIR effects on vascular and skin vascular functions, notably with longer time exposures and under different conditions (i.e., electric emitter, garments, temperature, exercise) using well-controlled designs that incorporate ·NO measurements and functional measurements of the macro- and microvasculature.

Authors’ contributions

MC designed the study, recruited the participants, collected the data, performed the statistical analysis, interpreted the data, and drafted the manuscript; MR and AFG interpreted the data and critically revised the manuscript; LM coordinated the research, designed the study, interpreted the data, and critically revised the manuscript. All authors have read and approved the final version of the manuscript, and agree with the order of presentation of the authors.

Declaration of competing interest

MC received a research grant from FIR arm cuff manufacturer Stimcare (Dieulefit, France), as part of his “Convention industrielle de formation par la recherche (CIFRE)” thesis work. AFG and LM are supervising the project but do not directly receive any funding. However, EPSI (Besançon, France), represented by LM, bought the pressure sensor with a part of the research grant. MR was externally sourced to critically revise the manuscript. Stimcare had no input into the study design, the collection and analysis of data, the writing of the manuscript, or the decision to submit the article for publication. None of the authors has any financial or other interest in the production and/or distribution of Stimcare products. The authors declare that they have no other competing interests.

Acknowledgment

The authors acknowledge all the participants.