This study was conducted in accordance with the Declaration of Helsinki, and the experiments were performed taking ethics, human rights, and the protection of personal information into consideration. This study was approved by the ethics committee of Osaka Kyoiku University (Approval number: 21051). All the participants signed a written informed consent before participating in this study.
The study participants included university students of physical education on campus. Thus, they were physically active. The inclusion criteria were as follows: participants were aged 18 years or older, fully understood the experiment, and gave their written consent to participate. We recruited participants by canvassing within the university, and as a result, they were young. The exclusion criteria were heart disease history, current arrhythmia, chest pain, exercise pain, and respiratory disease history. Forty individuals participated in a briefing session. Before the study commenced, the purpose and potential risks were carefully explained. Subsequently, 16 participants who volunteered to participate answered questions regarding their respiratory and cardiovascular disease history using the Physical Activity Readiness Questionnaire19. All the participants were nonsmokers and had no history of medical illnesses. The sample size was calculated using G*power 3.1, based on a previous study, assuming that VO2peak corresponded to 32.2 ± 9.0 and 43.9 ± 8.1 mL/kg/min with and without a cloth face mask20, with a 5% significance level and 90% power. Therefore, it was estimated that eight participants would be required.
Toward this goal, 11 healthy young men underwent an incremental load treadmill running test until exhaustion with and without cloth face masks. In this study, a randomized crossover design was employed. All the participants underwent an incremental treadmill load running test until exhaustion under two conditions: with a cloth face mask (CFM) and without a mask (CON) in random order. Each test was conducted on a separate day, in a random order, and at least 48 h apart. To minimize daily variations, both test conditions were conducted at the same time of the day for each participant within a 2-h time difference. Participants were briefed on the experimental procedures and practiced the test protocol 1 week before the study to familiarize themselves with the equipment and exercise protocol. After familiarization, participants were randomized into two groups and underwent the first running test. The second test was performed under different conditions from the first trial (Fig. 1). Participants were instructed not to consume caffeine or alcohol and not to engage in heavy exercise for 24 h prior to the test. The participants had their height and weight measured and performed voluntary stretching exercises on the test day. A 3-min warm-up was then performed by walking on a treadmill (3.0 km/h with 0% incline). After the warm-up, the participants attached a one-way expiratory mask (601M, ARCO, Chiba, Japan) connected to a mass spectrometer sensor via a pipe for expiratory gas analysis. Six ECG electrodes (Vitorode M-150, Nihon Kohden, Tokyo, Japan) were attached to measure the cardiac output (CO). To prevent falling, the participants were fitted with an upper-body harness. The test was then initiated, and the participants had to rest for 3 min before starting the exercise to measure the resting values. The experiment was conducted in October. The room temperature was controlled using an air conditioner; nonetheless, the room windows had to be opened in accordance with the university’s COVID-19 prevention guidelines. The room temperature was 25.0 ± 0.5 °C for all the tests.
In the CFM condition, a cloth face mask (DESCENTE Athletic Mask, DESCENTE, Osaka, Japan) was used (outer lining: 100% polyester, inner lining: 98% polyester, 2% polyurethane). Rizki and Kurniawan21 reported that cloth face masks can filter the air to a certain extent, and the polyester cloth face masks provides the most efficient filtration. Hence, the cloth face mask used in this study was expected to prevent droplet dispersal to some extent. After the face mask was attached, an expiratory mask for gas analysis was placed over it and secured with straps to prevent gas leakage. Before starting the test, the participants engaged in expiratory efforts until a positive mouth pressure of 50 cmH2O was detected to verify any gas leakage. Positive pressure was generated by shutting the gas pipe outlet connected to the expiratory mask (601M, ARCO, Chiba, Japan) with hands. Leakage was carefully checked for via sound, sensory, and visual inspections (such as whether the mask was lifted and whether air flowed from the side).
The Bruce treadmill protocol22 was used for the graded load exercise test. We adopted the Bruce protocol because several previous studies6,8,20,23 employed it in their treadmill exercise tests. Treadmill speed and incline were increased every 3 min after the onset of exercise until exhaustion was reached (Table 1). The criterion for exhaustion was the point at which the participant could not maintain the running speed and dropped by > 0.8 m. The participant was provided with verbal encouragement during the exercise.
Respiratory and metabolic responses were continuously measured during the exercise by analyzing expiratory gases using a mass spectrometer (ARCO-2000N, ARCO, Chiba, Japan) connected to an expiratory mask through a silicone pipe. Maximal oxygen uptake (Vo2), carbon dioxide elimination (Vco2), tidal volume (VT), respiratory frequency (fR), minute ventilation (VE), alveolar ventilation (VA), VE /Vo2, VE /Vco2, and end-tidal partial pressure of Co2 (PETCO2) were measured. The mass spectrometer was calibrated using two gases (ambient air equivalent O2, 20.93%; CO2, 0.05%; N2, balance and expired gas equivalent O2, 13.0%; CO2, 5.01%; N2, balance). To ensure that the Vo2 reached the maximum, participants met at least three of the following criteria: (1) a respiratory exchange ratio of ≥ 1.10 (43% of trials), (2) heart rate (HR) that reached 90% of the age-predicted maximal heart rate (220−age) (100% of trials), (3) rate of perceived exertion (RPE) of > 16 (100% of trials), and (4) the participant was unable to continue the exercise (100% of trials). (5) The Vo2 plateaued: a Vo2 plateau was the deviation from the extrapolated Vo2-time linear regression using 30 s data (the actual value was < 400 mL/min from the extrapolated value)24 (50% of trials). All the parameters were averaged every 60 s for analysis.
The cardiac response was measured using an impedance CO monitor (PhysioFlow Q-Link, Manatec Biomedical, Paris, France). HR, stroke volume (SV), and CO were calculated for each beat and averaged every 60 s for analysis.
Mouth pressure was measured by fixing a catheter tip pressure transducer (MicroSensor Basic Kit, Codman & Shurtleff, Inc., MA, USA). The catheter was covered with a plastic tube (diameter: 4 mm, length: 250 mm) and fixed with surgical tape from the nasal dorsum to the nasal apex to prevent the mask from contacting the sensor portion at the tip of the catheter. When wearing the face mask and expiratory mask, it was confirmed that the tip did not touch the skin or mask. The catheter tip pressure transducer was calibrated by immersing the catheter in a light-shielding pipe filled with warm water (37 °C) to a depth of 0–60 cm before the experiment to obtain a calibration signal. Mouth pressure was recorded on a laptop (Dynabook EX/55, TOSHIBA, Tokyo, Japan) at a sampling frequency of 200 Hz via an AD converter (PowerLab 8a/d, AD Instruments, Sydney, Australia) and analyzed using a waveform analysis software (Lab Chart ver. 7, AD instrument, Sydney, Australia). The absolute values were integrated from the obtained mouth pressure data and used as ∫Pm.
SaO2 was measured using a pulse oximeter (SpO2) (N-560, Covidien Med, Dublin, Ireland) placed on the forehead, which was recorded every minute.
The RPE was measured using the Borg scale25, and dyspnea was measured using the modified Borg scale26 by asking the participant every minute.
All the variables obtained in this study are presented as the mean ± standard deviation. All statistical analyses were performed using SPSS 28 for Mac (IBM, NY, USA). Normality was tested using the Shapiro–Wilk test. A paired t-test was used to compare the CFM and CON variables at maximal exercise intensity (Vo2peak, Vco2peak, VT, fR, VE, VA, VE/Vo2, VE/Vco2, PETco2, SV, HR, CO, ∫Pm, and SpO2) and time to exhaustion. Cohen’s d (d) was used for the effect size in the pairwise tests, and the effect size was determined as small, medium, or large for effect sizes exceeding 0.2, 0.5, and 0.8, respectively. Repeated measurements of two-way analysis of variance (Stage × Mask) were used for the last-minute mean values of each stage for Vo2, Vco2, VT, fR, VE, VA, VE/Vo2, VE/Vco2, PETco2, SV, HR, CO, ∫Pm, SpO2, RPE, and dyspnea. The Bonferroni method was used to adjust for multiple comparisons. For the effect size, the ηp2 was used to analyze variance, and the effect size was determined to be small, medium, and large for effect sizes of 0.01, 0.06, and values exceeding 0.14, respectively. The significance level was set at 5%.
This study was conducted in accordance with the Declaration of Helsinki, and the experiments were performed taking ethics, human rights, and the protection of personal information into consideration. This study was approved by the ethics committee of Osaka Kyoiku University (Approval number: 21051). Participants signed a written informed consent before participating in this study.