"Facemask Use During High-intensity Interval Exercise in Temperate and Hot Environments"

Mask use in public settings is recommended to limit the spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) that causes the disease known as COVID-19 and endorsed by the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO). Wearing a mask during exercise may have implications for recreational exercisers and athletes that train and compete at a high exercise intensities as it has been shown to limit ventilation (VE) and increase end-tidal CO2 compared to a no-mask control. For example, N95 masks are designed to fit more tightly around the nose and mouth than other types of masks and have been shown to reduce markers of maximal exercise performance compared to a no-mask control for individuals performing an incremental ramp protocol on a cycle ergometer.

In contrast to N95 masks being reserved for health care workers, surgical masks certified by National Institute for Occupational Safety and Health are easily accessible to wear during exercise to reduce the risk of COVID-19 spread. Indeed, these masks fit more loosely than the N95, but did not reduce maximal power (Pmax), relative oxygen consumption (mL/kg/min), or test duration in two studies where participants performed a progressive maximal exercise test on a cycle ergometer. Further, surgical mask use during a progressive maximal cycling exercise protocol did not change cardiovascular, metabolic, and pulmonary measurements when compared to a no-mask condition in healthy individuals, with the exception of a decrease in maximal heart rate (HRmax). Another recent study using a progressive maximal cycling exercise protocol showed no change in (HRmax) using cycle ergometry when surgical mask use was compared to a no-mask condition. While this type of progressive maximal exercise test is frequently used for clinical and laboratory settings to assess cardiovascular, pulmonary, and metabolic markers, this type of exercise protocol is not often employed by athletes seeking to improve performance or recreational exercisers taking action to improve fitness. Because this exercise protocol is not used by athletes or recreational exercisers to train, it is difficult to translate the results of mask use during progressive maximal exercise tests to commonly practiced forms of exercise. In fact, a recent meta-analysis concludes that wearing a mask elicits a minimal impact on cardiovascular, respiratory, or metabolic variables across a wide array of exercise intensities and modalities, however, high intensity interval exercise (HIIE) was not included in this analysis, nor were hot environmental temperatures. Thus, the physiologic or metabolic responses while wearing a medical-grade surgical mask are not yet known with HIIE in both temperate and hot environments.

High intensity interval exercise is a popular time-efficient training method that involves short high intensity bouts followed by either active (low/moderate intensity exercise) or passive (no exercise) rest bouts. High intensity interval exercise has been shown to be a safe and effective way to increase physical fitness or performance in a diverse population that include exercisers in recreational, competitive, and clinical settings. Further, as HIIE can be performed in either an indoor or outdoor environment and using a mask may increase the physiological and perceptual strain of the exercise depending on the temperature and humidity. In a hot environment (40°C and 20% humidity), no difference was found in skin temperature under or outside a KN95 mask in healthy males that performed 45-minutes of moderate intensity physical activity that mimicked movements required in occupational settings. Also, no difference was found in whole body thermal discomfort, and an increase in perceived breathlessness by 36% was observed. In addition, surgical mask use during 30 minutes of mild uphill treadmill exercise did not increase risk of heat stroke in a hot and humid environment. To date, no research study has investigated the effects of surgical mask use during HIIE in different environmental conditions.

Therefore, the purpose of this study was to investigate the effects of surgical mask use during HIIE on cardiovascular, metabolic, thermoregulatory, and perceptual responses in temperate and hot environments.

Methodsː In a randomized fashion, ten healthy participants completed two HIIE sessions in a 36°C hot (HIIE-HOT) and two HIIE sessions in a 23°C temperate environment (HIIE-TEMP) while wearing (MASK) and not wearing a surgical mask (CON).

Procedures Anthropometrics and maximal Oxygen uptake measurements Before the experimental trials, participants visited the Exercise Physiology laboratories for anthropometrics and VO2max measurement. Body height was measured using a stadiometer (SECA, seca 216, Chino, CA, USA) and weight was measured using a digital scale (Cardinal DETECTO 758C, Webb City, MO). Then, participants completed a 5-minute warm-up at a self-selected exercise intensity prior to the VO2max test on an electrically braked cycle ergometer (Lode, Excalibur Sport, Groningen, The Netherlands). During the VO2max test, HR was measured using a Polar HR monitor (Polar, Polar H10, USA) with each test designed to last between 10 and 12 minutes. The starting resistance and the continuous increase in resistance (between 20-40 watts) throughout the test was based on the participants' self-reported fitness. Termination of the VO2max test occurred at volitional fatigue or when the participant could no longer maintain a cycling cadence of 60 revolutions per minute or higher. Gas-exchange was measured using a metabolic analyzer (Parvo Medics Inc, TrueOne 2400, Salt Lake City, UT) and data were analyzed using an 11-breath rolling average20. Primary criteria for a VO2max was categorized as an observed plateau that varied by less than 150 mlO2/min21. To determine a plateau using primary criteria, the mean of the absolute difference from each neighboring data point in the last 30 seconds of the 11-breath rolling average VO2 was compared against 150 mlO2/min21; if the average value was smaller, the test was categorized as a VO2max. In the absence of a plateau, a VO2max was confirmed if two of the three secondary criteria were met: 1) an RPE ≥17, 2) a respiratory exchange ratio (RER) of 1.1 or greater, and 3) a HR within 10 beats of estimated heart rate max using the Jones et al. equation (202 - 0.72 x age). The highest power output recorded from the VO2max test was used to calculate the high intensity and active recovery bouts for the HIIE sessions.

High Intensity Interval Exercise Trials Before all HIIE sessions participant's hydration status was assessed measuring urine specific gravity (USG) using a handheld refractometer (Cole-Parmer, RSA-BR90A, Vernon Hills, IL). A USG measurement lower than 1.020 g/cc allowed participants to proceed to the HIIE sessions. If USG was greater than 1.020 g/cc, then participants consumed 500 ml of water, and were reassessed after 30 minutes. This water provision was only repeated twice if USG remained above 1.020 g/cc.

A randomized cross over repeated measures design was used to compare cardiovascular, thermoregulatory, metabolic, and perceived responses before and during HIIE across four experimental trials performed during two visits. For each visit, which was separated by a minimum of 72 hours, the participants completed two HIIE sessions separated by 3 hours of rest. Each HIIE bout consisted of 10 bouts lasting 30 seconds performed at 85% Pmax followed by 90 seconds of active recovery at 30% Pmax. The HIIE sessions were performed in a temperate environment (23°C, 25% relative humidity) with (TEMP/MASK) and without a surgical mask (TEMP/CON), and in a hot environment (36°C, 14% relative humidity) with (HOT/MASK) and without a surgical mask (HOT/CON). During the exercise protocol participants were allowed to drink water.

During HIIE sessions with a mask, all participants were instructed to wear the surgical mask snugly over their nose and mouth. The participants fitted the flexible metal plate within the surgical mask over the bridge of the nose and had the bottom of the mask cover underneath the chin. During the 3-hour rest period participants were provided a 350-calorie meal (7g of Fat; 64g Carbohydrate; 9g Protein) and water ad libitum.

Cardiovascular Measurements Before every HIIE trial and after 5-minutes of seated rest, resting blood pressure was measured using a sphygmomanometer, and stethoscope. A cardiac impedance device (PhysioFlow®, NeuMeDex, Bristol, PA) calibrated per manufacturer guidelines was used to indirectly measure cardiac output (CO), stroke volume (SV), and HR. These measurements were taken continuously using 10-second averaging provided by the PhysioFlow® software during each HIIE session.

Metabolic Measurements A near infra-red spectroscopy (NIRS) device (Moxy, Moxy Monitor System, MN, USA) was used to measure muscle tissue oxygenation every second during each HIIE session. The highest muscle tissue oxygenation value from each active rest period (MTO active rest %) was averaged for each HIIE session. Additionally, the lowest muscle tissue oxygenation (MTO during HIIE %) value per high intensity bout was recorded and averaged from each HIIE session. The difference of the average MTO active rest % and MTO during HIIE % was used to calculate the muscle tissue oxygenation difference (MTO difference %). All MTO measurements were taken on the dominant leg at a marked location on the vastus lateralis that would allow movement without interference from the NIRS device. A pulse oximeter (Caretaker, Caretaker Medical, USA) was used to measure SpO2 during HIIE. Blood lactate measurements (Lactate Plus, NOVA Biomedical, MA) were taken in duplicate at the ear lobe immediately before, and 5-minutes post HIIE.

Perceptual Measurement before and during HIIE Borg's rating of perceived exertion (6 rest to 20 maximal effort) scale (RPE) and thermal sensation (0 very cold to 8 very hot) was measured immediately before and every four minutes into HIIE. A breathlessness scale (0 mm no breathlessness to 200 mm maximal breathlessness)was used immediately before, at 10-minutes, and immediately after exercise to record subjective change in difficulty breathing. The highest measurements for both RPE (peak RPE) and thermal sensation (peak thermal sensation) were recorded per HIIE session performed. Additionally, RPE (RPE avg) and dyspnea scores were averaged over time for every HIIE session.

Thermal strain and hydration measurements During each HIIE exercise session continuous core temperature measurements were taken using rectal thermistors (Level 1 esophageal/rectal temperature probe, Smiths Medical, Minneapolis, MN, USA) connected to a thermometer (Precision 4000, YSI Incorporated, Yellow Springs, OH, USA) and monitored to ensure participant safety. Additionally, ∆ core temperature (i.e., the difference between peak core temperature and core temperature prior to starting exercise) and peak core temperature were recorded for each HIIE session. Sweat rate and dehydration were calculated from pre-exercise nude weight, post-exercise nude weight, water ingestion, and post-exercise urine output measurements27. Participants were asked to measure and report their pre-exercise and post-exercise nude weight using a digital scale (Cardinal DETECTO 758C, Webb City, MO) in a private room.