Jack Grealish recently posted photos of his pre-season training sessions in Greece. In one photo, he is wearing a face mask known as an elevation training mask (aka altitude mask). These masks became relatively popular a few years ago with several footballers, such as Gareth Bale and Dele Ali who were seen wearing them during training. Before I explain why some players train in the masks, I'll briefly describe why you would want to simulate training at altitude (what some mask manufacturers claim the masks do).
Why do athletes train at altitude?
At high altitudes (>2000 meters) there are fewer oxygen molecules per volume of air than at sea level. Therefore, each breath taken at a high altitude delivers less oxygen to the muscles compared to sea level; this leads to reduced oxygen delivery to the tissues (hypoxia).
The basic theory of living or training at altitude (or both) is that hypoxia provides an extra physiological stimulus, which elicits physiological adaptations to a greater degree than would be achieved at sea level. The desired adaptation from prolonged altitude exposure is an increase in erythropoietin (EPO), which stimulates the production of red blood cells leading to an increase in oxygen delivery to the muscles. Other proposed adaptations are increases in capillaries and mitochondria (where energy is made in the cells).
Why wear a mask during training? The terms altitude or elevation training masks are misleading. Elevation/altitude masks do not actually simulate altitude as they don't change the composition of the air, only the volume of air that can be inhaled and exhaled. How do elevation masks work? The masks provide resistance to breathing, making breathing more difficult. If you tried exercising with a facemask during the pandemic, you'll know what this feels like. However, these masks provide a resistance that can usually be adjusted. An elevation mask acts as a respiratory muscle trainer, like a dumbbell for your respiratory muscles.
Are elevation masks effective? Limited studies indicate there could be some benefits to wearing an elevation mask during exercise. Porcari et al. (2016) reported that the mask group experienced significant improvements in some markers of endurance performance. However, another study did not report significant differences in VO2max and pulmonary function between groups. One study reported some negative effects when wearing a mask during strength training (Jagim et al. 2017).
Based on current research, wearing an elevation training mask during exercise does seem to produce greater physiological adaptations or performance benefits than training without a mask. Wearing one of these masks during strength training could potentially be counterproductive.
Biggs et al. (2017). Effects of Simulated Altitude on Maximal Oxygen Uptake and Inspiratory Fitness. International Journal of Exercise Science, 10(1), 127–136.
Jagim et al. (2018). Acute Effects of the Elevation Training Mask on Strength Performance in Recreational Weight lifters. Journal of strength and conditioning research, 32(2), 482–489.
Porcari et al. (2016). Effect of Wearing the Elevation Training Mask on Aerobic Capacity, Lung Function, and Hematological Variables. Journal of Sports Science & Medicine, 15(2), 379–386.