In March 2020, the World Health Organization (WHO) announced that COVID-19 was a pandemic disease caused by novel coronaviruses (SARS-COV-2). Coping strategies during the COVID-19 pandemic mandated the use of medical protective masks for outdoor and indoor sports activities, but there is still much debate about whether wearing masks during exercise triggers safety issues [1]. To date, during the outbreak, the possible droplet and contact transmission of COVID-19 in public places and possible aerosol transmission within 3–6 feet have been reduced to some extent by wearing protective medical masks and maintaining a safe distance [2]. Many government authorities began to allow the gradual reopening of indoor sports centers, such as gyms, natatoriums, badminton halls, and billiards halls, in May-June 2020, but indoor exercise required wearing masks to ensure safety during the pandemic [3]. Therefore, especially facing the new COVID-19 variants (delta, omicron, deltacron), outdoor and indoor activities still require mask wearing to actively comply with government regulations and prevent possible risks of future pandemics.
Wearing a mask increases airflow resistance and prevents the body from receiving the required air at the maximum rate [4]. Past studies have found that wearing a cloth face mask while running on a treadmill significantly reduced overall exercise time by 14% and maximal aerobic (VO2 max) by 29% compared with nonmask, and higher levels of exercise can also cause discomfort reactions such as shortness of breath and claustrophobia [5]. Wearing a surgical mask while running caused a decrease in blood oxygen saturation, which reduced the anaerobic capacity of running and increased the metabolic burden on the body's cardiovascular system and other organs [1]. However, another study found that wearing nondisposable cloth masks or disposable surgical masks had no direct impact on sports performance during moderate intensity exercise or vigorous exercise [6]. Currently, studies on exercise with masks have not defined the exercise conditions that each participant can adapt to his or her own ability based on VO2 max. Therefore, whether wearing medical protective masks during exercise affects sports performance and increases the risk of injury still needs to be explored by standardizing VO2 max.
The biomechanics of running, such as the foot initial angle with contact surface, plantar pressure distribution, step length, acceleration, and vertical ground reaction force (VGRF), are adaptively altered by the fatigue state of the body [7]. Specially, VGRF is often used to investigate the risk of lower extremity injury and assess performance during running. Past studies found that when running on a treadmill at a speed of 2.9 m/s, muscle fatigue reduces the ability of the lower extremities to cushion impact forces from the contact surface, thereby increasing VGRF and loading rate of the impact force [8, 9]. However, other studies have found that fatigue caused by running on a treadmill at a speed of 2.7 ~ 4.5 m/s reduces the impact peaks and loading rates of the lower extremities [10]. Therefore, the influence of running-induced fatigue on VGRF at touchdown should be further demonstrated by considering the runner's speed. Moreover, previous studies have proven that the respiratory resistance when wearing a mask is affected by the tightness of the mask and the heat accumulation inside the mask, which reduces the overall ventilation of the body and causes extra respiratory muscle work to overcome the extra resistance [11]. The extra work may make the body more prone to fatigue, leading to higher impact and even a higher risk of lower extremity injuries during running. Therefore, it is necessary to further explore the influence of respiratory resistance on body fatigue when wearing a medical mask under the condition of adequate protection.
In the postpandemic era, it is still mandatory for most countries and regions to wear masks when exercising in gyms; due to safety considerations, most people still choose to wear masks for exercise. Whether wearing medical protective masks to prevent respiratory droplets or aerosol particles carrying COVID-19 virus from passing through will affect running performance by causing discomfort symptoms such as dyspnea requires further discussion to provide reference results. The purpose of this study was to explore whether lower extremity sports performance was affected by mask wearing and exercise duration during 15-minute treadmill running at a speed of 75% VO2 max. In this study, we hypothesized that medical mask wearing and running time would affect lower extremity performance and increase injury risk.