Here are some studies:
"Chen and Willeke [7] determined that a surgical mask had a 25% penetration rate of submicrometer-size aerosols at a flow rate of 5 L/min and a 70% penetration rate at 100 L/min [41]. While exercising, an athlete’s flow rate increases drastically, decreasing the effectiveness of surgical masks. Therefore, surgical masks used during periods of exercise may be far less protective in athletic populations. Currently, it is unknown whether the use of cloth or surgical masks by athletes will prevent the spread of the virus."
(
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7482372/
)
"Recent updates from the WHO indicate that people should not wear masks while exercising due to respiratory difficulties and potential microorganism growth caused by sweat droplets [51]. "
"The use of masks during a short exercise with an intensity around 6–8 METS, decreases O2 by 3.7% and increases the CO2 concentration by 20%. In some model of mask during exercise, 20 000 ppm of CO2 (2%) is reached and it can be uncomfortable and symptomatic for some of the subjects. " (20,000PPM CO2 is classified as CO2 poisoning with significant side effects from nausea to potentially blacking out).
(
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7275167/
)
"Generally, prolonged exercise at a maximal level may not be possible or safe while wearing a face mask. Special considerations should be made for athletes with underlying cardiorespiratory conditions - Johnson et al"
" is interesting to see that the collection efficiency has a peak of efficiency which actually is very small with a peak around a 3% and with approaching velocities around 0.6 m/s or thereabouts which is justified by the advection of the tiny respiratory micro-particles with the stream lines. This velocity is much more close to average walkers rather than runners where as seen in the figure the collision efficiency drops. - Arias et al" (FYI, the collision efficiency drops to .03% at .97 M/s which is 2.2 MPH. IE, Collision efficiency of respiratory droplets is greater at rest than in motion. The collision requires some airflow so the particles move, but not more than 1 M/s as then it is too fast. The risk of transmission in turbulent airflow is minimal.)
https://link.springer.com/article/10.1007/s42978-020-00071-4
"the odds of indoor transmission was very high compared to outdoors (18.7 times; 95% CI"
Lastly, in the Tour de France, ZERO riders contracted covid, even though they were riding in a peloton within inches of each other for hours per day. Four team staff contracted it as well as the race director, zero cases were reported in the peloton and crucially, zero transmissions occurred from the five positives.
https://academic.oup.com/jid/advance-article/doi/10.1093/infdis/jiaa742/6009483