Relationship between training intensity and risk of infection

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Summary: The researchers identify the exercise level at which aerosol particle emission increases exponentially and offer an explanation for why exercise intensity may be associated with transmission of infection.

Source: TUM

Before the study, it was known that the respiratory volume of untrained people increased from about 5-15 liters per minute at rest to over 100 l/min during exercise. Highly trained athletes even reach values ​​of 200 l/min.

It was also known that many people were infected with the SARS-CoV-2 virus when exercising indoors.

However, it was unclear how the exposure intensity is related to the concentration of aerosol particles in the exhaled air and the actual amount of aerosol exhaled by a person per minute and thus to the potential risk of spreading infectious diseases such as SARS-CoV-2.

However, this information is urgently needed in order to design cushioning measures for school gymnasiums and other indoor sports facilities, fitness studios or discos, for example, in order to avoid closures in the event of heavy waves of infection.

New methodology provides individually measurable aerosol values

A team led by Henning Wackerhage, Professor of Movement Biology at the Technical University of Munich (TUM), and Prof. Christian J. Kähler, Director of the Institute of Fluid Mechanics and Aerodynamics at the Universität der Bundeswehr München, have developed a new research method to investigate these questions.

Their test apparatus first filtered out the aerosols already present in the ambient air. During the subsequent ergometer stress test, the subjects breathed in the cleaned air through a special mouth and nose cover.

Training intensity was gradually increased from rest to physical exhaustion. The mask was connected to a two-way valve that only allows exhaled air to escape. The amount of aerosol particles emitted per minute was then measured and directly related to the current performance of the healthy 18-40 year old subjects.

Moderate aerosol emissions at medium exposure

This enabled the researchers to investigate for the first time how many aerosol particles per minute a person exhales at different levels of exertion.

The result: the aerosol emissions during exercise increased only moderately to an average exposure of around 2 watts per kilogram of body weight. Above that point, however, they increased exponentially.

This means that a person weighing 75 kilograms reaches this threshold with an ergometer value of around 150 watts. This corresponds to a moderate effort for a casual athlete, perhaps comparable to the training intensity of moderate jogging.

The aerosol emissions of well-trained athletes were significantly higher than those of untrained test subjects at maximum exertion due to their significantly higher respiratory minute volume. The researchers found no significant differences in particle emissions between the sexes.

Safeguards are important for high-intensity exercise

Although the aerosol experiments provide only indirect insights into the amount of virus in exhaled air, the study suggests useful starting points for managing indoor activities when a wave of infections combined with a poorly immunized population threatens to overwhelm the healthcare system.

“Based on our results, we distinguish between moderate endurance training with an intensity of up to 2 watts per kilogram of body weight and training with high to maximum intensity. Due to the strong increase in aerosol emissions during high-intensity workloads above this initial value, special protective measures are required if there is a high risk of infection with serious consequences,” says Prof. Wackerhage, who headed the study.

Training intensity was gradually increased from rest to physical exhaustion. The image is in the public domain

“Ideally, this type of training would be moved outdoors. If this is not possible, tests should be carried out to ensure that there are no infected people in the room. Participants should also keep an appropriate distance and a highly efficient ventilation system should be running.

“In addition, risks of infection are reduced by training at a lower intensity and by keeping sessions shorter. It might also be possible for fit, young athletes to wear masks while training.”

At low levels of exertion, such as light to moderate-intensity endurance training, Prof. Wackerhage adds, less protection is required and the risk of infection can be controlled by spacing and ventilation systems.

The research team is currently conducting experiments to compare aerosol emissions from strength and endurance training and to correlate them with the age and physical characteristics of the subjects.

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About this exercise and news from infection research

Author: Henrike Boden
Source: TUM
Contact: Henrike Boden – TUM
Picture: The image is in the public domain

Original research: Open access.
“Emission of aerosol particles increases exponentially above moderate exercise intensity, leading to superemission at maximum exercise” by Henning Wackerhage et al. PNAS


abstract

The emission of aerosol particles increases exponentially above a moderate training intensity, leading to superemission during maximal exercise

Many airborne pathogens, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), are transmitted indoors via aerosol particles.

During exercise, lung ventilation can increase more than 10-fold, and therefore exercisers exhale a larger volume of aerosolized air. However, we do not currently know how exercise affects the concentration of aerosol particles in exhaled air and the total emission of aerosol particles.

Consequently, we developed a method to simultaneously measure the concentration of aerosol particles in expired air, lung ventilation and aerosol particle emission at rest and during a graded exercise test to exhaustion. Using this method, we tested eight women and eight men in a descriptive study.

We found that exhaled aerosol particle concentration increased significantly from 56 ± 53 particles/liter at rest to 633 ± 422 particles/liter at maximum intensity. The aerosol particle emission per person increased significantly by a factor of 132 from 580 ± 489 particles/min at rest to a superemission of 76,200 ± 48,000 particles/min at maximum exposure.

There were no gender-specific differences in aerosol particle emission, but endurance athletes emitted significantly more aerosol particles than untrained subjects at maximum exertion. Overall, the aerosol particle emission increased moderately up to an exposure intensity of ∼2 W/kg and then exponentially.

Collectively, these data could partially explain superspreader events, particularly during high-intensity indoor group exercise, and suggest the need for strong infection prevention measures, particularly during exercise at an intensity greater than ∼2 W/kg.

Investigations into factors such as airway and whole-body hydration status during exposure to aerosol particle generation are needed.

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