How Indoor Cycling Affects Sweat Rate

With the arrival of winter, many cyclists switch from riding outdoors to training indoors. Platforms such as Zwift, TrainerRoad, Wahoo SYSTM and structured turbo trainer sessions allow athletes to maintain consistency and performance focus throughout the colder months. However, the physiological demands of cycling indoors differ substantially from those outdoors - particularly when it comes to thermoregulation, sweat rate, and hydration requirements. Understanding these differences is fundamental for performance, health and effective training adaptation.

INDOOR CYCLING AND THERMOREGULATORY DEMAND

Indoor cycling alters the body’s thermoregulation, which means greater effort for the heat dissipation mechanisms that prevent an increase in body temperature. During exercise, around 75-80% of the energy produced by working muscles is released as heat, with only 20-25% converted into mechanical output (Périard et al., 2021). This heat must be dissipated to maintain safe core temperatures and sustain performance.

Outdoors, heat is lost through a combination of convection, evaporation, radiation and conduction, depending on the ambient temperature, relative humidity, wind speed and solar radiation. The airflow generated by forward movement on the bike plays a crucial role in convective heat loss, as it improves endurance exercise capacity and reduces thermoregulatory, cardiovascular and perceptual stress during exercise (Otani et al., 2018). Indoors, this natural airflow is markedly reduced. As a result, the body becomes more reliant on evaporative cooling, yet evaporation is less efficient without sufficient air circulation. This leads to a more rapid rise in skin and core temperatures, accelerating the sweating response.

WHY INDOOR CYCLING ALTERS SWEAT RATE

Research consistently shows that the rate of perspiration is higher indoors when the intensity is similar to outdoor training. Without external airflow, convective cooling is reduced as heat builds up on the skin surface, increasing body temperature and causing sweating to begin earlier, driven by thermoregulatory needs rather than metabolic demands (Adams et al., 1992; Nadel et al., 1971). This accelerated sweating helps prevent further heat accumulation but increases fluid loss.

Typical indoor training environments, often with temperatures between 18 and 22 °C and limited ventilation, can create a more demanding heat load than cooler outdoor sessions of the same intensity (Gagnon, Jay and Kenny, 2013). Even when indoor temperatures are moderate, the absence of wind significantly reduces heat transfer, meaning athletes may experience higher sweat rates at lower indoor ambient temperatures than outdoors at warmer temperatures.

ENVIRONMENTAL & THERMOREGULATORY DIFFERENCES INDOORS VS OUTDOORS

Body temperature increases during activity because working muscles generate heat rapidly. Heat is lost through the skin by physical transfer (sweat evaporation, convection, and conduction) to the surroundings if the skin is warmer than the surrounding air. Sweat does not evaporate, and the body does not lose heat when the atmosphere is saturated with water vapour (i.e., 100% relative humidity).

The key differentiators between indoor and outdoor cycling relate to the dominant heat transfer mechanism:

Indoors, convection is significantly affected, making heat loss through evaporation the primary means of cooling. As exercise intensity or indoor humidity increases due to sweat accumulation and breathing, evaporation efficiency decreases further (Bright et al., 2025). This increases heat stress, even at moderate indoor temperatures.

THE IMPACT OF LIMITED AIRFLOW

The absence of airflow is a defining thermoregulatory constraint of indoor cycling. Studies comparing cycling with and without fans demonstrate higher core temperature at matched power output, increased heart rate and perceived exertion, greater sweat rate and earlier onset of fatigue (Cheuvront et al., 2010; Wingo et al., 2012; Fernandez et al., 2023).

Limited airflow reduces the possibility of sweat evaporating and, on the contrary, causes it to drip. From a thermoregulatory point of view, sweat that does not evaporate provides no cooling benefit. It is this inefficiency that contributes to greater total fluid loss indoors.

HOW SODIUM CONCENTRATION MAY CHANGE INDOORS

While there is strong scientific evidence demonstrating an increase in sweat rate during indoor training, the same cannot be said for sodium concentration in sweat. This is because sodium concentration is influenced by multiple factors, including genetics, hydration status, acclimatisation, and glandular reabsorption capacity (Baker et al., 2016). Evidence suggests that sweat sodium concentration may be similar or slightly higher indoors, although the magnitude varies substantially between individuals (Baker, 2017).

The mechanism explaining this possible increase is related to the physiology of sweat glands. When the sweat rate increases rapidly, there is less time for sodium reabsorption in the sweat duct, which can result in sweat that is richer in sodium (Baker et al., 2017). However, this response is not universal. In some athletes, sodium concentration remains stable indoors, increasing only fluid loss rather than electrolyte loss.

EVIDENCE-BASED HYDRATION CONSIDERATIONS FOR INDOOR CYCLING

The most scientifically aligned position based on current evidence is as follows:

Indoor training consistently elevates sweat rate and may modestly influence sweat sodium concentration, but individual responses vary and should be assessed over repeated sessions.

General key points to consider:

• Fluid replacement should aim to prevent > 2% body mass loss to minimise performance decline (Sawka et al., 2007).

• Sodium intake may be required for sessions longer than 1h, particularly when sweat rate is high, to support fluid retention and plasma volume (Shirreffs and Sawka, 2011).

• Monitoring individual sweat rate across indoor sessions improves the accuracy of hydration strategies, as inter-session variability is common due to environment, intensity and acclimation status (FLOWBIO).

APPLYING INDIVIDUAL DATA AND PERSONALISATION

Because sweat responses to indoor cycling vary between athletes, the most reliable way to optimise hydration is through individual data collection and repeated assessment. Tracking sweat rate and sodium loss across multiple indoor and outdoor sessions allows athletes to understand their typical sweat rate ranges indoors, whether sodium concentration changes materially indoors and how intensity, duration and room ventilation influence losses.

Tools such as the FLOWBIO Sensor can support this process by measuring sweat rate and sodium loss during training, enabling evidence-informed personalisation of hydration strategies based on an athlete’s own physiology. Moreover, the FLOWBIO App allows you to change the exact indoor temperature for the specific local environment.

CONCLUSION

Indoor cycling imposes a greater thermoregulatory burden than outdoor riding due to limited airflow and reduced convective heat loss, resulting in earlier sweating onset, higher sweat rates and increased dehydration risk. Sweat sodium concentration may also be modestly influenced, although individual responses vary. Given these physiological differences, hydration strategies should not be directly transferred from outdoor to indoor cycling. Monitoring individual responses and applying evidence-based hydration principles can help maintain performance, training quality and athlete health throughout the indoor season.

REFERENCES

Adams, W. C., Mack, G. W., Langhans, G. W., & Nadel, E. R. (1992). Effects of varied air velocity on sweating and evaporative rates during exercise. Journal of Applied Physiology (Bethesda, Md.: 1985), 73(6), 2668–2674. 

American College of Sports Medicine, Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 39(2), 377–390. 

Baker L. B. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine (Auckland, N.Z.), 47(Suppl 1), 111–128. 

Baker, L. B., Barnes, K. A., Anderson, M. L., Passe, D. H., & Stofan, J. R. (2016). Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences, 34(4), 358–368. 

Bright, F. M., Clark, B., Jay, O., & Périard, J. D. (2025). Elevated Humidity Impairs Evaporative Heat Loss and Self-Paced Exercise Performance in the Heat. Scandinavian Journal of Medicine & Science in Sports, 35(3), e70041.

Cheuvront, S. N., Kenefick, R. W., Montain, S. J., & Sawka, M. N. (2010). Mechanisms of aerobic performance impairment with heat stress and dehydration. Journal of applied physiology (Bethesda, Md.: 1985), 109(6), 1989–1995.

Fernandez, A., Wimer, G. S., Culver, M. N., Flatt, A. A., & Grosicki, G. J. (2023). Fan Cooling Improves Submaximal Exercise Capacity in an Indoor Thermoneutral Environment. Research Quarterly for Exercise and Sport, 94(1), 124–130.

Gagnon, D., Jay, O., & Kenny, G. P. (2013). The evaporative requirement for heat balance determines whole-body sweat rate during exercise under conditions permitting full evaporation. The Journal of Physiology, 591(11), 2925–2935. 

Nadel, E. R., Bullard, R. W., & Stolwijk, J. A. (1971). Importance of skin temperature in the regulation of sweating. Journal of Applied Physiology, 31(1), 80–87. 

Otani, H., Kaya, M., Tamaki, A., Watson, P., & Maughan, R. J. (2018). Air velocity influences thermoregulation and endurance exercise capacity in the heat. Applied Physiology, Nutrition, and Metabolism = Physiologie Appliqueé, Nutrition et Metabolisme, 43(2), 131–138. 

Périard, J. D., Eijsvogels, T. M. H., & Daanen, H. A. M. (2021). Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews, 101(4), 1873–1979.

Shirreffs, S. M., & Sawka, M. N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of Sports Sciences, 29 Suppl 1, S39–S46.

Wingo, J. E., Ganio, M. S., & Cureton, K. J. (2012). Cardiovascular drift during heat stress: implications for exercise prescription. Exercise and Sport Sciences Reviews, 40(2), 88–94.