Do you ever feel exceptionally sweaty after an indoor workout?
There’s no way your recent outdoor ride – of similar effort – created the same puddle of sweat, even if it was hotter and more humid.
Surprising data from Nix Biosensors explains why indoor workouts are often more sweat-intensive than outdoor training sessions.
Analyze Your Sweat in Real Time
Nix uses biosensors to offer athletes and coaches a better understanding of hydration monitoring. These biosensors provide personalized data by measuring fluid and electrolyte losses throughout a workout.
The Nix Hydration Bioseneor keeps score of your environment with the Nix Index. This value between 0 and 100 helps you determine how the current environmental conditions impact your sweat rate during a workout.
The Nix Index is based on relative humidity, wind speed, solar load, and temperature – wet bulb, globe, and ambient air. It also takes into account dew point and altitude.
The higher the Nix Index, the more heat your body produces, and the more fluid loss you’re going to experience during a workout.
When you log an indoor workout with the Nix Solo app, you’ll always receive a Nix Index of 59. The app applies the below fixed values for indoor training sessions.
- Temperature: 70°F
- Humidity: 50%
- Solar Load: 0%
- Wind Speed: 0MPH
How Training Affects Sweat and Hydration Needs
To better understand sweat rates, Nix tracked the Nix Index of cycling sessions (that lasted at least 45 minutes) from September 2023 to July 2024 in a recent study.
When you look at the outdoor workouts alone, you’ll notice that when the Nix Index rose, average hourly fluid and electrolyte loss rates also increased. In other words, as the environment got hotter and more humid, athletes sweated 6 to 15% harder.
This equates to roughly 10% more sweat for each 10-point increase in the Nix Index.
The athlete’s electrolyte needs followed a similar pattern – rising with the Nix Index. While not linear, electrolyte needs increased by approximately 16% with each 10-point increase in the Nix Index.
With the above information, a user who loses 800 mg of electrolytes an hour cycling with a Nix Index of 60 needs approximately 1,245 mg of electrolytes during extreme heat workouts. That’s 445 mg more electrolytes – slightly more than one-third of an LMNT packet.
Outdoor vs. Indoor Sweat Rates
When we compare indoor cycling workouts to outdoor workouts in extreme conditions, we see fairly similar fluid and electrolyte losses.
Why Are Indoor Workouts So Sweaty?
Now that we know indoor workouts are just as sweaty and taxing as their outdoor counterparts, it’s important to understand why they feel so much sweatier. This will help you create an efficient indoor cycling space.
Controlled Climate
Given the proximity to a water source, lack of exposure to more extreme outdoor conditions, and ability to stop at any point – compared to being on the road, far from home – many cyclists overlook their hydration needs during an indoor workout.
Air Flow
Another critical factor is the lack of – or significant reduction in – wind convection during an indoor session. During any workout, when temperatures rise, the body searches for a way to cool down, triggering sweat.
When cycling outside, the forward movement produces wind convection that promotes sweat evaporation and cools the body, which decreases sweat rate. When cycling indoors, even with a fan, the wind convection is lower than that typically produced by a 17 – 21 mph ride.
Additionally, when cycling indoors, all the heat dissipated from the body is retained within the room. Without the same level of wind convection, sweat rates will be higher to maintain the same core body temperature.
Tips to Stay Hydrated During Indoor Training
1. Manage and adjust your environmental conditions. Create airflow to cool your body and reduce sweat rates. Use a fan to direct airflow toward you and, ideally, position your trainer near an open window. Alternatively, set up your station in the coolest room of your house or turn down the temperature before a workout.
2. Know your personal sweat stats. Sweat is personalized – how you respond to an indoor workout will vary from a friend or training partner. Utilize a sweat test, in a variety of conditions, to understand your personalized fluid and electrolyte losses.
3. Bring fluids. Show up hydrated and have access to appropriate levels of fluids throughout your workout.
4. Don’t neglect electrolytes. According to the study above, cyclists lost approximately 1,500 mg of electrolytes hourly during indoor rides. That equates to just over 700 mg of sodium lost per hour. Failure to replace electrolytes can increase your risk for injury, muscle cramps, cognitive dysfunction, performance impairment, and – in more severe cases – hyponatremia.
By optimizing these hydration strategies during indoor cycling sessions, athletes can improve their performance, prevent dehydration, and recover more effectively – critical factors for anyone serious about year-round training and peak performance.
