Indoor training can be viewed as an unfortunate but necessary evil by some athletes, while others relish the opportunity to get in the ‘Pain Cave’ and log those hours on the turbo trainer or treadmill.
There's often an impressive (and slightly dangerous) pool of sweat on the floor whenever I've finished an indoor training session. So, does that puddle mean I sweat more when training indoors than outdoors? Let’s find out…
The effects of heat on sweat rate
The body controls core body temperature (CBT) to keep us alive and functioning, and we sweat when our CBT rises above a certain point.
The heat given off by working muscles has the greatest influence on CBT when exercising, so how hard you’re working has a massive impact on your sweat rate and more so than body fat, weight and overall size. This was emphasised by the findings of a 2015 study.
So, the average indoor workout is probably more intense than an outdoor session because we generally go for ‘quality’ over ‘quantity’ (unless you really enjoy being in your ‘pain cave’ for hours and hours on end). Therefore, this results in a higher sweat loss per unit of time.
Air Flow and Temperature
Two other important factors which will impact on how sweaty you get when training indoors are Air Flow and Temperature:
When outdoors, you’re moving through air so you get some airflow past the skin. Air movement causes heat to be drawn away from the body’s surface more effectively (via convection and sweat evaporation) and this cools you.
On a static bike or treadmill, you lose this airflow and the sweat tends to drip off you, making you more aware of it. And as there’s no natural cooling effect, you probably actually do sweat a little more to compensate too (unless you attempt to mitigate the lack of air flow by adding a fan to your indoor 'Pain Cave'...).
Your body tries to offload heat to the environment when you’re training. The bigger the gradient between the air temperature, and the lower the humidity, the easier it is for heat to be evaporated away. As many places we train indoors are already quite warm and humid, the gradients for heat loss and evaporation are less pronounced than outside, and this further hinders thermoregulation and drives sweat rate up.
So, whilst you don’t necessarily sweat significantly more indoors than outdoors, there are reasons why total sweat loss might be higher some of the time.
Tips for staying hydrated during indoor training
There are five simple steps you can take to ensure that you are well hydrated for your daily indoor training session...
Before: Arrive well hydrated
As we mentioned earlier, most people’s indoor training sessions are short and intense as we go for the old ‘quality over quantity’ approach. So, it’s important to make sure you start hydrated by doing some form of preload as this will maximise your ability to thermoregulate by sweating.
Aim to drink around 500-750ml (16-25oz) of plain water mixed with a strong electrolyte drink (we recommend PH 1500) a few hours before you start your session. Use a bit of trial and error over the course of a few sessions to refine this approach.
Before: Don't overdo your fluid intake
Having said that, there’s no need to go overboard on fluid intake in the immediate build-up to your session. Just try to stick to good hydration practices on a day-to-day basis.
Before: Add sodium
If you do find yourself low on fluid leading up to a session, it’s a good idea to add additional sodium to your drinks in the preceding hours as this maximises absorption of the fluids you do consume.
During: Drink to thirst during the session
This comes back to the ‘don’t overdo it’ point. Ultimately, don’t interfere with what you’re actually there to do (i.e. get sweaty on the turbo or treadmill) by trying to taking on unnecessary amounts of fluid.
Recovery: Continue to drink to thirst and maybe add sodium to your food and drinks
If you feel like you’re losing the battle to keep up with what you’ve sweated out during your session, the best way to 'top up' is through electrolytes or SweatSalt Capsules. By doing that you give your body the best chance of rehydrating fully before you go through it all again the very next day.
How to stay fueled during indoor training
Similarly to hydration, lots of athletes overlook the fueling needs associated with indoor training because the sessions are typically shorter than their outdoor counterparts.
There’s nothing worse than mentally preparing yourself all day for a hard TrainerRoad workout or Zwift race, feeling rested and ready to go, only to experience the dreaded “bonk” mid-workout and you’re done for the session.
Currently, the scientific recommendations for fueling don't differ from indoor to outdoor training or racing.
That being said, there are a few factors associated with indoor training that may alter your strategy slightly...
The effects of heat on carb intake
Indoor sessions often involve a focus on quality over quantity. The (usually) higher intensity of indoor training means that the primary fuel source the body uses is carbohydrates.
Having adequate carb availability is therefore crucial to ensure you hit the goals of your indoor workout. Another key thing to consider with indoor training is the associated increase in body temperature mentioned earlier in the hydration section. Because of this, a great proportion of blood is shunted to the skin to aid cooling, thus reducing the amount left to serve the gut.
This can lead to a reduction in the gut’s ability to absorb carbohydrates, and reduce the quantity you can tolerate when compared to cooler conditions. This can cause gastrointestinal issues like bloating and diarrhoea.
Tips for fueling indoor training
Essentially, the amount of preparation required and the amount of fuel you use will depend on the intensity and duration of your session (and what the rest of your training schedule for the week looks like).
For example, if you’re planning to do a long treadmill run with some efforts first thing in the morning, and a hard swim set later that evening, you’ll need to restock your glycogen stores faster than if your next session was 24-36 hours later.
Think of fueling during your sessions as preparation for your next workout, even if the current one perhaps doesn’t need fueling.
Before: Arrive well fueled
Ensuring your muscles and liver are stocked with glycogen before any session should be a priority so you can get the most out of each session.
Eating a carbohydrate-rich meal two-to-three hours prior to your session will top off your glycogen stores so you start your workout optimally fueled. This may look something like a bowl of porridge oats, a portion of pasta or some chicken and rice.
There might not always be time to eat this far ahead of a session, so aim to eat something a little closer that’s easily processed by the gut (e.g. a piece of toast with jam or a banana) and then top this off with a final carb hit...
Before: Final carb hit
If you’re planning a particularly long and/or hard workout, or you weren’t able to have a particularly large meal beforehand, taking ~30g carbohydrate within the last 15-20 minutes before your session can really help. This will peak your blood glucose levels, which your working muscles will use for energy at the beginning of exercise, and spare some of your stored glycogen for later in the workout when you really need it.
During: Consume carbs in proportion to your session intensity and duration
Short duration, high intensity: During sessions lasting under 75 minutes and/or those which have high-intensity intervals where carbohydrate utilisation will be high, you might benefit from a small intake of up to 30g of carbs during the session. This could be a gel or a carb drink mix which you might sip from, but the quantity isn’t too important. Whilst this carb intake likely won’t give you superpowers, it will help you maintain the desired quality of the session, particularly if you’re working really hard.
Furthermore, you can use shorter workouts to set the tone for those longer sessions, by carrying the same fueling habits over.
Moderate duration, moderate-high intensity: If you’re planning to go longer (up to ~2.5 hours), you’ll likely need to ingest a little more carbohydrate to match demand. Aiming for between 30-60g of carb per hour will help you meet these demands, but your body will still rely heavily on stored glycogen for energy.
Long duration, moderate-high intensity: Those who are really familiar with their indoor training setup, and wish to do workouts lasting 2.5+ hours, should aim to consume between 60-90g of carbs per hour. Those looking to use this session to “train your gut” should aim to build up your intake towards the upper end of this recommendation over time. It’s important not to take too much carb too quickly, because your gut won’t be able to absorb it and you’ll end up feeling sick or needing the toilet, so build up gradually over time.
Recovery: Refueling soon after will speed up the recovery process
If your next workout is within 24 hours, you’ll need to re-stock those glycogen stores fairly quickly, and doing so can take a little time, depending on how depleted they are.
Adding a reasonable quantity of carbs to your post-workout meal will help speed this process up, as the body is most primed to absorb carbohydrates immediately post-exercise, when stores are low.
Don’t forget to stay on top of your hydration and fueling when training indoors and following these simple steps will help keep you hydrated correctly and able to perform at your best.