Indoor vs. Outdoor Training: What is the difference


Many road cyclists wonder why they ride at higher power outdoors than they do indoors. Or in other words: Why does training on a smart trainer or bike turbo feel much more strenuous at the same power output? Is this just a feeling or reality? Sports scientist and former cyclist Patrick Marseille from HYCYS, formerly STAPS, laboratory in Munich ( knows the answer:

“First of all, we are always interested in what science says. Studies have shown that cyclists with the same rating of perceived exertion (RPE) ride at higher watt values outside on the road than they do indoors,” explains Patrick Marseille. These statements coincide with the perception of many cyclists. Here are the different reasons for this phenomenon: 

Temperature during indoor training 

After only a few minutes on the indoor trainer sweat begins to run. Some road cyclists lose buckets of sweat, i.e. amounts comparable to those of an uphill ride in scorching heat without cooling wind and shadow. “Our organism naturally works rather inefficiently when cycling. Only about a quarter of our total generated energy reaches the pedal. 

The rest is released as heat. This heat energy is transported via the blood to the skin where it is released. Sweating is our cooling mechanism”, explains Patrick Marseille, who also trains the amateur athletes of 2020 Team Alpecin. In combination with the airstream, this is a very effective way of cooling. When riding on an indoor trainer, however, there is no airstream and therefore no cooling effect.

The sweat produced only cools to a limited extent. Another “heat factor” is the room temperature which is usually higher than the temperature outside. If temperature is not elevated right from the beginning of the session, we’ll certainly warm up the room during training as we emit body heat. 

If we generate more power indoors over a longer period of time, our organism can no longer keep up with the heat regulation, which means that the core body temperature increases. To prevent us from overheating, our organism – often referred to as the central governor in this context – regulates the production of energy and heat. All of this can lead to a considerable loss of performance. 

Fluid loss during indoor training 

Water plays an important role in the thermal regulation of the human organism. Not only does our body consist of 75 percent water, but “water also makes up about 90 percent of our blood plasma volume. The blood plasma ‘absorbs’ the heat and transports it to the skin, to the periphery of the body. The fluid loss due to intensive sweating reduces the plasma volume,” says the HYCYS coach. 

“Heat is dissipated less effectively to the skin”. As oxygen transport must continuously be ensured, too, while maintaining the same power output, the heart has to work harder to pump the blood that is getting more viscous through the body. This leads to a heart rate increase – the “cardiac drift” which can be the first indicator of poor fluid management during indoor training. 

In order to know how much one should really drink in action, it is worth determining the approximate sweat rate. Simply weigh yourself before and after indoor training – naked and dry – and determine the difference.

It will never be possible to completely compensate for the fluid loss from intensive indoor bike training, as our intestines cannot absorb as much fluid per hour as we lose while physically active on the bike trainer. However, you’ll get a rough estimate of how much you should drink after the training session.

As a rule of thumb, an athlete should drink 1.5 litres of fluid per kilogram body weight lost after the training session, distributed over the day. 

(c) Kathrin Schafbauer

Sitting position and riding behaviour during indoor training 

Another reason why indoor training is more strenuous than outdoor training is that the athlete hardly varies his or her sitting position on the bike (trainer). Here’s some physics: “Newton’s third law of motion states that when a force is applied, there is always a counterforce acting in the opposite direction. 

Applied to cycling this means that the force acting on the pedal lifts the athlete out of the saddle a little bit every time”, explains sports scientist Marseille. If the athlete has strong and stable trunk- and hip muscles, he or she can use them as an abutment and much less power is lost.

Particularly at high intensities or cadence a bobbing movement on the saddle and an unsteady upper body are signs of a weak core and thus of inefficient power transmission. 

When riding outside an athlete with rather weak trunk muscles will try to increase power and compensate for the muscular weakness at higher intensities by slightly shifting the body’s centre of gravity over the pedal. However, the very static position on the smart trainer reduces the leeway to compensate for the stabilisation problem.

(c) Kathrin Schafbauer

While athletes with a well-trained core have less “power loss” in this rigid position, beginners or everyday athletes with weak core muscles face greater problems and riding on a bike trainer becomes even more intense. 

In addition, more energy is needed indoors than when riding outdoors, as the static position addresses the working muscles in a more isolated way. Due to the fact that there are no roll and recovery phases, the muscles lack these short breaks. 

As muscles are not yet used to this during the first few indoor training sessions they’ll get fatigued earlier, but of course this also makes the bike training very effective. 

Kinetic energy during indoor training 

Let’s stay in the physics department for a little longer. The last factor, which explains why indoor training is more strenuous, is also a scientific one. “Kinetic energy is lower during indoor bike training. This energy results from the factors mass and speed.

When cycling outside, both the wheels and the entire system of rider and bike generate kinetic energy – depending on mass and speed,” says Patrick Marseille. 

This kinetic energy “pushes” the pedal rotation, so to speak, and is significantly higher on flat terrain than during climbing. That’s why it takes noticeably more work to re-accelerate the crank after a missed pedal stroke uphill than it does on a flat road at 30 km/h.

(c) Kathrin Schafbauer

Modern smart trainers try to simulate this kinetic energy with a heavy flywheel or some electronics. This works quite well in some cases; this energy ensures that less work is required per pedal revolution to accelerate the crank. 

But the gear choice also plays an important role. If you ride with a low gear ratio, the flywheel accelerates to a lower speed, so that the kinetic energy is also lower. The athlete notices this immediately if he or she misses a stroke and the bike stops immediately.

Consequence: He or she needs more energy to accelerate again. If you ride a higher gear ratio, the flywheel will accelerate to a higher speed, so that riding is usually a little easier and the kinetic energy of the flywheel “pushes” you a little more.

Although modern smart trainers do simulate the kinetic energy of riding outdoors quite well, athletes still need to apply a slightly more constant push and pull phase over the whole pedal rotation to keep the crank accelerated – compared to riding on the road. 

The best tips for indoor training 

All these factors explain why riding indoors is so much more strenuous than outdoors. Here are a few tips to create better training conditions: 

  • It is important to minimise the influence of the individual variables as much as possible. For this purpose, training should take place in a room that is as well cooled and ventilated as possible. Set up a fan in order to slow down the rise of the body core temperature and to ensure good heat regulation.
  • Before and during indoor training, athletes should make sure they drink sufficiently to compensate for fluid loss. During sessions of more than 60 minutes, athletes should also pay attention to an appropriate mineral intake – especially sodium. Water is absorbed most quickly by the body if the drink contains a few carbohydrates in addition to sodium. 
  • Modern smart trainers with heavy flywheels improve the riding experience, as the flywheel reflects the kinetic energy as realistically as possible. 
  • Strong hip- and trunk muscles ensure optimal power transmission in the static riding position. 
  • The muscles – especially in the legs – need a few hours of indoor training to get used to the changed kinetics and the additional effort. 
  • Nevertheless, athletes should adapt their training intensity to indoor training. Depending on how well these tips can be implemented, power should be 10 to 30 watts lower than outdoors. 

If you want to be on the safe side, it is recommended to do an FTP test on the smart trainer or the bike turbo after an adaptation period in order to determine your individual training zones for indoor training. The mere transfer of “outside values” to indoor training will not work in most cases, as training will constantly be a bit too intensive.