According to a recent CTS survey, about 80% of cyclists and triathletes complete at least one cycling workout per week indoors. Throughout the year, the ratio of indoor to outdoor cycling workouts varies with changes in weather and daylight hours. However, one trend we’ve noted is that indoor cycling is a year-round component of training for more athletes than ever before. This leads to many questions about the real and perceived differences between indoor and outdoor Functional Threshold Power (FTP) and power training zones.
If you are training indoors and outdoors, do you need to use different FTP values, and therefore, different power training intensity ranges?
Indoor vs. Outdoor Functional Threshold Power
First off, to review, functional threshold power is the maximum power output you can sustain for 60 minutes. (Read more about FTP, how to test it, and how to create FTP-based intensity ranges. Or try this podcast or this one.) After testing and establishing an FTP value, many cyclists and triathletes observe that the perceived exertion associated with that power output is different when riding indoors vs. outdoors.
Typically, an FTP value resulting from an outdoor test or outdoor ride or race data is higher than the value established from an indoor test. As a result, trying to complete an indoor FTP effort based on an FTP value established outdoors may feel harder than expected.
Let’s say a cyclist can complete 3 x 12-minute intervals at 265 watts and an RPE of 7/10 outdoors. Moving the same workout indoors, 265 watts may feel like an 8-9/10. Instead of feeling strenuous but sustainable, the intervals feel like a race-pace time trial. Put another way, an RPE of 7/10 indoors might correlate to a power output of only 250 watts instead of 265.
Same Fitness, Different Performance
Functional Threshold Power reflects the output you can express at the pedals. Your fitness comes from the physiological adaptations to training stress. The amount of oxygen you can deliver to working muscles and the capacity of mitochondria to break down fuel to usable energy don’t change when you ride inside or outside. You are the same athlete with the same aerobic engine.
However, your performance – what you can do with your fitness – can be affected by mechanical and environmental factors that change as you move indoors and outdoors. This is the reason there is no convenient conversion factor between indoor and outdoor training ranges. Indoor efforts don’t always feel harder than outdoor efforts at the same power output. There are athletes who perform equally well – in terms of power and perceived effort – indoors and outdoors. Some athletes even ride better inside than outside. Often, it comes down to how much time you spend training in each environment.
Is FTP different for indoor and outdoor cycling?
It can be, but the differences are not normally so extreme that outdoor and indoor workouts need separate training zones. Remember, lab tests and field tests are used to create training zones – not to pinpoint a singular target power output. No matter which training intensity protocol you use (Coggan, Friel, CTS, etc.), ranges often span 5% or more of your FTP test result. Typically, we see that the difference between indoor and outdoor performance can be accommodated within ranges. This, of course, assumes your FTP and training ranges are up to date.
Let’s look at an example. The CTS SteadyState workout (which targets power at FTP) has one of the narrower intensity ranges in for any workout in the CTS Library. To start, we have athletes complete an 8-minute CTS Field Test. For a 285-watt test value, we first multiply by 0.9. (Another common FTP test is a 20-minute time trial, with the average power multiplied by .95.) That gives us a value of 257 watts. Next, we multiply by .96 and 1.0 to determine the SteadyState workout power range, which gives us 246 – 257 watts. For many athletes, the difference between completing a SteadyState workout indoors vs. outdoors is smaller than the target range. Completing the interval at the upper limit is not necessarily any better than completing it at the lower limit. And keep in mind, there’s no magical switch that says an interval is worthless at 245 or 258 watts.
In extreme cases, external factors affect a cyclist’s performance so significantly that using the same FTP value changes the intended training objective. In other words, we’re using power output (external measurement) to target a specific physiologic stress (internal process). If riding at 265 watts indoors creates a significantly different stress than riding 265 watts outdoors, they are not achieving the same objective. When that’s the case, we try to address the following factors to close the gap between indoor and outdoor performances so we don’t need different indoor and outdoor power ranges.
Factors affecting indoor vs. outdoor cycling power output
If you have the same physiology no matter where you are riding, what causes the differences in performance and perceived exertion? And just as important, is it possible (or necessary) to eliminate or minimize these differences?
Fixed vs. Free Movement
How you move with your bike plays a big role in how you produce power. Outdoors, your bike can sway underneath you, which changes the alignment of your core, hips, and legs in relation to the pedal. How much you sway with each pedal stroke can depend on core strength, hip stability, cycling position, and intensity level.
When you ride indoors, the bike is likely in a much more fixed position. Some indoor trainers and smart bikes include features that allow some side-to-side sway, but many are completely fixed. Riders with weaker core muscles, weaker glutes, and lower hip stability tend to sway more outdoors. As a result, these athletes often see a larger decline in power output at a given intensity indoors because they can’t sway as much.
What to do about it: Ideally, work on strengthening your core, glutes, and hip stabilizers. Doing this will improve performance both indoors and outdoors. In the meantime, consider trainers or accessories that allow some sway while riding indoors.
Heat, Airflow, and Dehydration
Heat is the enemy of endurance performance and staying cool is one of the biggest challenges during indoor cycling. Even with strong fans it is difficult to replicate the airflow a cyclist experiences across their entire body riding outside. As skin and core temperature increase, power output will start to drop. This is largely due to a trick of the mind that defensively reduces motivation to continue. It can also lead to a genuinely reduced capacity to produce power if prolonged heat exposure leads to acute dehydration.
Of course, the heat problem works both ways. In hot environments like the summers in the southern United States, riding in an air-conditioned indoor space can lead to improved performance. However, even in a cool room, airflow from fans is still crucial.
What to do about it: Be sure to use fans indoors and focus on fluid intake during indoor workouts. During shorter indoor workouts (60-75 minutes) you won’t need to consume calories you should still consume fluids.
Equipment Differences and Calibration
Some differences observed between indoor and outdoor performance are simply due to equipment. Many riders use a crank or pedal-based power meter during outdoor rides and an ergometer (i.e., smart trainer) indoors. Although manufacturers boast about the accuracy of their power measurements, it is common for athletes to have consistently different power outputs on different devices. When you’re using only one power meter that’s not much of a problem; consistency from one session to the next is more important than absolute accuracy.
What to do about it: If you are going to use more than one power measuring device for your cycling training, make sure to run the calibration procedures on the devices regularly. Over time you may observe a consistent difference between devices for efforts at similar heart rates and perceived exertions. When that’s the case, if the differences are significant, some coaches may choose one device as the standard and adjust the Training Stress Scores® of workouts done with the other power meter.
Changing your bike fit can dramatically change your ability to produce power. This is the reason most athletes aim to keep their outdoor and indoor riding positions the same. As mentioned above, the fixed position of a bike in a trainer can alter your position on the bike, or at least how you move in relation to your bike.
Screen position can also affect power output. Athletes often position screens used to display Zwift, Trainerroad, etc. higher than their normal line of sight for outdoor riding. This can change their head position, hand position, and trunk angle while riding.
What to do about it: If you plan on competing outdoors, do your best to replicate your outdoor position on your indoor setup. If you are planning on competing in e-sports, real-world aerodynamics don’t matter. In that case, you could adopt a cycling position that optimizes power output and/or comfort with no regard for aerodynamics.
Finally, the performance difference between indoor and outdoor cycling can be influenced by motivational cues. Outdoors you have the sensation of speed and the feeling and sound of the wind. If you’re riding in a group, you have real people to keep up with or challenge. Some athletes struggle to replicate their outdoor performances indoors without these motivational cues. In contrast, some indoor cyclists may benefit from the lack of external distractions or can leverage motivational cues designed into gamified indoor cycling apps.
What to do about it: Sync your most important workouts with the environment that provides the greatest motivation. This often leads athletes to schedule certain types of workouts for indoor sessions and others for outdoor rides.
The Key Takeaways
Whether you ride indoors or outside, you are the same athlete. Differences between indoor and outdoor performances come down to your ability to use or access your cardiorespiratory fitness to produce power. Typically, you will perform best in the environment and with the equipment you utilize the most. Small differences should be expected and can be accommodated without needing separate indoor and outdoor FTP values or training ranges. Larger differences can often be mitigated by addressing external factors that limit performance. In cases where large differences persist, separate indoor and outdoor ranges may be warranted.
By Chris Carmichael,
Founder and Head Coach of CTS
Training Stress Score is a registered trademark of TrainingPeaks.