It can be demoralising when you are unable to do your workout to the targets that you have set and it can be less effective if your targets are too easy. It isn’t unusual for people to have different thresholds for workouts on the indoor trainer compared to those outdoors.
Yes, you should use data that is relevant to the environment you are training in by testing specifically for each situation. Test your power profile both indoors and outdoors and if there is a significant difference, use the relevant values for each planned workout. Test regularly to make sure that you are using up to date values.
It isn’t clear why the values are different but it is clear that they are and for the purposes of planning and executing good workouts that is enough for most of us.
There is some speculation that it is harder to produce good powers on an indoor trainer because the resistance is different to the road and you don’t have the same momentum to carry you forward. This is said to result in a different power profile. However, with modern smart trainers it is questionable whether this is the reality. It is also common for people to find it easier to produce power on climbs than on the flat and of course, the way the resistance works is different in these situations. To a large extent, you will be better in the environments that you are most used to and if you do most of your workouts on the trainer, you may produce better powers on the indoor trainer than you do outdoors.
The important thing is to plan your training optimally and make sure that you are making improvements that are relevant to you chosen goals and do relevant tests to set your training sessions and to track your progress. It is no good aiming to smash out a great 20 minute power on the trainer if you are aiming for a multi day bike packing race, although having that as an interim goal may be relevant in some cases.
A while ago I read in interesting study about how thresholds vary significantly with fatigue. It may seem a bit irrelevant because it is based on tests of riders in the Vuelta d’Espagne but I think the principles are relevant to everyone. The paper is called “Decrement in Professional Cyclists’ Performance After a Grand Tour” by Jose A. Rodríguez-Marroyo, José G. Villa, Raúl Pernía, and Carl Foster, here is a link to it.
The paper shows that fatigue results in reduced power at aerobic threshold, anaerobic threshold and VO2max, which I think is something to bear in mind if for example you are struggling at the end of a hard training block or in an ultra-endurance event.
In an ultra-endurance event, bike packing race or stage race you may be stressing your body more for the same power at the end of the race than you are at the start. This means you may be dipping into your anaerobic reserves later in the race if you work to the same pacing strategy that you used early on.
In some respects, heart rate is a good way to determine whether you are on target but not in all cases.
Heart rate is a great tool for longer efforts and for endurance rides that aren’t very variable.
If you find that your heart rate is typical of what you see at your FTP but your power is not, this may be an indication that you should test your FTP for the specific environment. This is a great way to get a sense check on whether things are going well.
Unfortunately, for short efforts that are above your FTP, heart rate isn’t much good because it will just continue to rise as the effort goes on. It is therefore lower than the target average at the start and higher at the end. Heart rate is a great tool for determining recoveries between intervals though; if your heart rate isn’t going down between efforts it may well be time to go home or give yourself a bit longer recoveries.
Heart rate thresholds are also subject to the same influences as power when you are tired. If you are tired, your heart rate may not rise as quickly as normal for a given intensity of effort, in fact, it may not be possible to get it up to higher levels if you are really tired. It may also be excessively high on some occasions, so it can be a bit unreliable.
Heart rate will also be different to normal in the heat and at altitude and in these situations it is important to learn how you react because everyone is different.
Be positive and look after yourself and most importantly, enjoy and learn from the experience.
How often should I test my FTP? As a general rule it takes 4 to 8 weeks to make physiological changes, so testing every 6 weeks is a good rule of thumb. However, if you are just starting out or coming back after a break it is a good idea to test more frequently because things change quickly as when things are new or when you are getting back into things.
How do I improve my threshold powers? You can improve your powers by training specifically at your target durations as well as doing lots of easier riding. For example, if you want to improve your FTP you need to do some interval sessions at your FTP, some shorter efforts above it and some longer efforts a bit below. These would be known as Threshold, Maximal Aerobic/VO2max and Tempo respectively.