Smart trainers have greatly improved the indoor training experience over the last few years. I do not care for the virtual world myself but can see why many athletes really enjoy these features to help with motivation. I do utilize a smart trainer and ERG mode for some of my training as I can be very exact with work loads - especially on/off type work or steady state efforts that are hard to find terrain suitable for uninterrupted long intervals. Also, I am not comfortable training on my local roads during commute hours or in severe weather and may opt for the trainer to stay as safe as possible - safety first. I do see some downsides though and wanted to share those observations in hopes of helping you get more out of your training.
Pacing is the biggest issue I note. I typically ride with local new athletes when we first start up for some field tests and to see how they look on the bike as far as comfort, skills and so on. Many new athletes have been using Zwift or following a Trainer Road plan and are often quite strong when I review recent training files. The first thing I note when we do a short 60 second or a longer 20-30 minute effort on a hill is the lack of pacing skills, often starting way too hard and then dropping way off and repeat, yet their training files from indoor sessions are spot on perfect for pacing. ERG mode is doing it for them and their brains do not know how to pace yet. Pacing is a critical skill for all durations other than a max sprint. Critical power tests are often used to establish FTP and training zones, but the lack of pacing skills often leads to an inflated FTP. This is partially why I prefer actual blood lactate tests with new athletes for accurate training zones the more I look at field tests. I find that the poor pacing on the short durations under-estimate an athlete's anaerobic abilities and leads one to think that their aerobic abilities are stronger than they are = the number one mistake in training - an overestimated FTP. All of the technology can be used to help us calibrate our brains for better pacing skills and feel is still very important in training and racing. Try it without ERG mode and see if you have the skills or need to work on it by doing some intervals without ERG mode.
(The top image is an athlete’s workout using ERG mode and the second from an outdoor field test. Note the vast differences in power fluctuations and how heart rate also fluctuates from poor pacing skills. Both files are from the same athlete. Yellow line is Power and Red line is Heart Rate.)
(These images are 1 minute power tests from an experienced athlete (top image) compared to an inexperienced athlete (bottom image). Note how power fades for each athlete at a maximal effort and I think you can appreciate that one drops off much more and is more erratic than the other. We need to know true max abilities at the ~one minute duration to properly estimate FTP as using 95% of 20 minute power without knowing true max abilities at 1 minute misses the boat. Yellow line is actual power and red dashed line is Regression Power.)
Cadence is another issue I note. Things get very hard on a trainer in ERG mode when cadence drops into the 80 rpm range and spinning at higher cadences naturally feels better. I believe spinning above 90 rpm in most scenarios is very efficient and encouraged in training but the real world is messy and we often need to be able to generate power from lower cadences as the terrain varies, out of corners or the pace changes in races. I can often spot the trainer only riders in races. They spin, spin, spin and look so smooth. They also tend to lack that punch from a deceptive seated lower cadence and power to get a gap or make a critical selection.
Bike handling is the third thing I would like to talk about as it is another critical skill for riding in any mass start event such as fondos, gravel or group rides. It is not only racers that need these skills to be safe and have more fun. Riders get very strong using ERG mode and can keep up in events but are often riders I want to stay away from because they are twitchy and maybe not the safest to be around. We hear about how a rider wins Paris-Roubaix using Zwift and might think Zwift does it all. We need to remember that this rider is a professional and has spent years developing pack skills and can blend right back into the pack easily since the skill set was developed riding outside with others. I have raced bikes a long time and feel I can make the transition from a trainer to the pack but do feel a bit rusty the first time so am sure to get out and ride with others in a controlled setting to firm up my pack skills so I am as safe as possible for myself and the others I am riding or racing with.
I am not saying to avoid ERG mode or virtual training and riding completely. I am saying to be sure to mix it up and do some trainer interval sessions without ERG mode to learn pacing skills and to also be sure to get outside to practice skills and generate power at varying cadences. I understand schedules and the weather challenges, so a plan of attack might be structured on the trainer M-F with 1-2 sessions in ERG mode and the others making your own power versus pushing against resistance - there is a difference. And, use the weekend to go with the flow and enjoy allowing the real world terrain to dictate the pace and cadence a bit. There seems to be this misconception that we need to ride at X duration at X power to improve fitness and that is only doable on the trainer or following very rigid work to rest ratios. I am pretty sure cyclists were quite fit with a high FTP 50 years ago without all of this technology. There are many ways to improve fitness and I believe mixing in all of the options leads to a well rounded and successful athlete. Mix it up and get outside when you can.