Pedaling a bicycle, on the surface, can look like a straightforward act. You turn the pedals, which turn the cranks, which turn the chainring and off you go! However, there are some nuances that should be considered when you are choosing how, and where, to do your workouts. While if you're to read a blog, or watch a YouTube video on training for cycling the chances are that it will be all about power, or the amount of energy you can put into the cranks, I would suggest that how you create that power is just as important.
Power = Force x Velocity
Power is a measure of the rate of work, measured in Watts, and a Watt is equal to one joule per second, so if you are pedaling your bike at an average of 200 watts for an hour, you that would be equivalent to 720 kilojoules, or roughly 720 calories. We use power in everything we do from going up the stairs to drinking our coffee in the morning. And while the amount of work required doesn't change, the rate at which we do it (how quickly) does. Bringing this back to cycling, Power is Force (how hard you push on the pedals) x Velocity (how quickly you turn the cranks) and you can go faster by either pedaling harder, pedaling faster, or both. We accomplish this through a combination of gearing and cadence.
What is Erg Mode
Erg mode, or fixed resistance mode, is a convenient feature on most smart trainers that will allow/force you to ride at a constant power output by adjusting the resistance (force) according to your cadence (velocity). This can be a huge help to keep you on target when you are doing structured workouts. Anyone who has ever trained with a power meter outside will know how "jumpy" power can be and how hard it can be to hold the wattage within a narrow range. With erg mode, you can just set it and forget it. As you lose focus and start to soft-pedal it will increase resistance, if you speed up your cadence it will decrease resistance producing perfectly straight lines of power. With the introduction of smart trainers, and the ability to use erg mode, it has allowed workouts to become much more complicated, oftentimes doing "ramps" or other "shapes" in which you increase your wattage in 5-watt increments, a precision you could never achieve without the help of a machine. With that said, should you ride in erg mode? I would say 99.9% of the time... NO, and here is why.
False Precision
Power meters and physiology are just not that precise. With most power meter manufacturers your power meter has a stated accuracy of +/- 2%, meaning that at 200 watts it can be high or low by about 4 watts on any given day. Secondly, your body just does not know the difference between 2-5 watts, which is why we use "zones", levels, or ranges to prescribe training. Your body only really sees 3, maybe 4 "zones" where measurable changes take place, and as long as we are within the range we are accomplishing what we are after. So, when you are doing an endurance ride it doesn't really matter if you're at 65% of FTP or 67% of FTP, and ramping up and down within the zone REALLY doesn't matter, except maybe to make it more interesting and help pass the time.
Flexibility is Key
In addition to variance in power meter accuracy, our bodies are extremely complex machines, always trying to maintain homeostasis among several different systems and in turn, the amount of effort it takes to sustain a certain power can change from day to day with recovery status, stress, hydration, etc. or even within the same workout as you warm up and/or fatigue. Because of that, training needs to be flexible and have a margin of error so you can adjust accordingly based on how you are feeling on any given day. And to the point above, starting your endurance ride at a few watts less than prescribed isn't really going to have an impact, however, forcing yourself to hold a higher power than you're prepared for that day can quickly turn into a downward spiral.
Erg mode makes it easy to miss improvements
By not riding erg mode, and riding to the perception of effort (RPE), you will organically see improvements in power output over time. If you started your endurance rides averaging 130 watts and over time you are averaging 145 watts at the same perceived effort and heart rate, then chances are you have gotten stronger, a sign we might have missed if you're always riding to the watts.
Be an active participant in your training
Another important point with Erg mode is that it allows you to be a passive athlete, just along for the ride in your training as opposed to an active participant and the driver. Erg mode allows you to tune out and mindlessly pedal as it adjusts the resistance to maintain a constant power output. That is not how things work out in the real world. Maintaining constant pressure on the pedals requires focus and the ability to tune into the sensations you are feeling, a very important skill in racing and pacing, one that should be practiced and honed. However, you are robbing yourself of that experience when you put it on cruise control as you answer texts and emails while catching up on your favorite show. There is a lot to be gained by tuning into the sensations of burning in your legs, breathing rate, and pressure on the pedals.
It's just not natural!
One thing I think we have lost sight of over the last 5+ years is that being fast in cycling is not just about power, there are a lot of other factors that go into being fast as well. For example, it's not just the power, it's also how you create it. You can create the same power by pushing hard on the pedals while pedaling slowly, like you do when climbing a steep hill or riding through sand, or by pedaling fast with less pressure on the pedals like you do when you are on a slight descent. Below we can see 3 charts depicting max 20 minute efforts with erg mode, on a dumb/fluid trainer, and outside on a slight grade with Force on the vertical axis and cadence on the horizontal axis. Notice how scattershot the outdoor ride is where the force and cadence are all over the place despite a seemingly constant power output. On the dumb trainer, there is much less variability, and in erg mode even less so. Think of this in terms of the demand on your muscles. So if one of the principles of endurance training is "specificity" it would stand to reason that we should train in the same way we race. By locking yourself into a constant cadence and resistance you are doing yourself a disservice because you are not conditioning your muscles to handle the pedaling demands of the event. This would be the same as training for a flat or rolling time trial by always riding up a 6% grade, chances are you would become really good at producing power in that situation, but lose the ability to maintain power on the flats and rollers that require less force and a higher cadence. Finally, when we get tired outside, we produce less power and slow down. As we're climbing a hill, if we get tired, we slow down. But in the case of erg mode as we get tired the power stays the same but the cadence gets slower, and slower, and slower. This would be the equivalent of the hill getting steeper as you got more tired, or a personal trainer adding more weights to the barbell as your reps began to slow.
When should you use erg mode?
it's not all bad news, there is a time and place for erg mode! I think erg mode is great for setting a cap on the intensity for recovery rides so you don't get carried away once you're feeling good. It can also be a really useful tool for specific cadence work when you want to set a cap on the power. Also, if you have a bike with a 1x on the trainer and are lacking gears, it can provide the resistance you need to be able to hit your targets as your preferred cadence. However, going back to the idea of specificity, I would argue that there is value in figuring out the gearing you will be racing on and adapting to it by learning how to pedal faster or get comfortable producing more force so you can maintain your speed outdoors.
While erg mode may be a convenient way to get in a structured workout and allow you to execute the targets with surgical precision, it can ultimately hinder your cycling performance by robbing you of significant learning opportunities. To get the most out of your indoor cycling workouts, don't worry too much about hitting the exact target, but rather focus on getting in the range within +/-10w and then work on keeping the effort steady through consistent pressure on the pedals.