For me as a cycling coach this is an extremely important subject matter and one I need to repeat over and over with my own clients, the importance of consistent endurance or Zone 2 training. But why its so important? The majority of cyclists that come to me have plateaued on a regime of mainly Zone 3. That’s that tempo zone where its a little uncomfortable, the group conversation has gone a bit quiet, but it's not too hard that it hurts. It’s that Strava chasing zone, putting your riding buddies under pressure, pushing the hills, and feeling like you’ve done a great workout. However, you are not getting any training adaptation! So let's look at the physiology.
Across the power zones your body uses different pathways to generate energy (ATP) that is required for the muscle to work. A short sprint uses the creatine phosphate pathway, a longer sprint the glycolytic/anaerobic pathway and finally a long effort, the aerobic pathway. In very simplistic terms your glycolytic/anaerobic pathways are a source of energy when you go above FTP, using carbohydrates as a fuel source. Below FTP, your aerobic pathways use fat as the primary fuel source. Fat converts to the energy currency ATP, much SLOWER than carbohydrates, and that’s why at lower intensities your body can use fat. But as soon as you require big quantities of ATP, like when you do a surge, fat is too slow to be converted, and carbohydrates are then used. There’s an infinite amount of stored fat, carbs unfortunately have limited stores in the body and run out fairly quickly. Your body does not engage with each of those energy systems consecutively, and that’s why training each one of them, needs a very specific approach. Zone 2 where you are utilizing your aerobic pathways, you’re burning fat maximally and your body uses minimal carbohydrates to keep you going. Zone 4 and above is where we start hitting FTP and the body uses mostly carbohydrates. Zone 3 is a bridging zone, where your muscle generates energy by “stealing” some ATP from Zone 2 and “stealing” some ATP from Zone 4. So it becomes a mixture of pathways.
The body soon realizes it can get energy quicker from carbs in Zone 4 so it starts shutting down the aerobic (Zone 2) pathways. So what does that mean? You never stress your aerobic pathways and you never stress your anaerobic pathways while sitting in Zone 3. So NO adaptation, since adaptation comes through stressing the body and pathways. The easiest way to explain this is looking at the FatMax and CarbMax chart that is generated from an INSCYD Metabolic test.
The bottom X axis is power output, the green line is the amount of fat used and the red line the amount of carbs used. So we get a picture of the amount of fat, and the amount of carbs used at different power outputs (off topic: this is a great way to put together a race nutrition strategy if you have those numbers). The green shaded area is what we call Fatmax, the point where your body uses the most amount of fat. To use the most amount of fat all mitochondria are being stressed maximally, the only way to burn more fat in simplistic terms is to have more mitochondria. Fatmax correlates very closely with Zone 2. Once you move away from that green shaded area with increased power, look at what happens. Fat utilisation actually drops, the maximum amount of mitochondria utilising fat as a fuel drops BUT carbohydrate usage increases. So in Zone 3 we start using less fat, with less mitochondria, taking away the adaptation stress that leads to a better aerobic engine. Instead we start using the glycolytic pathways which don’t stress the mitochondria at all, AND at a level that doesn’t stress and cause a training adaptation in that pathway – that only happens when we get to Zone 4 and above. So zone 3 as you can see is really of very little value…… Lastly let's come back to the statement earlier around limited carb resources. We all have excess fat to burn, no matter how lean you are, but carbs are limited to 400g to 500g in the body. When carbs run out, the body cannot generate energy quick enough for efforts requiring carbs. So the higher we can get our rate of fat utilization, the more carbohydrates we spare in the long run and the faster and longer we can ride with our limited carb resources. So hopefully you now understand why I go on an on about Zone 2 riding and its importance in developing an all round competitor (and even more important if your main event is longer than 4 hours).
You may be great on the sprints and short efforts, but without a well developed aerobic engine, you are not going to maintain. Keep the consistency!