Cyclocross and Aerobic Training

With cyclocross season fast approaching, many athletes are starting to get serious about training for the cycling’s cooler weather discipline. In a recent article, coach Zack Allison covered the importance of anaerobic capacity in cyclocross. Anaerobic capacity is a massive part of ‘cross, but that doesn’t mean you can neglect the aerobic engine and its role in cyclocross performance.
As Zack mentioned, it’s rare to be knee deep in a cyclocross race wondering what energy system you’re using. Cyclocross is essentially a huge collection of repeated and intense anaerobic efforts. Training repeated anaerobic efforts in order to increase the capacity for work over threshold is a very important aspect of training cyclocross and specificity, training for the exact type of efforts you’ll do in a race, is imperative in training. Why, then, do you see national level athletes like Jeremy Powers and Christian Helmig doing long endurance rides in the late summer? Some cyclocross  pros even ride long Gran Fondos and the like in preparation for cyclocross season.
The answer to this question comes from how the body uses energy. Instead of being purely anaerobic or purely aerobic, the body actually shifts energy usage on a spectrum. That’s one reason you can’t feel the burn in your legs at the start of a hard Vo2 or anaerobic capacity interval: your body is using energy from a system that doesn’t immediately produce the burning sensation. The availability of that energy is very limited and as the shift goes from anaerobic to aerobic, you begin to feel the effects of the body trying to keep up the supply of oxygen in order to provide the energy that you’re using.  The following chart shows how the spectrum shifts between energy system contributions:
aerobic training energy system contributions
As you can see, the longer an effort is, the more aerobic energy matters. Now, this chart clearly doesn’t account for multiple repeated efforts. Cyclocross is made up almost exclusively of such efforts. The other side of anaerobic capacity is the actual recharge and recovery of that system. There are two primary ways that aerobic energy and training can help cylcocross.
The first, and simplest, way that aerobic training increases cyclocross performance is that it reduces the anaerobic contribution to each effort of a given level by increasing an athlete’s FTP.  Assuming a given anaerobic capacity, a higher threshold adds power to the effort by allowing the aerobic system to contribute more before tapping into that capacity. For ease of example, assume an athlete’s anaerobic capacity contributes 300w to a 60s effort. This output is fixed because the rate of energy use in the anaerobic system is fixed. If an athlete’s aerobic contribution makes up 200w (in other words his or her FTP is 200w), then the total output is 500w for 1 minute. That same athlete, with an FTP of 250w, could now put out 550w for the same minute at the exact same cost. While this is an oversimplification, it helps describe how FTP contributes to power even during anaerobic efforts.
The second way that aerobic training can improve cyclocross performance even with the anaerobic efforts of the sport is that it can improve the recharge rate of the anaerobic system. In much the same way as FTP contributes to overall power output, it also improves repeated efforts by providing faster oxygen utilization. This allows the energy in the anaerobic system to be replenished more quickly and reused more quickly. The pyramids below give an easy visual of the difference between an athlete with base training and one without.
Aerobic foundation
That effect also applies between workouts, so there’s a third, more obscure benefit to including aerobic training, be it “base training” with long rides or FTP training with harder aerobic efforts done at or near FTP. That has the added benefit of allowing athletes to require less downtime for recovery between workouts. Constant high intensity intervals build fatigue at a greater rate than a combination of different types of efforts, allowing more training volume overall, or alternatively better recovery from the higher intensity training through the aforementioned improvement in oxygen utilization.
Scheduling a workout structure is very complicated, but it’s important to manage and attend to each component of your training, anaerobic and aerobic together, in order to maximize performance, even in anaerobic intensive disciplines such as cyclocross. You’ll notice the pros don’t slack on aerobic preparation, and neither should anyone else serious about performing their best in cyclocross. Regardless of available training time, it’s possible to address the aerobic system and improve.
Mitchell SidesMitchell Sides recently graduated from the University of Texas at Austin with a BSEd in Exercise Science and is the newest addition to the team at Source Endurance. He’s transitioned from a self-proclaimed overweight couch resident into a member of Texas’s first UCI continental professional cycling team, Elevate Pro Cycling. He specializes in coaching road cycling and is inspired by the mentorship aspect of the sport. Learn more about Mitchell.