The Power Meter Handbook: A User’s Guide for Cyclists and Triathletes

if the pro triathlete in Figure 5.1 decided he wanted to become a road cyclist, he would need to place greater training emphasis on the low-duration end of the chart at the left. That means more short intervals and explosive sprint repeats. He could probably expect that over time his Power Profile would show a change in fitness, with a chart that would begin to look more like the roadie’s.
    In the same manner, your Power Profile reflects who you are as a cyclist at the current moment, but it is malleable with training. Within the confines of genetics, you can create the fitness you need that is unique to the demands of your races by emphasizing training with appropriate Intensity Factors.
HOW WELL DO YOU PACE?
    Pacing is closely related to how you expend your most precious commodity: glycogen. Glycogen is the storage form of carbohydrate, and you have a limited supply. If you’re good at replacing expended glycogen stores, you should come to the start line of a race with a full tank. A topped-up glycogensupply will last roughly two hours depending on your unique physiology and how you pace the race. Anything beyond that duration will require the intake of more sugar—sports drinks, bars, gels, blocks, or whatever you like to use. These “exogenous” (outside the body) sources are not as efficiently used by your body as the “endogenous” (inside the body) glycogen stored in your muscles and liver at the start line. When you start running low on your stored endogenous fuel, fatigue begins to set in.
    Pacing is also closely related to the body’s acidosis. Whenever you increase your power output, your body produces more acid, which limits muscle activity and causes the burning sensation experienced at high intensity. Going above your FTP for an extended time creates a lot of acid, which contributes to the fatigue you experience later in a race—depending, of course, on how well you paced it.
    You may have figured out your exact Intensity Factor for the race and therefore what your race power should be. You start the race—a time trial or bike leg of a triathlon—by watching your power and staying in the prescribed range. But then someone catches and passes you, so you speed up briefly. Or you hammer up a hill. This may happen repeatedly throughout the race, with each such episode accompanied by a short surge in power. In addition, you try to maintain goal power on several steep downhills. That’s also a surge, as you will see shortly.
    Surges waste glycogen and create additional acidosis. By repeatedly speeding up and slowing down, you don’t go any faster than if you rode steadily. But those surges needlessly use a lot of glycogen while increasing your blood’s acidity. The longer the race is, the greater this two-headed fatigue problem becomes.
    You finish the race, however, with about the Intensity Factor range for which you were aiming. So why was the last part of your race slow and painful? Well, despite your average IF being in the proper range (while perhapson the high end), you didn’t pace evenly. You ran low on glycogen and had your muscles swimming in acid in the latter stages of the race. No wonder it was yet another sufferfest.
    The problem was not that you raced at the wrong power but that you didn’t stay true to that power throughout, owing to constant surges. That brings us to a tool that can help you identify such unsteady riding in what are meant to be steady-state races: the Variability Index™ (VI).
Variability Index
    Steady-state races, such as time trials and triathlons, should be ridden … well, very steadily. When the race is over, you can determine how steadily you paced the race by checking your VI, which is a comparison of your Normalized Power and average power. Recall that NP is average power normalized to reflect the metabolic cost or sensations of fatigue experienced during a ride. In effect, it gives more value to the power spikes you create. The more spikes in power there are and the bigger the spikes are, the higher the NP is. By comparing NP and average power, you can isolate the spiked portions and therefore know how steadily (or not) you rode. You make this comparison by dividing NP by average power. The result is your VI for a race, a workout, or an interval.
    Let’s use an example to better understand this. You race in a time trial or triathlon, and afterward, while checking the software or head unit numbers for the race, you see your NP was 256 watts and your