Endurance Analytics

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The Measure of a Cyclist

August is wedding season here in Europe and we were recently involved with having a suit fitted and tailored. The process, done properly, involves an awful lot of measurements that together could do a pretty good job of describing the form and physique of the customer. No one measurement defines the perfect suit and this got us thinking about how, in the modern world of power meters and sport science, no one metric is sufficient to define the ability of a bike rider.

So what is the measure of a cyclist? There are romantics and journalists who might turn to words such as "class", "panache", or "tenacity" but let's suspend the BS for a few minutes and look at the set of metrics that we have come to understand are required to comprehensively define the ability of a cyclist. Even for natural number fans there is much more to consider than one simple "watts per kilo" - equivalent to an off-the-peg but poorly defined "size 50 suit"...

Critical Power (Watts)

Critical Power (CP) is what’s left when we strip away the power a rider can produce on a short term basis, “in the red zone”. That means it’s equivalent to power that can be produced aerobically, through “aerobic energy pathways”, and for a relatively long time. The gold standard model for identifying a rider’s CP, from just 2 or 3 short field tests, is the Monod Critical Power Model. Of all rider metrics CP is the most important for the vast majority of cycling events.

Anaerobic Work Capacity (Joules)

Anaerobic Work Capacity (AWC) is the size of a rider’s “red zone”, or the amount of energy they can produce over and above CP, before they need to slow down and recharge. Because AWC is an amount of energy it’s measured in joules. Since a joule is “1 watt for 1 second” AWC can be spent very quickly, at a power output way above CP, or more slowly, for example riding just a bit above CP for several minutes. The Monod Critical Power Model is also the tool of choice to identify a rider’s AWC. Some sport scientists refer to AWC as W’ - “W prime”.

AWC Recharge Velocity (Tau)

When a rider’s power output dips below Critical Power he isn’t spending his anaerobic work capacity, so if it’s depleted then there is an opportunity for the body to recharge it. Consistently riding above and then below CP isn’t particularly efficient, but the demands of a race or considerations of a pacing strategy may win the argument. Speed of recharge is key, and recent research (primarily by Dr Phil Skiba) has provided a framework in which to model AWC recharge. The lower a rider’s power output, relative to critical power, the faster AWC will recharge. Another determinant of that speed is a mathematical function known as the “Tau function” which can either be based on a general population of cyclists, or calibrated to the individual using power based field tests.

Velocity of Oxygen Dynamics (Seconds)

Imagine a rider accelerates from a standing start and upto Critical Power - the level at which all power is still assumed to be coming from the aerobic pathway. He’s never exceeded CP, so did he use any AWC? The answer is yes, because it takes a bit of time for his aerobic system to fire up to full capacity, and in the meantime the energy that’s not being produced aerobically has to come from somewhere. The faster a rider’s aerobic system can accelerate the better, because AWC is preserved and the cost of recharging it is avoided. This can be important in events with highly variable power demands, or for events that involve hard efforts straight from the gun, such as the pursuit. Acceleration of the aerobic system is typically modelled as an exponential function where velocity is explained by it’s half life in seconds.

Neuromuscular Power (Watts)

Sprinting requires explosive efforts that rely on an energy system distinct from the aerobic and anaerobic pathways, as well as a rider’s ability to activate and deploy muscle. Aside from very specific track applications we’ve yet to be convinced of the need for a formal model of neuromuscular power. The metric here is simply “best average power over 5 seconds”. Perhaps more than others this metric is sensitive to the quality of a rider’s power meter (how frequently does it capture data, and how does it measure angular velocity of the crank?).

Fatigue Index (%/Tx2)

The Critical Power model suggests that if we strip the power a rider can produce from anaerobic sources then we arrive at one number – critical power – that he ought to be able to maintain aerobically, and indefinitely. Of course this isn’t realistic – ride beyond an hour or two and everyone will fatigue, even though their aerobic engine isn’t actually getting smaller. So what’s going on? Lots of complex processes and factors contribute to fatigue but rather than try to model all of these factors we like the concept of the “Fatigue Curve” – a model which simply fits a curve to the tendency for a rider’s power output to decay with time, though at an ever reducing rate. The underlying mathematical function gives us a very useful summary number – by what percentage does a riders power output reduce when ride time doubles – and we call this %/Tx2.

Body Mass (Kilos)

The more a rider weighs, the more power he needs to climb at a given speed, accelerate at a given speed, or even to move across a flat road at a given speed. That last bit may be surprising, but it’s a fact that the amount of power needed to roll across any road is a function of tyre rolling resistance and the amount of weight pressing down on those tyres. There is nothing complicated about measuring rider weight, but it is still an essential metric.

Aerodynamic Drag (CdA)

At race speeds some 80 – 90% of a rider’s power is being used to overcome aerodynamic drag, so some measure of how aero or not the rider is must be important. A rider’s “coefficient of drag” multiplied by “frontal area” (CdA) is the standard metric. The whole of a rider’s morphology – including height and weight – will have an impact, as well as flexibility and core strength. We can do a good job of estimating rider’s CdA from height and weight alone. With a suitable field test, involving power data, we can do an even better job. No description of a rider can be complete without some estimate of CdA.

So there you have it – 8 measurements that together describe the ability of a cyclist. Not all are easy to measure, and not all are important to every type of event, but the good news is that the metrics important to the most popular events are also the most measurable using nothing more complicated than a power meter. Now what can we do with these metrics? First, this kind of total description of a rider can be invaluable to coaches looking to deliver improvements having excellent specificity for target events. But by extension these metrics have great applicability in CPL performance modelling.

One way to define performance modelling is simulating the performance of a given rider, on a given course, under given conditions, and the better we can define the given rider then the further we can go. By defining a rider in terms of the above metrics performance modelling can deliver the best possible insights in terms of performance prediction, performance benchmarking, goal setting, optimal pacing, equipment evaluation, and more.

In conclusion we would encourage all riders and coaches to take a multi dimensional view of testing and defining ability, using at least the metrics relevant to current goals. And of course, with benefits like the above, to embrace the considerable potential of performance modelling.

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