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September 1, 2013
My 80-something kilograms put me at the heavier end of the spectrum when it comes to cyclists. Guys like me have always been associated with sprinting or time-trialling, but personally I’ve always been able to put out my best power on the climbs rather than on a flat time-trial. Why would there be a difference?
For example, when I climb I can hold a consistent 400 watts for 15 minutes. I can’t do this when I’m on a TT bike though — I can only manage about 330 watts for 15 minutes. It’s not just me – my mates and I have always debated about why this might be the case and to be honest, none of us really knows.
My go-to guy when it comes to the physics and biomechanics of cycling is a gentleman named Raoul Luescher. Raoul owns Luescherteknik and has a long history in elite sport and sports engineering.
The AIS started researching this phenomenon back in the early 2000’s. Raoul and his colleagues noticed that Brad McGee could float around the track at 650 watts and make it look effortless. However, they questioned why he couldn’t climb when he clearly had good power.
McGee isn’t a a diminutive climber, but with a race weight in the low-70kg range, his power-to-weight ratio would suggest he had everything he needed to be able to climb.
I sat down and spoke to Raoul about this and a significant factor is inertial load.
“Basically, you’ve got a mass component, and you’ve got an acceleration component. To accelerate that mass, it takes energy. The kinetic energy is 1/2 mass times velocity squared.”
I don’t want to bore you with the physics, but if you’ve ever wondered why you’re good at one type of riding, and pathetic at another, this is something that may interest you.
Quite simply, kinetic energy is the energy of motion. In this case, the energy contained in you — and your bike — when you’re already in motion. The measurement units for energy (either potential or released) are joules. Power is expressed in watts, which is a joule per second.
So if I’m 80kg and I’m riding at 20km/h, my kinetic energy will be roughly 1,230 joules (Ek = ½ x 80kg x 5.55m/s2, where 5.5m/s is the same as 20km/h).
If I’m doing 50km/h, my kinetic energy will be nearly seven times higher — 7,720 joules — because as mentioned in the equation, velocity needs to be squared.
If you’re travelling 50km/hr on a flat road and you stop pedalling, you’re still moving quickly and won’t slow down very much. You’ve got all this energy which helps to overcome the drag and rolling resistance. So you’ve got a lot kinetic energy in the system, but the forces that are retarding you are relatively minor, so you don’t slow down very quickly.
Now, when you’re climbing, you are moving at a slower pace so you have less kinetic energy. When you stop pedalling on a climb you slow down very quickly. This is because you’ve got less energy in the system, plus the resistive force of gravity is significant.
If the total resistive power is 400w it would take 19.3seconds (7720/400) to slow down to a stop from 50km/h on the flat as compared to 3 seconds (1230/400) on the climb.
What does this mean? Well the way you pedal in a time trial is different to the way you pedal when you’re climbing. It might not seem obvious when you’re pedaling, but it’s all about motor pattern recruitment.
When you’re time trialing, once you’re up to speed you’ve got a lot of energy in the system and as the pedals go around they’re merely topping up the energy required to sustain a fast pace. In a TT (high speed, flat road, high kinetic energy), the duration that your muscles have to fire is very small. You’re basically firing the muscle for a very short period of time every pedal stroke, but very quickly.
When you’re pedaling up a climb (low kinetic energy, traveling slowly, gravity holding you back), your legs impart force on the pedals for a much longer duration throughout the stroke, even though your cadence might the same as when TT’ing.
In short, your motor patterns are significantly different between time trialing and climbing.
When rotating the pedals quickly (i.e. using fast-twitch muscle fibres), you are dipping into your anaerobic energy systems much more than when you’re using slow-twitch muscles (because the muscles are firing quickly). If you don’t have enough fast-twitch fibres you can’t generate the leg speed needed to stay on top of high inertial loads. Your muscles simply can’t fire quick enough.
One of the best examples of athletes with phenomenal leg speed are BMX riders who will often hit more than 220rpm and generate 2,400 watts for six seconds on 180mm cranks. When their legs are moving that quickly, the loads are actually insignificant. The biggest load is moving their legs, not pushing down on the pedals. Power is calculated by force multiplied by velocity, and when velocity is so high there is a lot of power being generated. BMX and track sprinters will do “no chain” drills to hone their leg speed (try it – it’s much more difficult that it sounds).
Part of your fast-twitch/slow-twitch make-up is genetic. You can change some of your fibre makeup through training but part of it you can’t. The other component is about self-fulfilment – it’s about getting good at what you enjoy doing.
If you’re naturally better at low-kinetic energy riding situations (i.e. climbing), you’ll gravitate to climbs, because when you’re better at something it’s usually more fun. And if you spend all your time climbing, your TT-ing is going to suffer.
It’s important to note that we aren’t comparing fast-twitch vs slow-twitch in the classical sense here. You might think of triathletes as the ultimate slow-twitch specimens and the fast-twitch game being ruled by track sprinters. The differences between fast and slow-twitch that we’re talking about here are much more subtle.
When doing the research, they found found that a rider could ride a TT and a climb with the same power, and same cadence, but there could be a 10bpm variation in heartrate, as well in differences in oxygen consumption and lactates. It comes down to an athlete’s predisposition of muscle fibre make-up.
There are subtle differences in how a rider moves his or her legs and the speed at which he or she moves them. The pedal stroke is not uniform. There are many accelerations and decelerations throughout the movement which are dictated by the load.
The simple way to understand it is that in higher kinetic energy situations, you have less time through the pedal stroke to apply the force. Even though you might be riding at the same cadence while TT-ing and climbing, and even though your legs are travelling through the same arc, it doesn’t necessarily mean your legs are applying force and contracting in the same manner and sequence.
You can’t necessarily feel these subtle differences, but they can be measured using an electromyography (EMG) which detects electrical activity in the muscles.
The simplest way to think about this whole situation is that your muscles need to contract quicker in a high kinetic energy situation (i.e. time trial) than in a low kinetic energy situation (i.e. climbing). Time-trialling recruits more fast-twitch muscle fibres even though you may be at the exact same cadence and same power as when you’re climbing. Depending on your physiological make-up, you will likely be better at one than the other.
Of course a Grand Tour contender needs to be good at both TT-ing and climbing. While the two disciplines aren’t mutually exclusive, many riders are so good in one direction that they’ll never be able to excel in the other. This is what makes a Grand Tour rider so special.
Thank you to Raoul Luescher for his expertise in helping me with this post.