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What Does Cadence Have to do with Anything?
Written by Dr. SkibaRecently, I have been getting a lot of e-mail asking questions about cadence. Specifically: What cadence should a person select? What is going on in my body at different cadences? Why does it matter? This article will briefly cover these questions and get you on the track to understanding what science knows of these issues.
First, we are going to start with the definition of a word that a lot of people use to mean a lot of different things. However, it means just one thing in the realm of exercise physiology. The word is "efficiency". In terms of your body, we use it to express the ratio of the amount of work you are doing to the amount of oxygen you require to do that work. So, we would say that two people of like size and ability, with a similar cadence, who ride their bikes at 20 mph using 4 liters of oxygen per minute each are equally efficient. If one guy required 5 liters to keep up with the other guy, we would say he was less efficient than his friend. This has important ramifications, because the guy using less oxygen is exerting himself less and is probably going to do a lot better and perform at this level a lot longer.
The question becomes, "So what pedal cadence is most efficient? 60? 70? 80?" This is an important point. First of all, people will select their most efficient cadence, and they usually do it all by themselves. For many people, it is in the ballpark of 60 rpm. That is, at around 60 rpm, they are using the least amount of oxygen to do a particular task. Now, this does not mean that the most efficient cadence is the best cadence to use. Your goal is not to do your race using the least amount of oxygen that you can. Your goal is to get to the finish line as fast as you can. Rather than asking what is most efficient, we should be asking "what is the best cadence to maximize my performance?" As it turns out, this cadence is usually higher than the most efficient cadence, maybe closer to 80 or 90, and again this will vary depending upon the person.
We need to consider what is going on in your muscles at different cadences. We need to think about what kind of muscle cells are being used at different cadences and why. Let's imagine you and I are identical twins, and we are cruising on our bikes, and our power meters both read 200 watts. Fortunately, we are both incredibly rich, so we both happen to have portable oxygen meters and we know we are using the same amount of oxygen. You are spinning your pedals at about 80 rpm, in a reasonable gear. I am using a huge gear, and end up grinding along at say 40 rpm. What happens? Well, we are doing the same amount of work, and using the same amount of oxygen. However, per turn of the pedals, I am using twice as much force. I need to maintain my speed with only 40 pedal turns each minute. So, my muscles have to exert more force per turn than yours. You are doing twice as much pedaling, but each turn only takes half the force that mine does.
Now, recall that you have two basic types of fibers in your muscles: the slow twitch cells and the fast twitch cells. (For a comprehensive review on the subject, check out my book). The slow twitch guys are a little whimpy, but they can go a really long time before they get tired. The fast twitch guys are really strong, but they get tired quick. What happens in your body is that your brain recruits these guys based on the task. When you were spinning along at 80, your brain said, "He's doing a lot of pedaling, but he is never asking the muscles to put out a maximal effort. We can do this with mostly the slow twitch guys."
Consider my case. My brain looked at what I was doing and said, "This clown is trying to push a really hard gear, and each turn of the pedals is taking a lot of work. Not only do we need the slow twitch cells, but we need to call in the fast twitch guys to help out." The fast twitch guys burn mostly carbohydrates, that is, the glycogen stored inside them as well as the Gatorade you drink/gels you eat. Remember, the fast twitch guys also get tired more easily, and this is at least partially due to using up the glycogen.
So what happens? Well, I would tire out and need to slow down as I wore out my fast twitch muscle fibers. I would be unable to continue putting out so much work per pedal turn. You, on the other hand, would probably go longer. As your slow twitch muscles fibers tired out, your brain would sequentially ask the fast twitch guys to come in and take up the slack. So, you would also get tired, but probably much later on.
Thus, we have a good reason for why race cadences are so much higher than the most efficient cadence. You are being smart about your fuel choice, because you have a lot more fat storage in your body than you do carbohydrate storage. You are also reserving some muscle cells for when you need them, rather than just beating them all up at once. In other words, on a subconscious level, your brain is deciding on your best strategy to get to the finish as fast as it can. It is performing a complicated balancing act between oxygen use, muscle fatigue, and fuel supply.
The moral of the story is this. Most people will pick the best cadence for the job they are trying to do, so it doesn't make a lot of sense to try to force the issue and ride a particular cadence. If you are just cruising the neighborhood, your oxygen needs are low, you don't need many muscle cells to do the job, so the low cadence is fine. You are doing your thing with your slow twitch muscles, and not very many of them. If your buddies pull up beside you and challenge you to a race, you are going to start doing a lot more work, and your brain will change your cadence accordingly to help you have the best performance. The important thing is not to over-think matters. Your brain knows what it is doing, just stay out of the way!
References:
Kautz and Neptune. Biomechanical determinants of pedaling energetic: internal and external work are not independent. Exerc. Sport Sci. Rev. 30(4): 159-165. 2002.
Ahlquist et al. The effect of pedaling frequency on glycogen depletion rates in type I and type II quadriceps muscle fibers during submaximal cycling exercise. Eur. J. Appl. Physiol. Occup. Physiol. 65(40): 360-64. 1992.
