So, my mind was wandering on my ride today and I came up with another explanation as to why cycling efficiency has a J or U shaped curve in relation to cadence. This is not the only potential mechanism but I think it is probably in play. I wanted to bounce it off you and see what you think.
First, for any given force, it takes more energy to contract the muscle the faster it shortens. The contraction that takes the least amount of energy is an isometric contraction as there is no shortening. The reason for this is it takes energy to make and break those actinomyosin bonds necessary to shorten the muscle. So, higher pedal speeds require higher muscle shortening speed and, hence would be less efficient. That would be in play at the high cadence end. I believe this has been described before as a potential contributor to the curve.
But, what about the low cadence end. Slower pedal speeds require higher muscle forces to generate the same power. This means the pressure in the muscle will increase and at some point will increase such that it interferes with blood flow, at first partially as it gets above systolic pressure becoming more severe as it gets above mean arterial pressure then above systolic pressure. The more restriction in blood flow probably has an effect on pH (or other effects) that could reduce contractile efficiency and lowering efficiency at lower cadences. Has any work been done to correlate the relationship of intramuscular pressure to arterial pressure to cycling efficiency? I couldn’t find any via google.
Although I don’t think this is the only mechanism at play here, these two effects together could be part of the explanation as to the shape of this curve. Comments?
Since the usual suspects haven’t come forth and pointed out how idiotic my thoughts are (again) I can only conclude that my thoughts are reasonable. Thanks for the feedback.
I am very weak this year, and find myself firing the cranks more quickly to try to make up the power. 100-110 RPMs.
I think last year I was more of a 80-90 RPM kind of guy.
There is no one single most efficient cadence. It will depend on a lot of factors including how you are made up and what power you are riding. The question here goes to the mechanism to describe why there is a most efficient cadence. It would appear that my tag is correct, exercise physiologists don’t do mechanisms.
So, higher pedal speeds require higher muscle shortening speed and, hence would be less efficient.
Whether you are riding at a low cadence or high cadence wouldn’t the length of muscle shortening be equal? The leg will go through the same kinematic sequence. I think you also have to look at power compared to torque, At a higher RPM it takes less torque to create the necessary power as well. Does less torque translate to less fatigue?
thinking about this though since power is linear with cadence the combined torque would probably be equal, however, is it like TSS where there is a exponential relationship between fatigue and torque?
If you want power, higher cadence, if you want endurance lower cadence.
Just think of cars, in a 1/4 mile race they are not worried about saving gas so they go as high of rpm as possible to create the most power.
If you were doing a 4 hour long car race and you couldn’t refill your gas tank then you have to start to look at gas conserving principles and ride at a lower rpm.
You also can’t compare road racers to TTers, they have to deal with quick accelerations which you can’t do if you are at 80rpm. They also don’t sustain 100+rpm for the entire 4 hour ride.
further more if you are training for ironmans then you should train your body to be really efficient at ~85rpm, if you go out and do a sprint mid training you probably don’t have the muscle adaptation to ride at 100+rpm.
So, higher pedal speeds require higher muscle shortening speed and, hence would be less efficient.
Whether you are riding at a low cadence or high cadence wouldn’t the length of muscle shortening be equal? The leg will go through the same kinematic sequence. I think you also have to look at power compared to torque, At a higher RPM it takes less torque to create the necessary power as well. Does less torque translate to less fatigue?
thinking about this though since power is linear with cadence the combined torque would probably be equal, however, is it like TSS where there is a exponential relationship between fatigue and torque?
Well, you are correct that the muscle shortening, and hence the number of bonds made and broken per revolution, would be the same regardless of cadence but the number of bonds made and broken per unit time would vary with cadence (it also take some energy to relax and stretch the muscle). Since power is based on work done per unit time the efficiency should drop at higher cadences.
But if you are riding at a higher cadence the time is shorter no?
If you are riding at 80 rpm or 100 rpm and at the same average power, then the work done per time should be equal
the only difference I think is if you are at 100 rpm you are having short bursts of lower torque compared to longer bursts at higher torque (but less bursts nonetheless). That is where my question regarding whether or not torque and fatigue is an exponential relationship. If it was linear then it should all equalize.
But if you are riding at a higher cadence the time is shorter no?
If you are riding at 80 rpm or 100 rpm and at the same average power, then the work done per time should be equal
the only difference I think is if you are at 100 rpm you are having short bursts of lower torque compared to longer bursts at higher torque (but less bursts nonetheless). That is where my question regarding whether or not torque and fatigue is an exponential relationship. If it was linear then it should all equalize.
Well, I don’t believe this is a linear relationship. If it were I don’t believe the curve would look the way it does.
Frank, why do you need to look to physiological factors to explain the drop off in efficiency at higher RPMs? Couldn’t it have to do with mechanical efficiency? At some point the speed of turnover equals the leg speed (limited by contractile speed) and the feet no longer apply force to the pedals. That might be really high like 130rpm, but at some point below that you probably begin applying power for less and less of the crank rotation.
As for the lower bound limit, what you said sounds right to me. Lactate buildup, oxygen depletion due to discontinuity of blood flow.
Frank, why do you need to look to physiological factors to explain the drop off in efficiency at higher RPMs? Couldn’t it have to do with mechanical efficiency? At some point the speed of turnover equals the leg speed (limited by contractile speed) and the feet no longer apply force to the pedals. That might be really high like 130rpm, but at some point below that you probably begin applying power for less and less of the crank rotation.
As for the lower bound limit, what you said sounds right to me. Lactate buildup, oxygen depletion due to discontinuity of blood flow.
That is another aspect to cause the drop off in efficiency, the foot must be accelerated up to the pedal speed before one ounce of pressure can be applied. But, experienced track cyclists can attain unloaded cadences of over 250 rpm so 130 is not the limit (although it might be close to the limit in the untrained).
I am sure there are several issues at play here all adding up to the whole. I was simply asking a question about one of them from those who, supposedly, have spent a life-time thinking about some of this stuff.
keeping power constant, a car operating at low or high rpm uses similar amounts of gas.
slightly more at higher rpms (ignoring cam timing and tuning issues) due to more friction.
which is probably true for the human knee and hip as well =)
that said, theres more to this than just efficiency. the most efficient cadence is MUCH MUCH lower than you would want to use, even for an ironman
If you want power, higher cadence, if you want endurance lower cadence.
Just think of cars, in a 1/4 mile race they are not worried about saving gas so they go as high of rpm as possible to create the most power.
If you were doing a 4 hour long car race and you couldn’t refill your gas tank then you have to start to look at gas conserving principles and ride at a lower rpm.
You also can’t compare road racers to TTers, they have to deal with quick accelerations which you can’t do if you are at 80rpm. They also don’t sustain 100+rpm for the entire 4 hour ride.
further more if you are training for ironmans then you should train your body to be really efficient at ~85rpm, if you go out and do a sprint mid training you probably don’t have the muscle adaptation to ride at 100+rpm.
that said, theres more to this than just efficiency. **the most efficient cadence is MUCH MUCH lower than you would want to use, even for an ironman **
It is? Do you have a reference for that?
I think the mistake you are making is looking at studies done on untrained cyclists who are putting out very low power. I think you will find that the most efficient cadence goes up as one puts out more power and that the most efficient cadence for an IM is close to what some actually ride in an IM (although I will admit that many ride at way to high a cadence because they are trying to ride like the pros even though they don’t put out the same power as the pros).
Whats interesting is that without adaptive training, most people are most efficient when they do a task as they would intrinsically.
If you give someone external cues (ie dictate their XXX) their efficiency will decrease in a relatively U shaped manner (generalization).
As you’ve pointed out, this is the reason that most non-trained cyclists exhibit best results at lower cadences.
However, if allowed ample time to adapt to the stipulation, they will become more efficient in the new parameter. Whether or not they are ultimately more efficient than their initial trial is a matter of the effectiveness of the new parameter. Nonetheless they will adapt. Muscle length will adapt to form more appropriate length-tension relationships etc.
Frank- Some counterpoints
I’m not aware of the energetic requirements of the speed of muscle contract with respect to crossbridge formation/ ATP usage.
However, If the same work is being accomplished with a higher rate, the force must be decreased. This is something you mentioned.
Keep in mind that Muscle tension / force is not generated by individual muscle fibers pulling harder, but in the recruitment of more motor units and more thus “calling in more troops”. If the energy savings you propose? exist, would they counter the increase in muscle fiber recruitment needed to produce a slow speed / high force contraction?
To completely occlude blood flow you need a relatively strong isometric contraction. This is a situation you would never see while biking. Even with a low cadence of 70rpm, the must contracts and relaxes in less than a second. While I could understand a slight decrease in bloodflow with extreme low cadences, the range through which normal riding occurs would probably increase blood flow through the “muscle pump” action. Muscle pump is used by cadets who are / were taught to isometrically contract their calf muscles during long periods of standing to improve venous return and decrease pooling that lead to fainting. Additionally its a large part of venous return especially given the one way valves.
Keep in mind that Muscle tension / force is not generated by individual muscle fibers pulling harder, but in the recruitment of more motor units and more thus “calling in more troops”. If the energy savings you propose? exist, would they counter the increase in muscle fiber recruitment needed to produce a slow speed / high force contraction?
To completely occlude blood flow you need a relatively strong isometric contraction. This is a situation you would never see while biking. Even with a low cadence of 70rpm, the must contracts and relaxes in less than a second. While I could understand a slight decrease in bloodflow with extreme low cadences, the range through which normal riding occurs would probably increase blood flow through the “muscle pump” action. Muscle pump is used by cadets who are / were taught to isometrically contract their calf muscles during long periods of standing to improve venous return and decrease pooling that lead to fainting. Additionally its a large part of venous return especially given the one way valves.
No, no, no. Complete occlusion of blood flow does not require an isometric contraction. It occurs in the heart with every beat of the heart whether at rest or at VO2 max. All it requires is a sufficiently strong contraction to cause the intramuscular pressure exceeds the mean arterial pressure.
The point I was trying to make was that while blood flow may stop for a fleeting moment, The cyclical nature of the movement would mean you wouldn’t achieve substantial occlusion unless the contraction was sustained for longer periods via a very slow cadence.
The point I was trying to make was that while blood flow may stop for a fleeting moment, The cyclical nature of the movement would mean you wouldn’t achieve substantial occlusion unless the contraction was sustained for longer periods via a very slow cadence.
The “occlusion percentage” actually goes up with higher cadences. This is because it takes a finite amount of time for relaxation to occur before blood flow can occur again and this time is fixed regardless of cadence. And, the stoppage is more than for a “fleeting moment”. This is well established in cardiac physiology where most of the experimental work has been done. The physiology would be exactly the same for skeletal muscle.
For my education, do you references to studies in which this is examined? Coronary artery occlusion or other?
This is a pretty standard physiological principle. A discussion can probably be found in any basic physiology textbook directed to the medical student level, certainly any basic cardiac physiology textbook directed to physicians.
I did find one internet reference that went to this from a cardiac anesthesia textbook. Read the part, Determinants of Coronary perfusion, on page 376