How Cadence Impacts Power Output And Knee Joint Stress

I am going to specifically talk about how cadence impacts power output and stress loads on your knee joints. Pay attention, because this could save you from a knee injury and help you become more energy efficient at the same time. I’ll nerd out with some equations and calculations, but stay with me and I’ll sum it all up with some take aways at the end, so it is easy to understand.

Power Is A Calculation Of Torque And Cadence

First of all; power is actually a number derived from torque and angular speed. I am sharing the equations with you below to clearly show you how cadence impacts power. Believe it or not, I actually remember this stuff from engineering school over 15 years ago, but I did double check myself with a google search, because google is the new text book, right…?

Watts = Torque x Angular Speed

Torque: N-m

Angular Speed: radians/sec

Horse Power = (Torque X Cadence)/5252

Torque: lb-ft

Cadence: rpms (revolutions per min)

In other words… power will increase as cadence increases, assuming the same amount of torque applied. Or, the same power output can be achieved with less torque and a higher cadence. I know there is a caveat to this as there is a limit to how fast anything can spin, but bear with me and I will circle back after discussing torque.

Torque = Force X Length

Force: load applied to your pedals

Length: length of the lever arm, i.e. your cranks arms

So as I said earlier, power increases as cadence increases, assuming the same amount of torque input. Or, the same power output can be achieved with a higher cadence and less torque. The caveat is that torque will drop off at some point, since it is reliant on the amount of load that can be applied to the pedals and it is increasingly difficult to apply the same load as cadence increases. We all have our limits here, but there are things that you can do to impact it. This is where cadence spin up drills and explosive power workouts come into play. I’ll come back to this in the real life examples below.

Real Life Examples Of How Cadence Impacts Power

Cadence Impacts On Knee Joints And Tendons

Let’s say that you are going to do a hill interval workout at 260 watts and 60 rpms… I am pulling these numbers directly out of some hill repeat workouts I was doing in one of my build phases this past year. You can find a link to that training report at the bottom of this write-up.

Anyways… If you put down 260 watts at 60 rpms, that means you are putting 41 N-m of torque into your drivetrain. That translates into 31 lb-ft of torque, for those holding out on the english system. Assuming a 175mm crank length, this means you are putting 52 lbs of force into the pedals. Again, I’m doing the math for you to bounce back and forth between the metric and english system, but I want you to understand the point.

So here is the point… If you put out the same 260 watts, but at 90 rpms… You are now only putting 35 lbs of force into the pedals vs the 52 lbs of force at 60 rpms. This means your knee joints are seeing 33% less compression force and your muscle tendons are seeing 33% less tensile stress (tension) at the higher 90 rpms and 40% less at 100 rpms. This is a really big deal. I put out about the same moving speed, with the same power output and same heart rate, but using a third less muscle force to do so at 90 rpms vs 60 rpms. I think this clearly shows how cadence impacts power and stress in and around your knee. Here is a chart where I did another short session for a quick look at the numbers again.

I also did a workout 2 days later, using the same power numbers at around 95 rpms. My heart rate was about the same for both workouts. Pay attention here… There was no difference in my heart rate, between spinning at 60 rpms and 95 rpms for the same 260 watt power output. My cardio vascular system did not work any harder spinning at 95 rpms, than it did at 60 rpms, but my muscles put out 35% less peak force and my joints saw 35% less stress while spinning at 95 rpms vs spinning at the 60 rpms. Yes, my muscles had to move faster, but the peak loads were less.

Vertical Jump Height Paralleled With Cadence

To draw another parallel to this is, just because you can squat or leg press a lot of weight, doesn’t mean that you can jump higher. Chances are you will be able to jump a little higher as you increase leg muscle in general. But, to really make big gains in your vertical jump, you must be able to exert that leg force at a high rate of speed.

Think of vertical speed like you would cadence. Your quads contract, exerting tension in your tendons that force a rotation around your knee joint to produce a force on the ground through your lower leg, making you jump. You will jump higher as you develop the ability to produce force in your quads at higher rates of speed, causing faster rotation at the knee joint that results in your legs straightening out faster and you jumping higher. That is what power production is all about.  The ability to produce a lot of power comes down to the ability to exert force at higher rates of speed.

Cadence Impacts Power Output Of Your Car Engine

Just like a car engine, where power starts to drop off at some point before the rpm redline, our legs will do the same. I know my ability to put out more power will start dropping off somewhere over 110 rpms. I couldn’t tell you the exact rpm point as I haven’t experimented specifically with power at those levels. But, even if I can produce more power at 110 rpms vs 90, I am approaching my redline and am going to burn out if I try to spin out over 100 rpms for too long. That longevity obviously shortens as rpm increases.

The same thing will happen to your car engine if you try to run it at its peak power point for extended periods of time. It will start running too hot, overheat and then a multitude of bad things can happen from there. I’ve destroyed more than 1 car engine in my life, but that’s another topic… I’ve destroyed my legs many more times these past few years.

To The High Cadence Naysayers

That being said… There is always that person that says, well not everybody can spin at 90 or 100 rpms and that creates too much stress on my cardio system. If you are overworking your cardio at 90 rpms to put out the same power that you could at a lower cadence, then you need to stop doing hill repeats. Go work on your cadence skills and improve your cardio before your ruin your knees.

So why would I do hill repeats at 60 rpms then…? First, I only do them at 60 rpms, because I’ve worked my way down to sixty rpms over the past couple of years. I’ve adapted my muscles and joints to those efforts over time. That being said, my force and power output is far from stellar and I need to improve it. Now that I have spent enough time on the bike to adapt, I am using that slow cadence near my max force ability to add muscle mass, build strength and later turn it into more power with short max power intervals.

Key Takeaways On The Importance Of Cadence

Hopefully all of that made sense. I love this stuff and have had fun putting it together. At one time during engineering school, I had a small interest in switching out of Mechanical Engineering and into Biomechanical Engineering, but I think I was afraid of the biology part at the time. However, as I was diving deeper into understanding fitness and became a certified personal trainer, I couldn’t help but draw the lines between human body movements and principles I learned in engineering dynamics. I really think it helps all of this make more sense and really breakdown the impacts of various inputs on the body such as how cadence impacts power and forces on your knee joints.

At any rate, be sure to read through the takeaways below. Hit me up if you’d like clarification on anything or have any comments or questions in general.

Improve Your Cadence Skills With Spin Up And Pedaling Technique Drills

Work on your cadence skills so that you have the ability to efficiently spin at higher rpms. If you are already strong, then you will be able to put out more power naturally as your muscular skeletal system adapts to spinning at a faster cadence. This is why I include some level of cadence work in the training plans that I put together.

High Power At Low Cadence Puts Increased Loads On Your Knee

Don’t immediately drop your cadence and go out for hill repeats. You have to ease into it and make sure you adapt your muscles and joints over time to the repetitive high loads that your knees will see at lower cadences. I have done some low cadence work at 60 rpms, but I’ve worked my way down to that and really monitor my knee for any subtle pain during and after the workouts.

I was doing these workouts to increase my strength or force output abilities. To get the real gains out of the workouts, I followed up with shorter and higher rpm all out sprinting power intervals in the following weeks and I was definitely seeing gains in my power and climbing. I have a long ways to go as I have always had chicken legs, but it is a weakness that has held me back and I am working on it. It has been really cool for me to see gains in my leg muscle mass and strength.

Understand Your Sweet Spot Between Cadence And Force Output

I personally feel my sweet spot balance between power and cardio efficiency at this point is somewhere around 92 to 94 rpms. I can make more power at higher rpms, but I can’t hold it for as long and those moments should be reserved for attacks to bridge a gap, beating a group to the entry of singletrack, not getting dropped on fast race starts or finish line sprints. Outside of that, 92 to 94 rpms seems to be my balance between a lower continuous force output and the ability of my cardio and my muscular skeletal system not spinning too fast and spiking my heart rate.

You’ll Go Just As Fast

As I mentioned above, my moving speed was about the same while putting out the same power at 2 different cadences with the same heart rate. The difference is, I was putting out less muscle force and less stress on my knees by spinning at 90 rpms.

Understand And Improve Your Threshold Heart Rate

Work on your threshold heart rate. If you are newer at the sport, then you will likely be able to increase your threshold heart rate a little bit each year. I’ve been able to increase mine a little bit each year. I suspect that I won’t be able to raise it much more at this point, but I am confident that I can maintain it for quite a few more years as long as I maintain my intensity levels. As you get older, then you will have to work to maintain that threshold. I know this is an entirely different conversation, but it is worth mentioning.

Also, the whole 220 minus age thing that I hear people use all the time for max heart rate is just a ballpark. Do not try to back out your threshold heart rate, just by using a percent of your ballpark heart rate max. Actually understand where your threshold heart rate is at with testing, it will take less than an hour to get some actual numbers. As a side note, I bet I can ballpark somebody’s threshold heart rate after looking at some .fit files from a couple of their shorter races or Strava segment chasing rides better than trying to back it out of the old 220 minus age and percentages.

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My Training Plans

Training Report That Includes Mentioned Workouts

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