What's the lowest safe cadence on a climb?

Question

I recently installed a power meter and just used it to get up some rather steep 15%+ climbs.

At my weight and lowest gearing (34f 30r) at a sustainable power level for my fitness (225w) this translated to a seated 35-40rpm cadence.

Higher cadence would result in heart rates of 190bpm+ which I couldn't sustain over the climb without stopping.

This is MUCH lower than what I usually spin at (95-100) and while I made it up the hills, I am concerned that this could cause damage to my knees/body if I continue to do this without losing weight or increasing power. (Working on this :) )

I have searched, but there doesn't seem to be much on this topic.

A triple is not an option as I am doing this for fitness/fun and in the long run the compact I just upgraded to (50/34 from 46/36) will be enough once I get more KM into my legs.

These climbs were 1-2km in length with up to 130m elevation gain (Cat 4), but the question is more about doing this for extended amounts IE (Cat 1 / HC)

Standing for that long of a climb is not achievable for me at the current fitness level.

I didn't feel any discomfort in my knees, but everything I have read suggests high cadence cycling but I am not there fitness wise yet on these climbs.

Answer 1

Mountain bikers regularly run these low cadences for very short periods, often at much higher power output. The issue with causing damage to knees is more about the duration of the climb and how strong your stabiliser muscles are. (Too much time mushing causes chronic overuse problems, while weak stabiliser muscles can allow injury to happen), however they also have much lower gearing for sustained steep climbs.

If this is one or two short (2-5 minute) pushes over a longer ride, I would not worry about it, but be very careful about technique. If it's a couple of 15 minute periods over an hour long ride, I would be starting to look at options before problems start.

Options to consider would be changing the route or going to lower gearing – If this is a regular route, putting a triple on the front (with a 24), would get your cadence up to about 50-60, which is ideal.

If these are not an option, getting off and walking is not a bad thing. Don't let you ego get in the way of doing the sensible thing. You're already down to about 5-6km/h, walking won't be much slower, and as it uses different muscles than riding you are resting you riding muscles.

Also be aware that if you are lowering cadence preserve aerobic endurance, you are putting less force on your knees than if you increased cadence - so without changing the bike set-up you cannot fix the problem. The mantra of high cadence presumes you can go down to a lower gear and hence reduce the force on you knees.

Answer 2

Understand that the concern is not generally things like a muscle or tendon tear that can occur with, eg, extreme weightlifting -- off-road bikers might be susceptible to that sort of injury, but not a road biker.

Rather, the concern is the injury that may be done to joint surfaces and structures due to repeated force, above some "tolerable" level, applied to the joint. This can result in joint inflammation, tendinitis, and in some cases, a form of arthritis (with resulting permanent joint damage).

And, unlike the torn tendon, this sort of "repetitive strain injury" may not be apparent until some time (days/weeks/months) after the injury has begun in earnest.

Based on your "enhanced" description of the situation, you are likely on the cusp of a problem. Depending on your genetic makeup and physical condition you might be safe or might be in danger territory. At the very least it would be wise to tread carefully.

(I'll add that one is most exposed to joint injury when the knee is tightly bent, vs when it is straighter. This is one reason why the seat should be as high as reasonably possible, and one should avoid riding a seriously undersized bike.

Answer 3

Your options to reduce the force on your knees are to reduce bike + rider weight or get lower gearing. If you have a standard road bike cassette, there are climbing cassettes that will reduce the gearing slightly (around 10% or so).

Is it dangerous to your knees? Well, that depends on your personal physiology, how much force you are pushing, how much to you have trained for this, and how well your bike fits you.

If you can do this without your knees hurting, you are probably fine. You might also consider standing up; this changes the mechanics a bit and mixes things up. When you stand up, make sure you slow down or you will spike your heart rate.

Welcome to the steeps!

Answer 4

Low cadences are generally putting a lot more strain on the knees, as you're relying on power, given the gear ratios I suspect another issue.

I would work on your fitness on flatter terrain and build up to hills. Start within your cardio range and stay in it, do this regularly 3-4 times per week if possible. The problem stems from fitness/strength which hasn't accumulated enough for the hills. Your cadence sounds good otherwise. Your heart is more important than your knees.

Optimally the value of around 60-70 rpm to be on the cardio rather than anaerobic side of your workout. Anaerobic will mean you tire quicker and can't maintain sustained effort.

With knees equally as important are good bike sizing and proper setup. The setup is incredibly important, ensuring saddle height, saddle fore/aft position and bar height are optimal for your physiology, reducing strain, improving comfort and efficiency. Further more make sure your cleats and shoes are suitable, different brands suit different physiologies better. I had a pro fit which remedied a lot of ongoing knee pain, far cheaper than physio.

http://www.bikeradar.com/gear/article/technique-how-to-pedal-like-a-pro-12772/

Answer 5

I'd like to venture that its not the lowest cadence that makes or breaks safety limits,
its the lowest speed where you can maintain balance and steerage way.

I have a hefty recumbent, and my lowest speed up a significant grade is 4.7 km/h. At this speed I'm focusing almost exclusively on bike control, which is staying upright and moving forward in a straight line, and not stopping.

If I turn off the line too much, the bike is much more likely to fall sideways or require a large correction steering input.

Based on https://www.sheldonbrown.com/gear-calc.html your bike will be doing:
5.7 km/h at 40 RPM in a gear of 34:30 on 28mm 700c tyres.
8.6 km/h at 60 RPM, the same.

Answer 6

Regarding "could cause damage to my knees/body if I continue to do this without losing weight or increasing power".

For a given grade and a given gear (34f 30r) increasing power will not reduce the stress on your knee. To go faster (higher cadence - more power) in gear X and hill Y you need to push harder.

The only thing that is going to reduce pedal force is less weight or a lower gear. Same answer as Eric (+1+).

34f 30r is a fairly low gear. Your muscles may be out of shape but your knee is not out of shape. If you don't have knee problems you should be OK. Unless you are carrying a lot of extra weight.

I would go ahead and continue to train with 34f 30r and if you get knee pain then you are going to need to go with a smaller gear or a smaller hill.