Half of cyclists get knee pain. Most bike fits measure the wrong thing.

Overuse knee pain turns up in about half of all cyclists. That figure comes from a review of the saddle height literature, and it is the reason the bike fitting industry exists in roughly its current form.

The response is almost always the same. Something on the bike gets measured, compared against a number, and moved. A plumb line off the front of your kneecap. A knee angle at the bottom of the stroke. A fore-aft saddle position set to a rule with an acronym.

Those measurements are real, and they are taken carefully. The problem is what happens next — the assumption that because a number is precise, it is the number that matters.

Most of what gets measured in a standard fit has been tested against the forces inside your knee. Most of it does not move them.

The plumb bob, and what it actually does

The best known rule in bike fitting is KOPS — knee over pedal spindle. Drop a plumb line from the front of your kneecap with the cranks horizontal, and it should fall through the pedal axle. Fore-aft saddle position gets set to make that true.

Three separate pieces of work have now tested the thing KOPS controls.

In 2013, researchers put 21 competitive cyclists on instrumented pedals and had them ride at their preferred saddle position, then forward of it, then behind it. Moving the saddle fore or aft changed patellofemoral compressive force by 1 to 4%. Tibiofemoral compression moved 1 to 3%. The authors put those down to the 5–6° of knee flexion angle change that came with it — in other words, trivial.

A 2018 study went further, testing saddle setback across a range of around six centimetres using musculoskeletal modelling. Their finding, stated plainly: moving the saddle forwards was not associated with an increase of patellofemoral joint forces. The paper opens by noting that the evidence for saddle setback affecting knee pain remains inconclusive.

Then in 2021, a study of 40 adults riding 18 different bike positions — one from commercial fitting guidelines, seventeen deliberately deviating from it — modelled patellofemoral and tibiofemoral joint reaction forces and asked which variables predicted them. The forces were predicted by hip and ankle kinematics. And: horizontal saddle position and crank arm length were not related to JRFs.

Horizontal saddle position is precisely what KOPS sets. Three groups moved it on purpose, one of them by six centimetres, and the joint everyone is worried about barely noticed.

Worth being accurate about what this is. Nobody has run a prospective study following cyclists to see whether KOPS predicts who develops knee pain — that study does not exist. What exists is an absence of any evidence that it predicts anything, plus direct mechanical testing showing the position it dictates does not meaningfully change patellofemoral load. That is not quite the same claim, and it is worth stating honestly, but it is not a weak one either.

One thing fore-aft position does change: tibiofemoral shear force, which rose by up to 26% in the rearward position in the 2013 study. So saddle position is not inert. It just does not do the thing KOPS is sold as doing, and what it does change is not the joint the rule is aimed at.

What about alignment?

The other staple of a fit is watching the knee from the front — looking for the knee that tracks inward, the hip that drops, the ankle that collapses.

A 2022 study assessed pedalling kinematics in 50 cross-country mountain bikers, 26 with anterior knee pain and 24 without. Sagittal plane variables did separate the groups. Frontal plane hip, knee and ankle alignment showed no association with anterior knee pain at all.

The sagittal findings are worth reading carefully, because the direction surprises people. Larger peak ankle plantarflexion and larger peak knee flexion were associated with less likelihood of knee pain. Larger forward trunk lean was associated with more.

The problem underneath all of this

Here is the part that matters most, and it is the same problem we wrote about in runner’s knee.

A 2018 systematic review pooled the studies on knee pain in cyclists. It found that cyclists with knee pain do differ from those without — larger knee adduction, larger ankle dorsiflexion, altered activation of the vastus medialis and lateralis. But it also found no differences in knee flexion angle, tibiofemoral forces or patellofemoral forces.

And then the authors said the thing that reframes the whole field:

this limited evidence is based on retrospective studies comparing cyclists with and without pain, which limits the conclusion on how cyclists develop knee pain

That is exactly the flaw in the runner’s knee literature. Measure people who already hurt, find differences, and you cannot tell whether those differences caused the pain or arrived with it.

Which gives this post its real argument. We measure the bike because the bike is measurable — not because the measurements predict who ends up in pain.

So when you read that cyclists with knee pain show more knee adduction, hold it the same way you would hold “runners with knee pain have weaker hips.” It is a description of people who already hurt. It is not a demonstrated cause.

The one that does matter — and where it stops mattering

Saddle height is the exception, and it is worth knowing where the line sits.

The 2011 review found that a 5% change in saddle height altered knee joint kinematics by 35% and joint moments by 16%, and that patellofemoral compressive force appears inversely related to saddle height — lower saddle, more force. A separate study found a low saddle produced greater knee adduction moments, sustained for longer, with larger knee flexion angles. Raising it reduced both.

But the same review also reported that changes of less than 4% do not appear to affect injury risk or performance. And a study testing saddle height changes of ±3% found no difference in peak patellofemoral or tibiofemoral forces at all.

Those findings agree with each other, and together they give you something genuinely useful: there is a threshold, and it sits somewhere around 4–5%.

Get your saddle height meaningfully wrong — and for most riders that is a centimetre or two, not a millimetre — and you change what the joint absorbs. Fiddle inside that range and you are, on the current evidence, chasing noise. Which is the opposite of how saddle height usually gets discussed.

For what it is worth, the review’s own recommendation is to set saddle height by knee flexion angle, in the range of 25–30°.

Cadence: the running advice does not transfer

If you have come to cycling from running, or you are a triathlete, you have probably been told to spin more. In running, that advice has good evidence behind it — a 7.5% increase in step rate reduced peak patellofemoral joint force by half a body weight.

On the bike, it does not carry across.

A study measuring patellofemoral joint forces across varying workloads, pedalling rates, saddle heights and foot positions found that different pedaling rates or foot positions did not significantly change these forces. What did change them: workload and saddle height.

There are reasons to hold that loosely. It is six subjects, from 1987, and a null result in a small old sample is weak evidence of no effect rather than proof of none. Other modelling work has found that average hip and knee moments are minimised somewhere near 105 rpm, so there may well be a cadence effect on moment that simply has not been demonstrated on patellofemoral compression. And as cadence rises, more of the pedal force becomes non-muscular and inertial, so total joint force does not fall the way a simple model predicts.

But the practical version holds up: on a bike, the levers with the best evidence behind them are how hard you are pushing and a saddle that is not substantially too low. Not cadence.

Where the evidence disagrees with itself

Worth saying out loud, because most articles on this present a consensus that does not exist.

The 2021 study found joint reaction forces were least sensitive to minimum knee flexion angle — which it describes as the current gold standard for assessing bicycle fit. That is the same 25–30° knee angle rule the 2011 review recommends. Meanwhile the 1987 work did find saddle height moved patellofemoral force, which supports the review and cuts against the 2021 finding.

Three papers, three overlapping but non-identical answers. The honest reading is that saddle height matters at the extremes, the specific angle target is less settled than it is presented, and fore-aft position is the weakest of the lot.

It is also worth noting that two of the studies finding little effect used low workloads — 60 watts in one case, explicitly because the researchers were interested in rehabilitation. The 2021 study concluded that bike fit is of particular importance for those working at high intensities. Which fits what we see: fit problems that are invisible on an easy ride surface on the fourth hour of a hard one.

So what does a fit that works actually do?

The 2021 study answered this directly in its conclusion: optimizing joint kinematics should be the primary goal of bicycle-fit.

Not a plumb line. Not a number off a chart. How you actually move — your hip and ankle in particular, which were the variables that predicted joint loading when the rules did not.

And you cannot optimise joint kinematics without first knowing what range the rider has available. How far can the hip flex before the pelvis rotates? How much dorsiflexion is there at the ankle? How much of that range can you control under load? A position that requires motion you do not have will be held by something else, every pedal stroke, for hours.

Which is why our bike fitting starts on the treatment table rather than the trainer. Assess the rider, then adjust the bike.

When to get it looked at

If your knee has been grumbling for more than a couple of weeks and is not trending down, it is not settling on its own. The same is true if it is reliably arriving at the same point in every long ride, or if you have already changed three things on the bike and none of them helped — that last one usually means the problem is not where you have been looking.

And if you are deep in a build for Ironman WA, the volume you are about to do is exactly what turns a small, tolerable thing into a season-shaped one.

On Form Physiotherapy is Perth’s endurance sport clinic. For a physio-led bike fit, or a knee that is not settling, book an assessment or call (08) 6385 8029.

Sources

According to PubMed:

• Bini R, Hume PA, Croft JL. Effects of bicycle saddle height on knee injury risk and cycling performance. Sports Medicine 2011;41(6):463–76. DOI
• Bini RR, Hume PA, Lanferdini FJ, Vaz MA. Effects of moving forward or backward on the saddle on knee joint forces during cycling. Physical Therapy in Sport 2013;14(1):23–7. DOI
• Ménard M, Domalain M, Decatoire A, Lacouture P. Influence of saddle setback on knee joint forces in cycling. Sports Biomechanics 2020;19(2):245–257. DOI
• Gatti AA, Keir PJ, Noseworthy MD, Beauchamp MK, Maly MR. Hip and ankle kinematics are the most important predictors of knee joint loading during bicycling. Journal of Science and Medicine in Sport 2021;24(1):98–104. DOI
• Branco GR, Resende RA, Carpes FP, Mendonça LD. Association of cycling kinematics with anterior knee pain in mountain bike cyclists. Journal of Sport Rehabilitation 2022;32(1):40–45. DOI
• Bini RR, Flores Bini A. Potential factors associated with knee pain in cyclists: a systematic review. Open Access Journal of Sports Medicine 2018;9:99–106. DOI
• Tamborindeguy AC, Bini RR. Does saddle height affect patellofemoral and tibiofemoral forces during bicycling for rehabilitation? Journal of Bodywork and Movement Therapies 2011;15(2):186–91. DOI
• Wang Y, Liang L, Wang D, Tang Y, Wu X, Li L, Liu Y. Cycling with low saddle height is related to increased knee adduction moments in healthy recreational cyclists. European Journal of Sport Science 2020;20(4):461–467. DOI
• Ericson MO, Nisell R. Patellofemoral joint forces during ergometric cycling. Physical Therapy 1987;67(9):1365–9. DOI
• Redfield R, Hull ML. On the relation between joint moments and pedalling rates at constant power in bicycling. Journal of Biomechanics 1986;19(4):317–29. DOI
• Kautz SA, Hull ML. A theoretical basis for interpreting the force applied to the pedal in cycling. Journal of Biomechanics 1993;26(2):155–65. DOI
• Willy RW, Willson JD, Clowers K, Baggaley M, Murray N. The effects of body-borne loads and cadence manipulation on patellofemoral and tibiofemoral joint kinetics during running. Journal of Biomechanics 2016;49(16):4028–4033. DOI