What’s the Mechanism?

If you’ve read the previous blog article in this series then you’ll have spotted my dislike for the “if X, then Y” approach to bike fitting. The main reason I dislike it is because it’s never supported by an explanation as to why it works, or what the mechanism is behind why you see X in the first place. So, if you don’t know why it works, or what causes X, then what makes you think Y will solve it?

As an example, lets dig into an old wives’ tale of bike fitting – that the knee being in front of the pedal axle at 90 degrees through the pedal stroke causes anterior knee pain.

Thankfully, there aren’t many fitters that still believe this, but it also gives us a good opportunity to think about the thought process (or lack of) for those fitters that used to believe it. There’s definitely a group of fitters out there who want everything to be simple, and some education has tried to simplify fitting to make sure that all their students understood the concepts they were trying to teach. So, I get where the “if X, then Y” approach came from. But what I struggle to understand is how more people didn’t spot the flaw in it.

Blindly believing what you’re told makes life easy; there’s no grey, it’s just simple and everything’s clear. There’s a comfort to things being certain. But it’s not reality, and far too many fitters cling to that comfort rather than peak below the curtain to see what’s actually happening.

If you did start questioning why the knee being in front of the pedal axle causes knee pain then you’d quickly struggle to find a decent answer. Some people say it increases patellofemoral forces, but science has disproven that theory. A research paper published in 2016 (Domalain et al) and presented at the Science & Cycling conference that year, found that patellofemoral forces do not change significantly across saddle fore-aft changes within a 6cm range. The study also suggests that the forces are also quite low – certainly lower than the body experiences when running – and therefore not a risk factor.

But that study wasn’t printed until 2016, so fitters before that could have been forgiven for not knowing, right? No, because Ericson and Nisell found exactly the same thing in their study way back in 1987. That paper almost predates bike fitting, so almost all fitters working today started after that study was published.

There’s a great article by Keith Bontrager that finally killed the knee over pedal spindle concept for good, but it really shouldn’t have taken that long!

Even if you use pure logic and compare it to other activities where the knee extends beyond the pedal axle without knee pain – we don’t even have to step outside our own sport because it’s incredibly common in triathlon and time trial – you don’t see an increased risk of PF pain. If there was, entries to triathlons would be dropping off a cliff because of all the injured athletes. And triathletes are running, which is a much higher risk factor for knee pain than cycling, so why don’t they get knee pain?

Any time someone tries to answer that question the answer just points to another variable that’s more likely to resolve the issue:
“Triathletes tend to ride shorter cranks.”  Surely that suggests your solution? Shorten the rider’s cranks, don’t not move their saddle back.

“Triathletes tend to have a higher cadence.” Then suggest your rider with PF pain increases their cadence.

Why would you move the saddle back if the reasons people suggest triathletes don’t get PF pain suggest other interventions would be more successful?

The only logical reason I’ve seen to attempt to explain away PFP risk in triathletes is that they split their training across other disciplines and therefore reduce the repetitive strain compared to a pure cyclist. The problem with this argument is that I know a lot of Ironman triathletes that still ride more than some of the road cyclist clients I see with PF pain.

On the rare occasion when moving the saddle back helped, it was probably because it either increased their saddle height or changed when/where forces were applied to the pedal. Neither of which have anything to do with where the knee aligns at an arbitrary point in the pedal stroke.

This is why it’s important to understand what the mechanism is behind your interventions. Without it, you’re just guessing and hoping.