Monday, February 1, 2016

Anatomical Considerations in Bike Fit

by Alan Couzens, MS (Sports Science)

I was fortunate last week to have the company of John “The Whitster” Whittington and Mat “Matty Stein” Steinmetz to see if we could work together to come up with a solution for John’s ongoing posterior knee pain. John had been fitted numerous times by some very respected names in the bike fit world but continued to have issues with recurrent behind the knee pain during a good share of his rides.

It’s my opinion that John’s issues with the bike, and in fact, most athletes’ issues with discomfort on the bike despite being ‘fit’ come from a fundamental flaw in the bike fit paradigm. The most common methodology is to fit a rider to his bike rather than a bike to the rider. I’m not talking so much about fundamentals such as putting someone on an appropriately sized frame, but rather setting a cyclist into a restrictive position on the bike based on arbitrary numbers – angles of knee flexion, hip flexion etc without first looking at what happens to the cyclist’s body when he assumes such angles!!

It’s long been my stance that we should first look at the restrictions of the rider with respect to joint angles and range of motion and then we should ‘build the bike around the rider’ and his/her restrictions.
So, the first thing we did with John was to take him through a comprehensive range of motion assessment to look at what ranges of motion were ‘natural’ for him in a static position without some sort of compensation. We also looked at how these ranked with respect to normative data so that we could identify some potential hotspots that might be worthy of a work-around on the bike. Some of John’s results are shown below:

The ‘big hitters’, the scores that deviate more than 30 degrees from the norm are highlighted in yellow. John has limited range of motion in his left hamstring (surprise surprise), his left ankle and both quads.
As we might expect, these restrictions are significant enough to result in compensation on the bike. Typical ‘open’ hip angles on a road bike are in the range of 100-110 degrees. While this should be no problem for someone with a normal ROM in hip flexion of 90 degrees. John’s left leg came in at 126 degrees. Additionally, even with the knee bent to 30 degrees (a typical # for the bike), John was still at least 6 degrees short. Therefore this bears some consideration.

More importantly, typical ankle angles of dorsi flexion when pedaling are +10 degrees. With John’s restriction at the left ankle joint (max flexion of -10 degrees), this could cause some problems!!
So, the next step in the bike fit is to take a look at John on the bike to see if he is exhibiting any of the compensations that we might expect. For this we headed off to the capable hands of Matty Steinmetz at Retul.

The Retul system is quite simply the best motion capture tool on the planet when it comes to bike-fitting. It enables the fitter to see each and every joint angle during real time pedaling – a significant step up from the old method of stopping an athlete mid cycle and attempting to replicate a ‘real life’ position.
In this case we were looking to see if John’s current position was presenting challenges beyond his ROM capabilities and if any compensations involving the posterior knee may be resulting from John’s restricted range of motion in hip flexion, or dorsi-flexion of the ankle.

Sure enough, the keen eyes of Matty Steinmetz spotted that on the initial capture, John’s ‘knee forward of foot measure’ was only -4mm. Considering that in the Retul system, this measure is taken from the Fibula head rather than the patella, this meant that John’s knee was actually projecting beyond the line of the metatarsal head on the foot, creating a good amount of ankle dorsi-flexion, an amount of dorsi-flexion in fact that our good buddy Mr Whittington didn’t have!

This ‘maxing out’ of ankle dorsi flexion was leaving JW with no choice but to externally rotate the tibia to effectively shorten the foot length and decrease the ankle angle. This was clearly visible to the naked eye and was obvious to JW who commented that he was regularly hitting the chain-stay with the heel of his shoe! This was shown on the Retul chart as a lateral hip-foot average off set of -5mm. The problem with this, of course is that it made the Biceps Femoris muscle the big player in knee flexion and effectively lead to a case of tendonitis.

So, to rectify this we made a couple of changes:

  • We brought the seat back to move the knee further back from the foot and decrease the level of ankle dorsi flexion required.

  • We shortened & flipped the stem to preserve the open hip angle and not force JW into a reaching position that may challenge his hamstring flexibility after being placed further back from the bottom bracket.

As a result the ‘knee forward of foot measure’ was taken back 14mm (!) enabling JW to have a much more open ankle angle. Additionally, this change brought the hip & knee in line (hip knee lateral offset = 0) and enabled JW to ditch the external rotation that he had been using to compensate for poor ankle flexibility.
The real results of this change, however, are much more profound (at least to the whitster) – NO KNEE PAIN!

It is a great shame that, as triathletes, we tend to go to great pains to analyze and rectify gait issues on the run while writing off the bike as such a simple motion that will ‘figure itself out’. In actuality, a bad position on the bike is much worse than a restricted run gait because, in the case of the bike, both ends of the chain are relatively fixed at the seat and the pedal. In other words, if things don’t line up, something’s gotta give. Usually that ‘something’ is the cyclist’s knee!!

Fortunately, cycling in and of itself is not particularly challenging in terms of flexibility and given the right information, as to the individual’s personal flexibility ‘sweet spot’ adequate work-arounds can usually be established. Hopefully in the future more bike fitters will consider both the cyclist and the bike when coming up with the’ ideal’ bike fit.

Train Smart,

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