Cycling Biomechanics Optimisation

Whoa ... what does that phrase even mean? For starters, it’s so vague - just the term ‘biomechanics’ is so broad that it would be almost impossible to cover every single interpretation of its meaning! So, with this post, I’d like to explain what is meant by ‘cycling biomechanics optimisation’, and also to present a case study of a cyclist with initial poor biomechanical performance.  Power output on a bicycle is generated by applying forces to the pedals.
Unfortunately, humans are incapable of producing a perfectly even pedal stroke, which means we aren’t 100% efficient. What’s more, even trying to maintain smooth pedalling "in circles” could result in an increase in energy consumption. Therefore, we produce forces that are not contributing to power output and sometimes, especially in the upstroke, even brake the crank rotation. Thus, minimising the forces applied to the pedals while maintaining the same power output is one way to optimise cycling biomechanics. Another way is to make a cyclists symmetric. That means that he/she will apply forces in a similar (if not the same) way through both their left and right side, that is, in the same direction and with the same amplitude. Maintaining a symmetric pedalling technique results in a cyclic pattern with more stability and less energy loss.

In order to assess this particular aspect of cycling biomechanics, force pedals are required  which accurately measure forces in all three directions. With additional information on the pedal and crank angle, we can compute the amplitude of the effective force – that is, the force that’s actually pushing the pedals forward and is directed perpendicular to the crank. Effective force multiplied by the crank length allows the crank torque to be calculated which, when combined with the crank angular velocity information, allows the calculation of actual power output. The ratio between the effective and resultant force is called the index of effectiveness and is a relative measure. The absolute measure can simply be an average of the resultant force. Minimising the resultant force and boosting the index of effectiveness can be considered an optimisation of cycling biomechanics. 

But what about the symmetry? The easiest way to get an overview of how a variable changes is to display it on a crank angle scale, normally from the top dead centre (0/360°) over the bottom dead centre (180°) and back to the top dead centre. Any variations in a given variable can then be linked to a phase in the crank cycle and allows direct comparison between the left and right side. This is best illustrated using an example.  

Below are two screenshots taken from one of my bike fitting assessments with a recreational cyclist. The lower left graph is the index of effectiveness for the left (red) and right (blue) pedal as an ensemble average of all cycles in a 30-second time window.  

Before the bike fit

As can clearly be seen, the cyclist is doing something different in the last third of their pedal stroke. This particular cyclist had been using a modern power meter that provided an estimate of left-right power balance and, ever since, had been trying to improve the left side by pulling up with the ankle in that particular part of the pedal stroke. Interestingly, this is occurring just on the left side, with the right side appearing somehow "normal”. After delving deeper into the issue with the cyclist in question, he revealed that he was consciously trying not to pull, which was why he adopted this specific pattern. He also reported that his power meter was displaying that he was symmetric (what a great power meter!). 

During the bike fit, I was curious to see as to how any alterations in seat height could affect that asymmetry. Pedal kinematics revelled a 6° higher range of motion in the left pedal compared to the right, which could be an indication of excessive movement in the ankle, especially in the direction of dorsal flexion during the downstroke.  Increasing the seat height and putting the cyclist higher normally provokes more plantar flexion and limits that ineffective pattern called "ankling” (Zommers, 2000). After several iterations, we finished with the seat 1.5 cm higher which resulted in an improved pedalling mechanics (as seen below).  

After the bike fit

As the data demonstrates, force direction and amplitude are almost identical and we managed to go from a 25% difference in index of effectiveness to just a 4% difference. Furthermore, pedal range of movement decreased by almost 10° and was identical for both the left and right side.  

In this specific case, I’m comfortable to say that we improved the cyclist’s biomechanics. In the end, the relative measure of effectiveness of both sides was slightly lower value, however we eliminated the asymmetry. However, the effectiveness of the right side (the so-called ‘normal’ one) improved. In this article I only presented a small bit of changes, but with a change in body position (moving up and fore on the seat) we decreased the lateral force and lateral torque on the pedal. Consequently, this both affects shear forces on the knee and limits energy loss. Due to more plantar flexion, a decrease in Q-angle range of motion can be observed, which has been shown to minimise the risk of knee injuries (Bailey, Maillardet, & Messenger, 2003).  

Overall, this bike fit resulted in one very happy cyclist who, I’m glad to report, will be promptly getting rid of his current view of the left/right balance from the power meter!

Dr. Borut Fonda, Cycling Science program director

References
Bailey, M. P., Maillardet, F. J., & Messenger, N. (2003). Kinematics of cycling in relation to anterior knee pain and patellar tendinitis Kinematics. Journal of Sport Sciences, 21(8), 37–41. http://doi.org/10.1080/0264041031000102015
Zommers, A. (2000, September 2). Variations in pedalling technique of competitive cyclists : the effect on biological efficiency. Victoria University. Retrieved from http://vuir.vu.edu.au/15742/1/Zommers_2000compressed.pdf