- May 16, 2016 at 10:26 pm#2513Andy BrookeKeymaster
I thought I’d start the thread to carry over our discussion from Twitter.
As a brief summary, conversations on there included:
How to measure joint angles and whether they are reliable drivers of fit decisions or whether we should measure outputs like power, directional forces, saddle stability, etc
Whether there was any value in measuring joint angles at 90 degrees/3 o’clock position and whether indeed that was where peak force occurred – Borut mentioned he looks at pedal angle, which indicates ankle plantar flexer strength and heel-to-toe-box drop of shoe, but can be altered with cleat position.
The conversation originally started as a discussion on scientific papers that cover overuse injuries in cycling so if anyone has any good papers then please share a link below. Lotte had some good thoughts on identifying risk factors so papers related to that, although probably few and far between, would be really interesting.
Hopefully that gets things started but here are a few (slightly edited) thoughts I sent to Paul in an email…
“On the BDC tweets – I think the terminology can be a little misleading as it doesn’t correlate with the point in the pedal stroke furthest away from the saddle. For example, if a frame has a 73 degree seat tube angle then (depending on seatpost setback and where the saddle is positioned on the rails) the point furthest from the saddle would be around 163 degrees through the pedal stroke. After that point, the pedal is actually getting closer to the rider so knee and ankle angles should both be decreasing. I’m not sure anyone has ever studied it in great detail though and the pedal stroke is very inefficient through the bottom quarter so it wouldn’t surprise me if it didn’t correlate perfectly.
I think the 90 degree/3 o’clock position has been seen as important because it’s assumed that people push directly down and place a bit too much emphasis on the direction of gravity. But as you can see from that Broker diagram (although it’s a little old), forces at 90 degrees aren’t straight down but slightly forward. This is probably an effective force vs total force discussion.
I’d be interested to see what Borut has found as I’d think the most “effective” point in the pedal stroke (tangential forces as a proportion of all others) doesn’t correlate with the highest absolute tangential force.
I certainly don’t place any importance on the 90 degree position while fitting and think it’s probably more important to prioritise other things.”
Plenty to get your teeth into there…May 17, 2016 at 5:42 am#2514Borut FondaParticipant
Thanks Andy for starting this. I’ll skip straight to the forces section, as the answer “how to measure kinematics” has a very clear answer for me 🙂 See here
I think why Paul mentioned the 90 deg position is as it is assumed it’s where the peak force occur and from a knee load point of view could be more interesting than BDC.
Why I look at 90 deg pedal angle? Vast majority of cyclists have the most effective force direction at 90. This is where we should search for the peak force as well. However, most of the cyclists achieve peak force latter in the crank (100-120 deg). Now imagine one develops a force of 500N (300W) and the heel drops… this results in an ineffective energy transfer to the pedals. Yes, you could standardise the ankle angle measurement (marker placement error, anatomical landmarks, etc.), but to know exactly what happens with the pedal, you have to measure it at the pedal. Not to mention a more accurate way to assess the contra-lateral (a)symmetry in pedal kinematics (see photo below).
It begs the question… what to optimise? You see loads of “gurus” that the second half of the pedal should be pulling, etc. But a closer inspection revels that even with pulling (and increasing your index of effectiveness by 5x), you actually contribute very little to the absolute force. During push down, you generate about 300N of force during steady cycling (aprx. 180 W) and during pull up/second part, you have only around 50N.You improve for 10% the second half – 5N. You improve by 10% the push down – 30N. You see where I’m going? 🙂 At the end of the day is to generate as little force as possible to maintain the same power.
I look forward to the rest of the discussion.
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