Cadence and metabolism

A few days ago I was reading a physiology textbook and ran across this graph (from my phone, sorry about the quality):

This is the article referenced.

In case it's not obvious at first glance, these graphs say that the particular twitch (i.e. fast or slow) recruited for a muscle contraction depends both on its duration and force.  This makes sense from an economical standpoint: slow twitch fibers rely primarily on fat metabolism, and there's no need to get high force muscle fibers burning through glycogen for doing something as simple as typing on a keyboard.  (Don't know why that comes to mind.)

As in everything I read, I ask: how can I use this in cycling?  Maybe we can maintain more reliance on aerobic metabolism by pedaling at higher cadences.  Or maybe we can induce greater aerobic stress at high cadences.  This will be extremely useful in base training and general engine maintenance through the year.

Next I did a quick calculation, disregarding force evolution for a moment: if a muscle is engaged for 25% or 50% of a pedal stroke at 90rpm, how long does it contract for?  At 25%, 167ms, and at 50% it's 333ms.  At 120rpm we get 125 and 250ms.  It's looking okay right now, especially if the pedal force and total power output are kept low in addition to high cadence.  But the graph doesn't actually show us what motor units are engaged or even if this happens in leg muscles.  (First dorsal interosseus is in the hand.)  Let's dig deeper and see which muscles are engaged for how long in a pedal stroke.

Thanks for the fun colorful stuff, trainingpeaks.

I found a trainingpeaks article which led me to the above graphic, but I checked the references anyway.  This is one of the studies referenced.  Go to figure 6 for the meat and potatoes.  I suppose we can forgive the authors for working with triathletes, whose pedal strokes are very downward heavy due to their positions (different muscle recruitment is why you should always practice on your TT bike, kids).  Here is a second, most excellent article that reviews muscle engagement in relation to pedal and shoe conditions as well as seat height, and actually looks at two active and two former bike racers who can competently engage muscles entirely around the pedal stroke (look at subjects CDEF in figure 3, with cleats and normal saddle height).  Black regions of the graph represent muscle engagement.  In relation to fiber recruitment and aerobic stress, this shows that many muscles are engaged in half or more of the pedal stroke, indicating a good deal of fast twitch recruitment and glycolysis.

Because I make things more complicated than they need to be, I went through all of the above.  And only now do I look for articles studying metabolism in relation to pedal stroke.  This only does 50 vs 100 rpm.  Not a lot of bike racers pedal at 50rpm, so while informative in a proof-of-concept way, isn't exactly what we're looking for.  I dismissed several more articles on their ridiculously low cadences (down to 15rpm) until I came across this gem, which looks at 60, 80, and 100rpm in female recreational cyclists.  As predicted, HR and VO2 were higher at 80rpm than 60rpm, and higher still at 100rpm.

What we're learning is that despite how much it sucks for a rider like me who prefers high torque situations, high cadence pedaling during endurance exercise will strain the aerobic system more than at lower cadences.  This aerobic stress is, after all, why we base train.  Now we can make it more effective.  In terms of practical application, you should pedal at a higher cadence (if you have that option) if want to save your precious glycogen stores for later in a race.  It's something we all kind of knew anyway, but now we know why.  That's important to me, anyway.

As an aside, I ran across this article showing the neuromuscular adaptations cyclists have in pedal stroke efficiency.  It shows that our muscles don't work against each other as much as compared to untrained individuals.  With the same overall power output, we use less oxygen.  Someday I'll get a pair of Vector pedals and really get to go nuts with pedal stroke analysis.