The Fiber Profile Column

Whoa, big "whoop-de-doo" over on the BTR forum for the past few weeks where lots of undies got up in a bundle. Ahhh, geeeeeezzzzz, it's just a web forum about bike crap - glad I got the chance to open the johari window for myself a bit in that thread.

Refreshing, in fact, to state some of my values, my ideas on changing behaviors, and having the opportunity to represent what I believe (ya know, bike racing isn't a math problem!) despite being continually attacked by a select few.

Anyway, during the sharing of ideas, I linked to a great thread that took place a long time ago, where Kirk made some good observations back in January of 2006. Here's a particularly relevant post in that thread:

http://biketechreview.com/phpBB2/viewtopic.php?t=504&postdays=0&postorder=asc&highlight=fiber+column&start=48

Here's the relevant bits of that post for those averse to clinking on links and whatnot:

"For a given cadence, changes in power (force) are the result of changes in the motor-units recruited. More force, more power, more motor-units. Fatigue can also result in the recruitment of additional motor-units. When motor-units normally recruited at a certain power begin to fatigue (produce less force when they fire), additional motor-units are recruited to take up the slack in order to maintain the power (force). Because of the progressive, sequential nature of motor-unit recruitment with the needed force, increasing power (force) generally requires additional recruitment.

I look at an entire muscle as a column, a column roughly divided up into power levels I call fiber-profiles. At max power (say a 5s sprint), most of the column fires. At low power, only the bottom section associated with that power is used. As fatigue at a given power sets in, the fiber-profile "creeps up” the column, as additional motor-units are recruited to maintain force (added to the top). So, the fiber profile associated with a given power as fatigue sets in includes more motor-units than when fresh.

So, take the example of cruising along at 250w (let’s say that’s a tempo intensity), a certain fiber-profile (composed of individual motor-units) is used to generate the force required to produce 250w. Over time, fatigue will set in within that fiber profile. That fatigue manifests as reduced force. In order to maintain that 250w (force), additional motor-units, ones normally recruited at higher powers, are then brought into that power’s fiber-profile.

So, it terms of recruitment associated with 20MP (or any power), as long as the power is at that level or higher, I know that AT LEAST the smallest potential fiber-profile associated with that power is recruited…and as fatigue sets in…that fiber-profile only grows larger (more motor-units recruited…added to the top) to make up for the drop in force production in the fatigued motor-units. The fatigued motor-units are still recruited, their force production just drops. So, that 250w *may* eventually recruit some of the motor-units normally within the >95% of 20MP profile with sufficient fatigue, but it is unknown to me exactly when this happens. When I train at intensities > a given power, say 95% of 20MP, I know that a given fiber-profile is recruited, and as fatigue sets in, this profile creeps up the column.

So, I think that at least a minimal level of recruitment can be associated with a given power, and that by acknowleding this minimal profile, I can quantify the amount of stimulus seen by that minimal profile (although it may grow to include more motor-units) via my choice of 60s chunks which include that profile (bins or time in zone are similar). I’d love to hear of ways that recruitment order and force production can commonly get tossed out of whack so that limitations to this way of thinking can be identified. If these issues do exist, it may also influence the use of powermeter data in general, because if non-sequential recruitment of motor-units occurs, a given power number has less meaning (at least to me). There is always more to be learned!

Kirk"

Yeah, visualizing things in terms of a column of fiber recruitment makes sense to this caveman, especially considering that when one uses gears, cadence is confined to a relatively small range (outside of the transition to/from coasting).

Good stuff to think about, anyway.

Oh yeah, I've disabled comments on this entry, so, if you'd like to engage others on the topic, feel free to sign up and post your thoughts on the BTR forum:

http://biketechreview.com/phpBB2/index.php

keep it real out there, folks.