Hello all, not sure if this is a good place to ask my question, since it is partly a science question but I’m very curious about it so hope someone here has insight.
There is a lot of discussion about training adaptations and how there is a trade-off between intensity and endurance for aerobic exercise. I’ve heard people say “All roads read to Rome (or Tokyo)” meaning that for aerobic exercise, there are similar adaptations for exercise at all intensities Zone 5 and below (even if the strength of these adaptations varies a bit depending on the intensity). This is illustrated by a chart in Coggan and Allen’s “Training and Racing with a Power Meter” which lists a bunch of types of adaptations and marks (qualitatively) how much riding in each intensity level stimulates that adaptation. There is substantial overlap between levels/zones 2 through 5. This suggests some intensity-duration trade-off, where a lot of riding in Zone 2 might produce similar adaptations to a medium amount of riding in Zone 3 and a shorter amount of riding in Zone 4, etc (I’m not arguing it is a perfect trade-off at all especially in highly trained athletes).
It seems like there is a lot of evidence the above is true, but I’m curious how it ends up working in the context of Henneman’s size principle:
which says that there is a hierarchy of muscle fibers on a force production/fatigue resistance continuum, where fatigue resistant small motor units are firing all the time, and high force units fire only when you exceed a certain power threshold. I get the impression that a lot of peripheral adaptations to exercise are “local”, i.e. if a muscle fiber fires all the time it will get adaptations, but muscle fibers that don’t fire won’t. Is that right? If it is, it seems like there is a big physiological difference between riding at a low power for a long time compared to riding at a higher power for a shorter time. In one case the small motor units are working constantly and must get the maximum stimulus possible (is that how it works?). In the other case, adaptations are spread over more motor units, but less time is spent working them. When I picture this in my head it seems like different exercise intensities are producing very different adaptations even if they are all aerobic, because of uneven distribution across motor units. Does everything work out because if you work close enough to your limit for some duration of time eventually the smaller motor units tire and you start needing to recruit the bigger ones, so the adaptations spread around? Or do people who use different training strategies have very different distributions of aerobic adaptations in their muscle fibers?
A slight wrinkle: larger motor units will be increasingly engaged not when you need more power, but when you need more torque. So if you do the same one hour 160watt Z2 ride you’ll use more large motor units if you ride at 60rpm than at 90rpm.
However, if your 90rpm ride becomes a four hour ride, you’ll increasingly be using larger motor units towards the end of the ride because of the fatigue that you’ve induced in the smaller ones.
Motor unit recruitment is dependent upon not only the force required, but also the (intended) speed of movement. That is, the threshold at which a particular motor unit is recruited is lower for “ballistic” than for “ramped” muscle contractions (as Henneman himself termed them in his studies). Applied to cycling, the primary determinant of what fraction of your the pool that is recruited is the power output - whether the cadence (and hence torque) used to generate that power matters is actually still equivocal.
The other factor that matters is time/fatigue. As time passes/the initially recruited motor units fatigue, additional motor units will be recruited to maintain the intensity. Thus, by the end of a prolonged bout of exercise to task failure you will have recruited (and hence trained - as you indicated, adaptations only take place in the muscle fibers that are utilized) all, or almost all, of your motor units (and thus muscle fibers), even if the intensity was actually moderate.
Quadrant analysis is a(n imperfect) graphical tool I came up with about 20 y ago to help visualize such issues. TP seems to have lost the article I wrote describing it, but the same information can be found in TRWPM, or various places on the web.
In case anyone is still following this thread and interested, there was a short discussion of a related issue at the 33 minute mark of the latest Inside Exercise podcast with David Wright. The prompt was that there are exercise linked adaptations in a wide variety of non-muscle tissue, so why doesn’t it happen in the non-contracting muscle:
Basic premise (as I understood it) is that tissues that experience increased energetic demands get adaptations.
