Sprint Metabolism Myth — and coining a new term — Anaerobic Fatigue

Peter Weyand and Matthew Bundle recently published their research paper entitled “Sprint Exercise Performance: Does Metabolic Power Matter?” which calls into question how we look at sprinting metabolism.  Their conclusions immediately make a lot of sprinting books, articles and websites out-of-date.

So what exactly does “Does Metabolic Power Matter?” mean?  Well, first it means  all those charts and descriptions that describe how your body “fuels” the sprints are wrong.  Here’s a few links that need updating:

http://en.wikipedia.org/wiki/Energy_systems

http://www.athletesacceleration.com/sprinttrainingenergysystems.html

http://www.bodybuilding.com/fun/sprint-vs-marathon-energy-demands.htm

http://www.brianmac.co.uk/energy.htm

and the list goes on and on…

All of these articles are based on the old theory that the body’s ability to maintain sprinting velocity is controlled by anaerobic fuel limitations.  And that old theory is dead wrong.  The old thinking goes something like this: “first you body uses up all the ATP stored directly in the muscle.  That will last upwards to 2 seconds.  When that starts to deplete, the body starts using CP to generate more ATP which last another 6 seconds.  When that starts to run out, it starts using stored Glucose which produce Lactate and Hydrogen Ions, which can last around another 40 seconds…”  Many will say all the energy sources are used all the time, but at differing amounts…but even that doesn’t explain the evidence.

Weyand/Bundle reference a study that shows the “one-step creatine phosophokinase reaction that supplies ATP…is capable of resynthesizing ATP several times more rapidly than the contractile proteins within the muscle cells can us it.”  What that means is that your ATP stores never really run out.  So, the idea of one system being depleted and another system picking up where the first left off is not accurate.  Weyand/Bundle take the position that we really don’t know how the systems work in conjunction with each other.

The key proof of their position comes from the iEMG data they measured during treadmill sprint and sprint cycling tests.  These tests showed that iEMG activity increased as the sprint trial progressed.  For example, the treadmill test had subjects run at a non-sustainable steady 7.3 m/s pace (13.7 100m pace or 54.8 400m pace).  During this test the iEMG value for the Bicep Femoris might go from a value of 4 in the first few seconds to a value of 8 at 40 seconds.  More importantly, the force being generate on the treadmill remained constant — so even though high iEMG reading were being detected the force produces was the same.  These results were even clearer in the cycling trials.  However, when running at a sustainable pace (pace that can be sustain via aerobic metabolism) the iEMG data showed constant activation.

What this boils down to is that our bodies start by firing contractions on all the motor units it “needs” to get to the desired speed.  But immediately (for anaerobic activities), those motor units begin to “fatigue” and the body must recruit more motor units (which is what the iEMGmeasurement is telling us).  Exactly which motor units the body recruits and when is not clear (my guess is that our system is naturally efficient and cycles fresh and then fatigued motor units in some sort of optimized manner — likely recruiting the faster type II muscles first then optimizing afterwards).  What is clear is that there is a linear progression of the number of motor units recruited (or at least requested — anaerobic by-products are likely interfering with the contractions even though the recruitment signal is being fired from the nervous system).  Moreover, the slope of this recruitment line is directly related to the intensity of the effort.

The next question is why do these motor units fatigue?  If it isn’t that they ran out of fuel, why can’t they contract with the same force that they did in the first few steps.  Well, this isn’t totally clear.  But it is clear that certain muscle fibers fatigue more rapidly (those that can contract faster and with more force) and that anaerobic contractions do cause metabolic by-products.  Many coaches teach that the build up of hydrogen ions in the muscle interferes with the contractile impulses (often called Lactate Tolerance Training); however, the “interference” theory isn’t full verified.  Other by-products have been posed as the culprits and the jury is still out.  Also, I’m not sure it’s clear if the individual motor units contract with less force or if they simple misfire due to by-products.

So, what does all this mean to you training plan?  Well, probably not much.  It doesn’t really give us any insight into comparing short-to-long, long-to-short, mixed…  It does mean that we should use different terminology to describe training modes.  I prefer the term “Anaerobic Fatigue Training” (coined here perhaps?) over “Lactate Tolerance Training”.

As a coach, I like the UKA Sprint Classification for discussing sprint training.  If you remove the parts about energy systems, the classifications are still handy semantics for discussing workout focus.  It seems to me that instead of discussing the workouts by their “incorrect” metabolic label, we can now lump these classification into three distinct super groups: Anaerobic Speed Training, Anaerobic Fatigue Training, and Aerobic Fatigue Training.