I have had a big surge forward in recent weeks in my run training coinciding with my discovery (thanks to Michael Eccles) of Stephen Seiler's polarised training model. I have been listening today (link below) to a podcast concerned with how different motor units - slow to fast - are recruited in relation to low versus high power output endurance exercise. Motor units are the interface between the nervous system, which controls movement and exercise, and the muscles. The nervous system works to match exercise with the correct pattern of motor units depending on the demands of the task.
The podcast I have been listening to today makes the point that there are two means of recruiting the high force motor units: via high intensity intervals and with pre-exhaustion of slow units during long, slow training.
An aspect of my running that I have struggled to understand and resolve concerns how I am able to significantly increase run tempo after many long, slow km's with no outward sign of any increase in stress (e.g. heart rate) but starting a run at high tempo exhausts me. In other words, I can tolerate much higher tempos after a long, slow build up than I can begin a run with. I am certain this is a common phenomenon.
After today's listening, I believe I have an answer and it relates to motor unit capacity and recruitment. The first, and perhaps most important, point to make is this: power output after 60s of exercise is essentially aerobic power; the size and efficiency of the oxidative metabolic system, lungs and heart all the way down to the muscle fibers. This is a point that Seiler and his cohorts make over and over. Regardless of circumstance, build your aerobic capacity above all and that means training 80-90% of the time with a low heart rate (not above 70% of HR max). The remainder of the time (10-20%), train like you're being chased by a pack of wild dogs.
So what did I learn to explain my pattern of fatiguing less when I start slower and spend a bucket load before try to hit the accelerator? The answer is my recruitment pattern of slow versus high force motor units. Faster runs use more of the faster motor units. When I start fast, I am using a higher percentage of these than when I start slow. The problem is, they're also fast fatiguing and that explains the crash I experience during fast starts. The thing is, they're not supposed to be the only units in play - I'm not running that fast! The slower, more aerobically efficient units are also supposed to be engaged but I don't think they are; at least not in the numbers I need.
The clue to understanding what might be going on in my body is provided by the other phenomenon of my body hitting a fast 'steady state' after a long, slow warm-up. I don't think I have been training the slow motor units correctly and this has impaired the way my nervous system uses it's resources. By training with a higher than ideal heart rate, I have signaled to my brain that I want a greater proportion of high force motor units engaged at all times.
My long, slow runs are forcing my body to properly recruit the slow motor units and as these sub-serve the fast units, the whole system begins to operate efficiently. Without the slow units doing their job, the fast units quickly run out of gas.
What does all this mean for my training? It says that I have to spend most of my time running slowly both to grow capacity in the aerobic system and also to ingrain the correct recruitment pattern across all runs: slow to fast.
Polarised training and motor unit recruitment
