Clogging Up The Drain: Why You Are Forced To Slow Down


By Scott Semple

When I first started skimo racing, I started races way too fast. The gun would go off, and I’d bolt forward. I’d feel great for about 60 seconds and then as if I was running through molasses. I’d have to slow way down. With that kind of start, the rest of the race felt horrible.

This experience is so common, and not just in competitive athletes, that it is worth asking: What’s going on? Let’s break it down. By starting too fast:

* I was flooding my muscles with lactate and raising the acidity of my blood;

* Drowning in that metabolic soup, my aerobic system was overwhelmed. The more acidic my blood, the more I’d have to slow down to allow my aerobic system time to clean things up;

* Later I learned that starting much slower allowed for a more gradual increase in lactate. Although hard to do, starting slower allowed for a faster average speed; and

* To start faster, I need to train my aerobic system to absorb larger amounts of lactate.

When you ramp up too fast, your body recruits fast-twitch muscle fiber. Fast-twitch muscle fiber runs primarily on glycogen. It allows for a faster speed and a higher power output, but at a cost.

Glycolysis (the process of using glycogen for energy) produces negative byproducts. If those byproducts build up faster than your aerobic system can remove them, you’ll slow down. Your aerobic system needs time to catch up. Often that clearing process takes so long that your race will be over shortly after it’s begun.

But why is it *so* slow?

The upper limit of aerobic speeds is generally considered to be an activity level that produces a steady state blood lactate value around 2 mM/L. Above that, other steady states exist, but the higher levels of lactate reflect a greater and greater contribution from the anaerobic system (glycolysis and fast-twitch muscle fiber).

The Maximum Lactate Steady State (MLSS, also know as the anaerobic threshold or lactate threshold) is the highest speed where the muscles are producing and reabsorbing lactate at equal rates. Above this speed, lactate accumulates in the blood faster than it’s removed. When that starts happening, the clock is ticking. The faster the rate of accumulation, the shorter the duration. At a certain point, the only way the body can restore homeostasis is by slowing things down well below the MLSS.

The frustrating part is that the speed required to reabsorb a large amount of excess lactate is much slower than MLSS. It’s often even much slower than a purely aerobic speed. So if you start a race with a burst, you’ll have to continue at a much slower pace than if you had started without the burst.

Reabsorption speeds are so much slower because even aerobic speeds produce some lactate. If you’re running at your MLSS, your lactate levels will be at a steady state. Lactate is being produced and absorbed at the same rate. There isn’t any available capacity to absorb the excess lactate.

 Lactate is a fuel for the aerobic system. The more lactic throughput capacity the aerobic system has–i.e. the bigger the funnel–then the higher levels of lactate it can use, and the more anaerobic input it can tolerate. The end result is a faster overall speed.

So your body forces you to slow down. The large gap created between MLSS and the slower speed creates excess capacity. Your muscles can then start reabsorbing excess lactate.

How long does it take to reabsorb lactate?

This will vary by athlete, but here’s an example.

The other day I did an anaerobic capacity test to help me plan for this year’s training. Specific to my sport, I set my treadmill to a grade of 25% and selected a speed that I thought I could hold for 45-90 seconds. The goal was to produce as much lactate as possible, so I needed a short, maximal effort.

Once the test was over, I lowered the treadmill to 1% and ran slowly. I measured my lactate after several long intervals. Here are the results: Note- AFAP = As Fast as Possible

My aerobic threshold (~2 mM) for flat running is a blistering 11 kmh. (A little more than half the ~2 mM speed of a professional marathoner… Ouch.)

So off the couch, running at 7 kmh, my lactate values would be much lower than 1.9 mM. You can see that I had to go *way* below my aerobic threshold to restore aerobic levels of lactate. As described above, the reasons are several:

* Any activity level produces some lactate. If the blood is already saturated, even speeds at MLSS/anaerobic threshold are too fast to reabsorb excess lactate and I needed to slow considerable to see significant lactate clearance;

* To speed the clearing process, *extremely* slow speeds are required. So in a race context, if you start too fast, your average speed for the event will be much slower. If you start without the burst, you avoid the initial flood of lactate and the slow reabsorption period;

* Frustrated by the slow speed on race day, most people misunderstand the problem. They won’t go slow enough to correct it, which prolongs the problem and makes their race even worse.

You’ll notice that my second-last interval was at 5 kmh, but very little lactate was reabsorbed. The change from the previous interval was only 0.1 mM.

My first guess is that the speed was too slow for optimal reabsorption. There wasn’t enough muscle fiber recruitment to maximize the lactate recycling. Or I needed more time to get down to aerobic levels. I’ll have to wait for future tests to find out.

But how can so-and-so start so fast?

Some athletes are fast-starters. They can burst off the line at close to race pace and maintain a high speed for the duration of the race.

My hypothesis is that:

1)These fast-starters are very slow-twitch and don’t produce much lactate; or

2) They have very large “funnels”; or

3) Both.

For athletes that are very slow-twitch, their anaerobic capacity is naturally low. They can start at a high speed without overwhelming their aerobic system. For a pure slow-twitcher, it makes sense to use that advantage at the beginning of a race.

The disadvantage of a slow-twitcher is that they typically don’t have as much “kick” at the end of a race.

Three different size lactate “funnels”. The drawing probably overstates my and Trump’s funnel sizes relative to Jornet’s…

In contrast, fast-twitchers need to start slower and ramp up to race pace. “Slower” won’t feel slow; it just has to be slow enough not to overwhelm the aerobic system.

The high anaerobic capacity of a fast-twitcher is also an advantage, but only at the end of a race. In the beginning, it’s a liability.