I’ve been training at my MAF HR for a few months now. My top speed has decreased. Am I losing fitness?

It’s unlikely.

To explain why, we need to talk about what happens to the body when there is too much high-intensity training. The body begins to feel that the only way to survive is to keep up with the training demand at all costs. The body begins to sacrifice its health in order to produce more muscle mass (and to drive that muscle as hard as it can).

When we are in a stressed state like this, our athletic output can be far greater than what is healthy—than what we are “really” capable of. (This a milder version of why, when someone is in intense physical or emotional pain, they can express an immense amount of strength.)

So, why do you become slower when you’e been training MAF for a while? The body is no longer asked to compromise its integrity, and its stress has subsided. Because of that, the body is no longer willing (or even able to) produce a level of output that was only possible in a stressed state.

In other words, it's unlikely that your body's actual physiological ability has waned. In fact, it is probably increasing. What has waned is the muscular power that was only built because the body perceived itself to be constantly fighting for its life. Put another way, if that muscle power had NOT been challenging the body's health, it would NOT have waned. So, the fact that the body is allowing its power to wane means that it's getting a chance to develop the systems upon which that power should be built (the aerobic system).

Let’s explain this with a metaphor: A body that has too much muscle power for its aerobic system is like a 30-story building with a foundation that is only rated for 20 floors. The ceilings are constantly caving in, and productivity constantly stops while repairs are made. (In other words, injuries happen regularly). Finally, the building manager brings in a competent engineer. How is the engineer going to go about repairing the building?

1. Immediately removing those 10 extra stories in order to remove undue stresses to the foundation.
2. Repairing all the damage.
3. Upgrading the foundation to the desired rating.
4. Add floors that are supported by that rating.
5. If more floors are needed, upgrading the foundation first.

(This is essentially the calculation that the body makes when its stress levels have subsided enough for it to grow properly).