A while ago I read an excellent article titled Why heart rate always matters. It goes into great detail on a topic I’ve previously discussed here on running in systems: why the heart rate is always going to be an excellent representation of what is happening with the body’s stress response and energy metabolism. I think that some of the topics it discusses, as well as the excellent debate in the comments, are worth expanding on. Here’s an excerpt from it:

“Our fight-or-flight system often activates without any actual demand. When we get ‘stressed out’–engaged in a heated argument, mulling over a burdensome worry, or simply sitting in traffic–seldom is any physical task being undertaken. But the body is being activated. The engine is revving higher and tremendous sugar–the preferred fuel of fight-or-flight responses–is burned when under psychological stress, which is a major factor in ‘stress eating!’ We function as if we’re fighting an intense battle.

Stressed out and going for a run? Your body will perceive the cost of that run as higher (because it is already dealing with your life stress) and will activate a more intense energy system to cover all the demands. More energy cost!”

There was a particular comment in the article that I wanted to address:

“Very well written article and I agree with most of it.
However, I think you overstate the impact of activation level on energy expenditure…

…In my understanding, the energy demand dictates the energy production. And the energy demand is mainly dictated by the mechanical work of the muscles and all the side processes needed for that level of power output.
I agree, that the excitation level directly impacts the chosen energy supply system but as long as this system doesn’t actively provide energy, it’s [maintaining] cost will be relatively low.
Yes, a higher activation will have a higher energy demand but I don’t believe it’ll come anywhere close to exceeded mechanical [energy] demands.”

I agree with the commenter in that I, also, believe that the author was overstating the impact of activation level on energy expenditure. However, I think the author’s overstatement makes it difficult to observe 2 key implications of this discussion:

1. Activation level  (a.k.a. stress) changes the type of energy metabolism, which means that it changes the ratios of fuel (fat and sugar) that it uses.
2. Training stimulus is inextricably tied to activation level and energy metabolism. This means that the ratios of fuel usage have a much bigger say in how the body perceives the workout (as low-intensity vs. high-intensity) than the rates of fuel usage.

The point is that while the author does overstate the energy cost of the stressors he mentions, it doesn’t really matter—there’s things the athlete just can’t get out of training if their body is taxed in the ways the article mentions.

A lot of people think that low-intensity means “slow,” “easy,” or “consuming little energy.” It doesn’t. Low-intensity is when the workout is easy on the body—specifically, when the body is burning a majority of fats for fuel, and the sugar that is being utilized is burned wholly aerobically  (in the presence of oxygen). In other words, there is no substantive anaerobic work. Highly-trained endurance athletes, who burn fats at much greater rates than the rest of us, can run at very high speeds while remaining in a completely aerobic state. Such an athlete may be running blazing times in a workout that is for them, metabolically speaking, a low-intensity workout.

Now let’s look at higher intensities: In order to produce the energy necessary to approach your top speed, a lot of changes have to happen within the body. One of these is that the body has to go from burning a greater percentage of fats (which burn relatively slowly and so provide energy at a relatively lower rate), to burning a greater percentage of sugars (which burn relatively more quickly and so provide energy at a much faster rate). So, in order to get closer to your top speed, a greater percentage of your energy has to come from sugar.

In order to release more sugar to the bloodstream (to be utilized by the muscles), the body releases hormones called glucocorticoids—glucose (a.k.a sugar) releasing hormones. The main glucocorticoid is cortisol, which many will recognize as the main stress hormone. Another hormone that is release during the stress response is insulin, which helps muscle cells avail themselves on the sugar that cortisol released into the bloodstream. Cortisol and insulin, then, work synergistically to produce (and increase) sugar metabolism.

To recap: want to run closer to your top speed? You need to release more sugar. How do you do that? By getting more stressed. But because of some of the body’s more complex molecular mechanics—fodder for another post—the body can’t release a bunch of sugar and still be releasing fats. What would happen is that you’d just flood the bloodstream with unhealthy concentrations of both fuels. So, when insulin is released or when anaerobic function (which is dependent on sugar) increases, fat-burning drops. If sugar-burning goes up, fat-burning goes down (and vice versa).

This works the other way around too. If you get more stressed because, say, you had a rough day at work, or you got into an argument, you’ve got more cortisol and insulin running through your body. But it’s not like the body can decide to release (and use) sugar only when the reason for cortisol and insulin release is because of increased athletic demand (a.k.a. athletic stress). For any other stress (work stress, etc.), cortisol and insulin become released, and increase carbohydrate metabolism. Research on the metabolic effects of social stress in fish supports this idea.

This, incidentally, is why people get tired after a stressful day at work or an argument that stretches for too long. They didn’t use up all their fat-stores at work, obviously. But because the stress put them in sugar-burning gear, enough of their sugar ran out that they start feeling tired. It’s not that they ran out of fuel, but rather that they ran out of the fuel they’ve been stuck using.

It also takes a relatively long time for the cortisol to get out of your system—and when it does, it’s not like you can just pop back into action and go for a run. The adrenal glands, which put out cortisol (not to mention various other mediators of the stress response) have been used up. They’re tired, and will resist further activity. And since you use all the glands in the body to one (significant) degree or another during training, it’s not a good idea to train with exhausted or depleted glands.

Asking your body to work out when you’re already out of a major fuel and your stress glands are tired is an even worse idea: the “same” workout is relatively much harder for a tired gland that’s nearly out of adrenaline and cortisol than for a rested gland. Training after a period of stress is, in physiological terms, almost exactly like doing back-to-back training sessions. Effectively, you’re extending the period of stress.

And if on top of that, your blood sugar is low (as usually happens after a period of stress), you’ll be asking those tired glands to produce even more cortisol and adrenaline than they would usually have to: in their already tired state, it’s not enough to simply produce enough cortisol to maintain blood sugar levels—they have to make up for the lack of sugar in the bloodstream.

If on top of that, you’re “stuck” in sugar-burning mode because you still have all that errant cortisol and insulin flowing through your system (since you’re still stressed), you’ll be depending on sugar—which you’ve substantively burned through—for the duration of your training session. Because the body is inhibited from fueling itself with fats (due to the insulin in your system), it has to rev up those exhausted adrenals even more to provide the requisite cortisol.

Insofar as your body is stressed, it will respond to what is normally an “easy” workout as if it were a “mini high-intensity workout.” In other words, you can’t really have a “low-intensity training session” when you’re stressed (and expect to accomplish your goals in any sort of way). 

This is why doing MAF training—exercising under the aerobic threshold—under stress (or after a period of stress) produces such a dramatic drop in speed/power output at the same heart rate. When you’re under stress, exercising at a rate that looks anything like the aerobic training you do when unstressed would mean elevating your heart rate far beyond your aerobic threshold. Because aerobic work output is so reduced in a stressed state, it’s a much better idea—and a much simpler fix to the problem—to simply rest for the day and do your “easy” training session tomorrow.