This refers to the intensiveness factor involved in cortisol production.
To recap, here is a little something from one of my articles:
The 6 Training Variables That Increase Cortisol & Adrenaline
Cortisol and adrenaline go hand in hand. Anything that’ll raise cortisol will also lead to an elevation of adrenaline. That’s why you have problems sleeping during periods of high stress.
Resistance training will increase cortisol. This is actually needed if you want to train hard. Without cortisol there’s very low adrenaline, which means lower strength, speed, endurance, and drive. But too much – especially when combined with chronically elevated levels due to a stressful life – can lead to what we call overtraining.
When it comes to training, we have six main variables that can impact cortisol and adrenaline:
Cortisol and adrenaline can both be used to mobilize stored energy. The more volume you do, the more energy you need. As a result, you’ll need to mobilize more energy leading to greater cortisol and adrenaline levels.
This refers to how hard you’re pushing each set, not the actual weight lifted but how close you get to failure. The closer you get, the greater the stress response is and the more cortisol/adrenaline you produce. Going past failure with methods like rest/pause, drop sets, and the like push cortisol and adrenaline even higher.
These are the two big ones when it comes to cortisol in training. If you want to avoid high cortisol levels, both cannot be elevated at the same time. If you want to do a higher volume of work, the intensiveness has to come down – stopping each set a few reps short of failure. If you prefer to push your work sets extra hard, you’ll need to lower the volume. And there’s a middle ground where you can have both at a moderate level.
Other variables can also contribute to cortisol and adrenaline elevation, and they’re not to be overlooked.
3. Psychological Stress
This can happen if a set or max rep makes you nervous or even a bit anxious. If you need to get psyched up beforehand, or if you’re anticipating pain or discomfort before a workout, your cortisol and adrenaline levels will ramp up. This is actually your body’s way of preparing itself for battle. This is where those big weights come in, but also painful CrossFit WODs where you know you’ll hurt.
4. Neurological Demands
The harder the brain needs to work to coordinate your movements, the more it needs to be activated. And activation requires adrenaline. As such, when you use more complex movements, when you have to produce a lot of force or speed, try to learn an exercise you’ve never done before, or combine several complex exercises in a circuit, you increase nervous system activation via an increase in adrenaline (and cortisol).
The shorter the rest intervals the more “up” your body and nervous systems need to remain, and the higher cortisol and adrenaline stay during the workout. That’s why sessions with long rest intervals are often boring to many and why adrenaline junkies tend to jump from set to set with very little rest.
If you want to beat a partner, your record, the clock, etc. you will increase adrenaline. Anything that gives you a sense of urgency or gets you more motivated will raise adrenaline and cortisol.
TO GET BACK TO YOUR QUESTION…
The “may come at a cost” is the greater production of cortisol and adrenaline when you push a set to failure. On machine exercises and isolation movement this is not really a problem, but on compound, free weight lifts it can do more harm than good to go to failure.