1. The biggest interference effect of motor unit recruitment within the training session, set to set, is cardio demand 

2. So if you go into the next set, with existing cardio demand present (to a significant degree) then that adds a LOT to the perception of effort very quickly.  

3. So the degree of motor unit recruitment is going to drastically lower than compared to if you allow yourself full recovery between sets, eg there's no cardio demand present, or very little of it, when you start the next set.

4. This of course, varies by training status due to the fact that people with more muscle mass will increase lactate by a LOT compared to a person who doesn't have a lot of muscle mass.

5. Therefore beginners likely feel just fine after a set of 6-8 where someone like myself actually needs the full two minutes or more between a set of heavy curls at 8 reps.

6. One of the things we went over, was just knowing when you sense of "readiness" returned for the next set, and then to check what your heart rate was at.

7. Over the course of a week or so you'd get some idea of/if there's a correlation between heart rate and when it's time to do the next set.

8. Also remember that it just isn't heart rate alone, but you still need SOME TIME for metabolite clearance because if not, then that too will create a reduction in motor unit recruitment as well.

9. I have to bring that up because someone will inevitably say that they have amazing cardio and after a set of 6 reps they feel fine in 30 seconds.  Cool, that means either you have no muscle mass or just did some wrist curls.  

10. The point is to allow cardio demand to subside, and then for metabolites to clear that way you're not dealing with more of a decrease in MUR than you have to, and then all of your sets are as effective as possible.