One of the questions I’m asked most often has to do with “cardio” (which I always have to put in finger quotes when speaking, even on the phone.)
It seems people use the term “cardio” to refer to one of two things:
1) ones’ cardiovascular system (heart, lungs and blood vessels)
2) the specific energy system(s) (metabolic pathways) one intends to target through training
If you’re alive and reading this, I’m gonna go out on a limb and assume your heart, lungs and blood vessels are doing their job at least somewhat effectively, so this analogy will focus on #2.
Imagine 3 cars, all the same make/model…
Car A (ATP/PCr) can only drive in 1st gear.
Car B (Glycolytic pathway) can only drive in 3rd gear.
Car C (Oxidative Phosphorylation) can only drive in 5th gear.
ALL THREE are “cardio”. If they all begin at the same starting line, which one will win a race?
Short answer: it depends on the length of the race.
In short-distances, Car A will smoke the other 2 out of the gate. Quick acceleration is this car’s specialty.
Car B really comes into its own in mid-distance races. It takes a li’l bit longer to get rolling, but once the gears mesh, it’ll zip right past Car A right around the time it’s starting to burn out.
Car C takes more time to take from a relative standstill to highway speed, but once it’s up and running, it’ll go like it’s got the cruise-control set at 55mph on a Kansas highway with a full tank of gas. Cars A & B are still running, but the longer the race, the more dominant Car C becomes. (In a sprint, even though Car A will obviously win, Car C is still chugging along trying to get the engine and transmission to make the full power-transfer connection.)
Your rest intervals determine how much fuel each ‘car’ gets before starting the next lap (but I’ll save my ATP analogy for another day.)
With this in mind, cross-training is not as simple as ‘weights one day, treadmill the next’ – especially if the relative intensity of the weight training is low and done as a high-volume circuit with minimal rest. That’s really no different than driving Car 3 with your right foot on the gas one day, your left foot on the gas the next.
Cross-training done right incorporates aspects of ALL THREE cars in the race, so duration, intensity and rest intervals are all important variables to consider (possibly even more than the actual exercise(s) being used), .