Intensity in training has a different definition depending on whom you ask.
In bodybuilding, it means contracting the targeted muscles as hard as possible for the longest time under tension.
In metabolic conditioning, it means speed as a percentage of maximum possible for a given distance.
In strength training, it means a percentage of 1RM for a given lift.
CrossFit says that intensity is the single most important variable for results.
That’s actually correct, even when applied purely to strength training.
[There’s a minimum threshold of intensity required to build muscle and strength
even in high-volume training, and high intensity/low volume work preserves
fitness while allowing the fatigue accumulated in higher-volume phases to dissipate.
High workload + adequate intensity = increase fitness + increase fatigue = maintain preparedness
High intensity + low workload = maintain fitness + decrease fatigue = increase preparedness
That’s how periodization and peaking work.
And everything in strength programming can be applied to speed training.]
CrossFit has probably the best overarching definition of intensity,
which is power output given the parameters of the activity.
That has obvious transfer to metabolic conditioning and strength training
since measurable physical work is being done.
(It’s less obvious to apply to bodybuilding, but if we could measure the biochemical work
done by the muscles in question we could approximate a power-output summation.
We won’t worry about that here.)
In monostructural met-con, if you’re to do 8 repeats of a 100m sprint, beginning each interval every 3 minutes, maximal power output would mean fastest average interval time.
In mixed-modal met-con, it’s completing as much quality work as possible within a given time frame,
or completing a given workload as fast as possible without compromising movement integrity.
For strength training, we have other fixed variables – lifting tempo and reps, namely – so power really means minimizing time to complete the concentric part of each rep while maximizing load.
I would posit that you could also approach this from the other end, keeping load constant
and ending the set when power output (i.e. speed) starts dropping appreciably.
Basically, we’re not JUST trying to maximize the power of each rep;
we want to maximize the power of the set as a whole.
Let’s say for the sake of easy math that your 5RM squat with a 30X1 tempo is 200#.
(Make it kilos if you want to feel good about yourself.)
The distance for each rep is the same, so instead of foot-pounds we’ll use rep-pounds.
Let’s assume that it takes about one second to lift the weight on reps 1-3,
two seconds on rep 4, and three seconds on rep 5.
This means it takes 28 seconds to do 1000# of work, or an average of 35 rep-pounds/sec.
However, if we cut the set at three reps just before power output drops off,
we can hit a power output of 40 rep-pounds/sec.
As an aside, look at Prilepin’s table for the intensity zone that a 5RM squat falls under (80-89%).
Optimal reps per set? 2-4.
The Soviets were way ahead of us on this.
So to answer the question, why does intensity matter?
Because intensity brings results.