Dr. Wayne Westcott, the fitness researcher associated with the Navy Seals, performed a study of different combinations of sets and reps in different styles of training, and came up with an unexpected number. All of the conventional styles of training that worked well for him seemed to end up at about a 70 second duration (+ or - 10 seconds). You could vary the number of reps and the speed of the reps any way you wanted to, as long as you ended up with an exercise that lasted about 70 seconds.
Is there a biological connection between anything that goes on in the muscles and 70 seconds?
Yes there is. Working at their peak level, intermediate twitch fibers exhaust in about 60 seconds, give or take 10 seconds, which leaves the last 10 seconds for the fast twitch fibers to exhaust.
Now here is the interesting common secret that you will get from almost every fitness guru.
"You have to push hard through those last 2 or 3 reps."
Why? Why wouldn't the first 60 seconds be enough?
Remember, INTENSITY of EFFORT is key! Why is so much determination necessary for the last 2 reps, and not for the first 60 seconds or so, with standard exercise? And why does all the new muscle mostly come as a result of the all out effort done right at the end?
A Little Light on Muscle Recruitment
The reason has to do with how the motor neurons that engage the muscle fibers are wired. They are wired in a fashion that requires you to fire all of the Slow Twitch Type I fibers before you fire the first Fast Twitch Type IIa fiber. And to fire all of the Fast Twitch Type IIa fibers before you fire the first Fast Twitch Type IIb fiber.
That type of biological techno-speak mumbo-jumbo is what used to get me picked on by the other kids in school, so let me give you an analogy.
You have rented out this theater to put on your daughter's play, and you want to shine a great spotlight on her during her big scene.
But the guy who does maintenance for the theater doesn't speak the same language that you do, and turns out to be really stingy with lighting. He has been instructed to keep the electric bill, and the bulb replacement bill as low as possible, but still make sure that you get all the lighting you need.
So he's installed a system where the bulbs are arranged in a row. The dimmest, longest-lasting bulb is the easiest to turn on, you just reach over and flick it on. He stuck that one closest to you because it's dim and has a ridiculously long lasting bulb. That way he saves on electricity, and you've got a bulb you can use all the time.
He shows the system to people, and shows them how he wants them to first turn on the 1st bulb, then the 2nd, and so on, so that they are always using the longest lasting lowest power bulbs for as long as possible.
But some people start cheating and just turn on the bulbs in the middle, to get bright light quickly. He doesn't like that because the middle bulbs don't last as long, and take more power. He'd rather you use two of the dim bulbs instead of one middle brighter bulb.
So what he stretches the bulbs out along a catwalk. Now you have to turn on the first bulb to be able to see your way to get to the second bulb, and you have to turn on the second bulb to get to the 3rd, brighter bulb. And this way he is certain that you only use as much electricity as you need, and you only use the longest lasting bulb that you can.
But just in case, there at the very end of the catwalk he's put some flash bulbs, just for the most brilliant light you might need (though they are the shortest lived)
Even so he has been instructed to make sure you have all the lighting you need, so if this ever isn't enough, he's to upgrade all of the lights to larger, brighter bulbs.
At first he would sit in the audience and watch the whole play just to make sure that you always had enough light.
Then he notices something. If you never turn on the flash bulb, then you always had enough light. Only if you turn on the flash bulb, only then do you need more light. And that's his way to be absolutely sure that the lighting needs to be upgraded. If you use the flash bulbs then, and only then does he know that the lighting needs to be upgraded.
Nature Is Efficient, Not Perfect
Your body is a lot like that theater manager. You can't tell it what you want, and it doesn't always use a clear signal to figure out what you want.
Conventional, 20th century fitness training with weights, somehow got locked into the notion that it isn't safe to do an all out effort, even though sprinters, shot putters and Olympic lifters routinely do exactly that.
And if you follow the analogy they are much like the father in the theater turning on everything except the spotlight on his daughter, and then only turning the spotlight on after the other lights burned out.
The theater manager didn't care if the other lights burned out or not, he was used to the way most people turn on the lights. Most people go straight out to that last light, turning every light on along the way, if they want more light than the theater has.
Who am I talking about when I say, most people? I'm talking about the entire animal kingdom, and all of humankind prior to the advent of weights.
Nature doesn't care about the dim lights in the Slow twitch fibers. Nature wants to know if you need the Bright lights, and especially if you burn out the brightest one in the house. Because if you do, an upgrade to the lighting is an absolute requirement.
The quickest way to burn out the bright one is to turn all of them on right away, including the bright one.
That's what sprinters do. That's what shot putter's do. That's what Olympic lifters do.
But don't do it with free weights, without a power rack. Unless you have the skills of an Olympic lifter and are prepared to just toss the weights to the side when you get in trouble, then it's just too dangerous.
And that, more than any other reason, is why the whole "wear out the weak lights before turning up the power" method of training has evolved, because of the unsafe nature of barbells that are near your strength limit.
But with a bar that doesn't move, that worry is a thing of the past.
Effort is the KEY!
The same way the theater manager could tell that you really needed newer brighter lights when you were willing to walk all the way out the catwalk, with all the lights on, your body can tell that you are serious only if you turn on every motor unit, which takes a massive amount of effort.
This amount of effort is foreign to most people. And there's a reason, because it is effort.
And it actually works both ways. The reason nature has made it so hard is that nature is stingy, and doesn't want to build any stronger a body than it has to. That strong body just takes too many calories to keep going. So nature made it as hard as possible to turn on the flash bulbs.
That's nature's way of knowing that you really needed the upgrade.
Your slow twitch fibers are nature's equivalent of 40 watt bulbs, and your fast twitch fibers are nature's equivalent of a flash bulb.
And the effort you have to put into it is nature's equivalent of the catwalk that the theater manager set up.
So if you think of the dim bulbs as the Slow Twitch Type I fibers, the brighter bulbs as the Intermediate Twitch Type IIa fibers, and the brilliant but short lived flash bulbs as the Fast Twitch Type IIb fibers, and you think of the light switches that all had to be turned on along the way as the motor units, then you have an image of the way muscle works.
So why 70 seconds?
If the weight used were so low that the slow twitch fibers could mostly handle it, then when the intermediate twitch fibers finally gave out the fast twitch fibers would only blow a few bulbs.
What's wrong with the analogy?
The analogy is not perfect. Your body does look down the row of lights a bit.
It does upgrade the lighting a little bit even if you never get to the last flash bulb.
But the evidence from shot putting, Olympic lifting, and 100 meter dash sprinting is very clear. In similar events, the shorter all-out effort grows faster larger muscles than longer efforts at lower intensities do.