Substantial fitness comes from activity performed at an intensity that recruits all muscle fibers in a sequential fashion. This goal requires a specific time frame and heavy weight. You likely perform 3 sets of 8 reps when you lift. Ever wonder why? 90 seconds is the time it takes your slow-twitch fibers to recover.
You have different types of muscle fibers which fire and recover at different rates. Your muscle recruits slow-twitch fibers first, and when they fatigue, your intermediate- and fast-twitch fibers take over. Your fast-twitch will take days to recover once they are fired, but in 90 seconds the slow-twitch recovers and takes the load. So, you want to fatigue your muscles at a rate that will allow all your fiber types to fire and fatigue during the work out. You want to fatigue your muscles at a rate that will allow all your fiber types to fire and fatigue during the work out.
This means you can do the same work in one set of slow lifts! Slow strength-training requires practice and self-discipline.
The benefits of Slow lifts include a decreased risk of injury and increased amount of strength gains by avoiding momentum which diminishes motor unit recruitment.
Time Under Tension
Muscle develops from time under tension. The goal is to work your muscle for a certain amount of time. Many people assume the longer you work the better. To that end, they add more sets and more reps to add more time. But studies show additional sets do not improve strength. Why?
Time matters, but what matters more is how intensely you work. Your goal is failure. You want to fatigue your muscles and do it between 60 and 90 seconds. Why?
The why has to do with your muscle fiber types. You have three types: slow-twitch, intermediate-twitch, and fast-twitch. The slow-twitch is a lower-order fiber (less calorie cost for your body), fires first, and recovers quickly within 90 seconds. Think jogger, endurance. Fast-twitch is a higher-order fiber (takes lots of calories to maintain), fires very briefly with great power, and slowly recovers over 4-10 days. Think sprinter, power. The goal in strength training is to strengthen all your muscle fiber types for endurance, power, and to keep your hormones youthful and your body lean.
To recruit all your muscle fiber types, you must fatigue your lower order fast enough (before 90 seconds) that they are not recovered before your fast-twitch is recruited, yet slow enough (after 60 seconds) so that you are not recruiting all fibers at the same time and quitting before your slow-twitch can be fatigued. To get all fibers worked, you must fire them sequentially, and that takes a slow, steady lift that allows your slow-twitch to fatigue and your fast-twitch a few seconds to fire before failure.
So you can see where the notion of 3 sets of 8 reps comes from. A 2-second contraction with 2-second extension done 8 times over 3 sets gives you 96 seconds when your muscles are under tension! The problem is that the structure of sets also gives you breaks during which time your slow-twitch recovers and prevents you from reaching muscle failure in all muscle fiber types. Very few of us have learned to push ourselves to muscle failure. Yet, that is the true goal of strength training.
The way to approach this magic 90-second set is to lift continuously, with slow repetitions using a weight that is enough that you will not be able to do one more full contraction (reach failure) between 60 and 90 seconds. If you can achieve this formula, you can achieve a full body workout in 12 minutes.
It will take practice and desire. Start with a goal of 6-second reps. Slow down as much as you can without stuttering the move—keep it fluid.
Inspired by Body By Science (Doug McGuff, MD and John Little).