One of the supposed rules of fast muscle gain is that in order for a muscle to grow it has to be exposed to a certain load (e.g. under tension) for 40–70 seconds at a given time. This simply means that every set you do should last somewhere between 40 and 70 seconds. If you have never heard this nonsense before, I will give you a second to pick yourself up off the floor and stop laughing. To disprove this theory, I will point to the immeasurable number of lifters who have grown to enormous proportions without ever adhering to this rule. Taking 40–70 seconds to do a set of 6–8 reps is the most ridiculous thing I have ever heard of. Lifting at this slow speed severely limits the amount of weight you can use and thus decreases the effectiveness of the set. The only thing that long time-under-tension sets do is increase lactic acid production. This results in the burn feeling and may be what people are confusing with effective training. You can get a great burn by going for a swim or a long bike ride or by doing 1,000 crunches, but everyone knows this isn't going to do a single thing to help you build muscle.
Fast twitch muscle fibers are the ones that have the greatest potential for growth and are only stimulated when the load is heavy, the reps are fairly low and the set lasts about twenty seconds or so. So sets of five or six reps with 80% or more of your one rep max (1RM) are far more productive at eliciting fast muscle gain than sets of 10-20 reps with 50-60% of your 1RM done at slower speeds. That's not to say that you should never do slightly higher reps or use less than 80% of your 1RM but that that type of training should never be the main focus of your program.
I hate to admit this, but back when time under tension was first discussed and brought to popularity, I did extensive experiments with this concept in my own private training center. Yes, I got suckered. After reading everything I could about time under tension, I immediately modified every one of my clients' training programs. We experimented with longer time under tension sets of 40-70 seconds for over a year.
The results? Nearly every single client, to a man, got smaller and weaker. Those that didn't remained the same size and strength or made marginal gains, at best, that were far inferior to anything we had previously done. I admit it was a huge mistake, but trying things, making mistakes and adapting is the best way to learn.
The good news is that I have already done the experiments and made the mistakes for you. Learn from my misfortunes, avoid these deadly mistakes and you will be on the road to fast muscle gain in no time.
Source White Market