Time Under Tension (also called TUT) is a concept considered by many in bodybuilding to be one of the most important factors for stimulating muscle growth (hypertrophy). An important goal of resistance training is to create mechanical tension in the muscle. This tension is created when we lift heavy weights and our muscles contract against the load. The act of the muscle working against the tension then triggers some reactions in our body. One of the primary reactions is increased protein synthesis (growth).
Increased protein synthesis means our body adds protein to the muscle fibers, which thickens them. This thickening makes our muscles bigger and stronger. As the loads increase, so does the tension which should further increase protein synthesis and muscle size. So you would think all you need to do is lift really heavy weights. But as powerlifting and other sports have shown, moving a lot of weight for just a few reps doesn’t produce the greatest muscle gains.
There is also a time dependence to the tension applied. The muscle has to be under tension for a certain length of time. This is where TUT or Time Under Tension comes into play. The longer our muscles are under tension, the greater the protein synthesis. Of course it’s not as easy as just lifting a light weight for a lot of reps. As the load gets lighter, the tension goes down. We want MAXIMUM TENSION during the set for MAXIMUM PROTEIN SYNTHESIS which means using the heaviest allowable weights.
As usual people can’t agree on the exact amount of time under tension during a set needed for maximum muscle growth. Some may say as little as 20 seconds, others may say as high as 80 seconds or more. The generally accepted range for the most muscle growth usually falls somewhere between 30-60 seconds.
The trick is to find the right balance of weight and time under tension. Doing one really heavy rep will produce a lot of tension but a very short TUT. We want to use the heaviest weights we can move for that 30-60 seconds. Any less than that favors more strength improvements while any longer helps to improve muscle endurance more.
Assuming it takes between about 2-5 seconds to complete each rep, most people will need between 6-15 reps during a set to maximize both the load and keep the time under tension in the 30-60 second range. Obviously when fewer reps are used (6 vs. 15), the slower the reps need to be completed to extend the set so that it meets the 30 second limit.
6 reps X 5 seconds per rep = TUT of 30 seconds
15 reps X 2 seconds per rep = TUT of 30 seconds
The internet is filled with arguments about which reps are best and no one knows for sure. Since people have success with many different reps, it may not matter as much as people think as long as the reps used cause the time under tension to fall in the desired range while using the heaviest weights possible.
In general, staying in the 6-15 rep range in theory should yield the best results for muscle growth by allowing you to use the heaviest weights for the 30-60 seconds. The middle of that range (9-12 reps) is also arguably the most popular reps used by bodybuilders whose goal is maximum muscle growth.