Here's a good article that got me thinking about time under tension. The article covers muscle-building from a variety of angles, including some NASA research on what builds muscle. The article quotes trainer Alywn Cosgrove, who believes a muscle has to be subjected to 90 to 120 seconds of total tension in order to trigger growth. It seems likely that there is some minimum of total tension necessary for muscle growth; after all, bodybuilders usually do three or more sets of an exercise, not just one.
There's no hard science yet on how many total seconds of tension are necessary for muscle growth. It's also unknown if it's better for the tension to be continuous or broken into shorter segments with rest periods. Yet there is plently of anecdotal information out there on the subject.
Here's one transformation that took place on The Learning Channel. The challenge was to transform an out-of-shape professor into a male dancer. According to the press release, the subject was able to gain 10 pounds of muscle in six weeks. The weight training was 3 sets of 60 seconds of slower repetitions, 3 seconds up and 3 seconds down.
Here's another transformation by author Tim Ferriss. According to him, he gained 34 pounds of muscle in only 4 weeks. Who knows if there is any funny business going on with this claim, but the pictures do show he made tremendous progress no matter what the time frame. Ferriss performed compound movements for 8 to 12 repetitions, with slower repetitions of 5 seconds up and 5 seconds down, for a total time under tension of 80 to 120 seconds.
Finally, I just saw a ton of copies at the supermarket of trainer Jorge Cruise's new book, The 12 Second Sequence. Cruise has "discovered" that slow repetitions are effective at building muscle. He recommends 4 repetitions of 10 seconds up, a 2 second static hold, and then 10 seconds down, for a total of about 90 seconds time under tension.
From these three possibly legitimate sources, it would appear that a threshold of tension for muscle-building may exist, perhaps in the range of 80 or 90 seconds. I would like to see some good studies done on this, but in the meantime it seems like a good idea to pass this time threshold when exercising a muscle group.