These are great for strength. Although I've never done negatives exactly as suggested here, I've included them when I had a goal in mind and a great training partner to help me acheive it...
Studies show that the more control you use when lowering a weight, the more weight you'll be able to lift. One of the reasons for this is that the lowering phase loads the muscles that'll be used to execute the positive or concentric portion of an exercise.
Don't believe me? Try this test: Set up a power rack and lower the pins to just above your chest. Take your normal bench press one rep max and try pressing it from the bottom position. If you haven't trained bottom position bench presses, then I really doubt the bar will move at all!
When you bench press and take advantage of the lowering phase, your concentric muscles get warmed up and loaded. Thus, one way to improve your concentric strength is to improve your eccentric or negative strength.
I discussed negative training with strength coach Charles Poliquin recently and he revealed a great tempo speed that really emphasizes negatives. To illustrate, let's use the example of a barbell squat. Lower yourself to the bottom position in fifteen seconds. Once you get to the very bottom, explode upward as fast as possible.
Charles explained that one of the reasons this method works so well is that you'll have a natural urge to complete the concentric phase as fast as possible after taking so long to lower the weight. In addition, your concentric muscles will be fully loaded and ready to fire. Charles is having great success with this method and I recommend you give it a shot.
This technique can be applied to a number of exercises, but I suggest you stick with compound movements such as military presses, squat, chin-up, bench presse, and barbell row. Also, since there's such an emphasis on the negative portion, keep the rep range between one and three.
Here's one way to gradually break into this type of training:
Phase 1: Do six sets of one rep with five-second negatives and one-minute breaks between sets. Use a weight you can lift three times with solid form. Once you can complete all of the sets with the same weight, increase the negative portion to ten seconds.
Phase 2: Do six sets of one rep with ten-second negatives and one-minute breaks between each set. Again, use a weight you can lift for three reps with solid form. Once you can complete all six sets with the same weight, increase the negative to fifteen seconds.
Phase 3: Do six sets of one rep with fifteen-second negatives and one-minute breaks. Again use a weight you can handle for three reps with solid form. Once you can complete all six sets, add two more sets for a total of eight. Once you can do that, add another two sets for a total of ten sets per exercise.
Phase 4: Do ten sets of one rep with fifteen-second negatives and one-minute breaks. Once you can complete all ten sets, decrease the rest periods to forty-five seconds. Once you can complete all ten sets with forty-five second breaks, decrease the breaks to thirty seconds. Finally, once you can complete all ten sets with thirty-second breaks, increase the weight by ten pounds and start with one-minute breaks again.Pick one or two exercises you really want to improve with this method and try it for six weeks. I think you'll be very happy with the increases in strength and size. Also, there's no need to purchase a metronome and get anal about timing the negatives. Just count slowly or have your workout partner count the negatives.