Only doing one part of the exercise (concentric or eccentric) is inferior for growth. Studies looking at pure eccentric vs. pure concentric vs combined training found that for hypertrophy:
*Combined (eccentric + concentric) is superior
*Eccentric-only is 2nd
*Concentric-only is 3rd
That having been said, you CAN use pure eccentric exercises within a program that includes other types of work. And while it is “theoretically” inferior, because it can represent a novel stimulus, for a short period of time (2-3 weeks) it can actually be more effective than regular training.
That later point actually applies to many things and can lead to becoming a blind advocate of a method: you get better gains for a few weeks, not because the method is more effective, but because of the novelty of the stimulus which forces adaptation.
By the way, that’s the reason why I include a lot of methods and approaches in my programs and clients. Even the best method doesn’t stay the best forever. Once the body is habituated to that type of stress, the training loses its effectiveness. And “inferior” methods actually become better.
That is the limitation of short-medium term studies (6-12 weeks) done to evaluate how effective certain methods are: they don’t account for the habituation factor.
Personally I don’t really have clients do pure eccentrics. We will do eccentric overloads with weight releasers, but they still have to lift the weight back up (albeit with much less weight than the eccentric). But because we use around 30% more weight during the eccentric than concentric, the former is emphasized a lot more.
For example, I just designed a program for an athlete and he has to do sets of 6 reps with 90% of his max on the concentric and 60% on the bar. The eccentric is done as slowly as possible and the concentric as fast as possible.
The only times we use a pure eccentric (negative) is as part of a complex.
A1. Pure eccentric (4-6 reps with 90-95%)
A2. Stato-dynamic (pause at parallel, explode up, 3-5 reps with 50-60%)
A3. Isometric (squat down to parallel, hold 20-30 seconds)
A4. Regular reps x 4-6