If I bounce (slightly) at the bottom of an exercise I induce a faster contraction due to consequent stimulus of fast twitch but also elastic recoil. And if I dont bounce and maybe even pause at the bottom the movement is much more difficult causing much more of a strain to perform.
Which is the more beneficial in inducing growth and strength gains?
I'll agree with the recoil part, I don't see any "faster stimulation of fast twitch fibers" however. I would say that unless you are doing a competitive lift (read: OLY lifting) you probably shouldn't bounce out of almost any lift. Yes it is harder, forcing you to control the weight more and spend more time under tension which should in turn affect your strength gains. Bouncing may slightly increase the weight you can use, but you also subject yourself to greater opportunity for injury and normally less time under tension (aside from just the bounce) during the entire ROM.
About ten years ago , I was in gym working out with a guy who was a big believer in a little bounce at the END of the rep. One day I happen to be watching him as he was on the leg press.Rep after rep with his little bounce. Then out of nowhere I heard this loud scream and a deperate cry for help.As he was bouncing ; in the leg press , his lower body wouldnt extent. (no bullshit) He was basicly paralysed in the leg press machine. We had to unload the leg press machine. I think about 6 plates. He tried to get himself out of the machine ,but his lower body wouldnt move. Someone called the paramedics , and of course it took them foreever too arrive. When they finally came , they had to shove a needle or two in him. (I cant remember where) After 10-15 mins of crying ,he got some mobility back , but was still in the strangest position.
About a month later , he returns with a new philosophy: NEVER BOUNCE ON WEIGHT BEARING EXERCISES
There are a bunch of lessons to be learned from this -
Don't bounce leg presses.
More generally, don't bounce with your spine in a rounded position (this is why people screw up their backs leg pressing)
Still more generally, don't release intrabominal pressure or muscle tension when you 'bounce'
'DON'T BOUNCE' isn't one of them.
From a the perspective of training for strength, though, I would be interested to whether there is any advantage to pausing or descending slowly during training. As an oly lifter, of course, I never do either of these things. But are they beneficial for powerlifting?
Pausing increases the time under tension and assures cleaner (usually) technique. A slower eccentric move (descent) does a couple of things. Again it allows for greater time under tension--remember that more time spent under heavy weight (to a point) lends itself better to hypertrophy gains and strength. Slow eccentric moves also allow for greater cross bridging (basically more muscle fibers get involved) and therefore potentially greater force production. This is why doing "negatives" on an exercise leaves you so sore--more damages muscle fibers. Remember, you can ALWAYS lower more weight than you can lift because of this cross bridging effect.
Powerlifters will use this type of idea for force production, becuase it allows for greater control over the weight and also the ability to move more weight. Think of trying to squat something extremely heavy (max box squat, not OLY squat). If you had to rely on a bounce to get you out of the hole, there would be a momentary lapse in muscle tension and you would never get the weight up. If you slowly descend and stay tight on the bottom you stand a greater chance of moving the weight up.
I don't believe in bouncing out of the hole (not because I'm worried about staying tight but because I don't want to injure myself), but on both the squat and the bench you should lower the weight as fast as you can under control. The longer you take to lower the bar, the weaker you will be.
Secondly, you can stay tight while taking advantage of the stretch reflex. Especially in the bench, it's the tightness in fact that allows you to get a good stretch reflex (a relaxed muscle has to be stretched a lot further to get the same elastic force production). Rather than letting the bounce do work for you and just going limp, work with the stretch reflex which will give you good speed out of the bottom.
If you see a powerlifter going down slowly, it's usually because he can't go any faster due to the supportive gear he's wearing.
I like to bounce when I'm in a ballistic phase of training. I just wrote a thread on this about what happened to me while bouncing at the top of my squat. Long story short, I ended-up bouncing the bar off my neck, and all the way down my back with my hands still gripping the bar. Luckily, I didnt dislocate my shoulder; although I did get some strained ligaments.
When you bounce you are taking advantage of the stretch reflex. Box squats are a tool powerlifters use to help increase their Squat. The box causes a "break" in the concentric/eccentric motion, thereby removing the advantage of the stretch-reflex...or ability to bounce. The answer is you have to train both. When to train both is where most arguments arise. My understanding from some Champion Powerlifters is at least 1/3 of their time is spent training without the "bounce".
Additionally...the more advanced lifters lower the weight more slowly. The faster the eccentric...the harder it is to "overcome" the weight to raise it. The slower the eccentric, the easier it will be to overcome.