I guess I find the ongoing controversy with loaded stretches so bewildering given the popularity of flexibility training and plyometrics.
Lyle is correct, in the sense, that if you were to hold muscle fibers in a passive stretch, the response won't be all that significant without a super-lengthy amount of time. However, that is not the actual intent of loaded stretches and other forms of stretch.
LS and PS are methods to trigger the stretch reflex. The stretch reflex causes a neural signal that increases the effective tension. That signal can get very, very high. I read one piece a long time ago where it said that the reflex (albeit in a very extreme reflex reaction) can create a burst double the tension that the muscle was originally producing. Of course, you did that, you also run the risk of producing pulling the muscle and/or surrounding connective tissue. And that is why your PE teacher told you not to bounce while touching your toes.
You basically have two schools with that reflex. Flexibility training teaches you how to relax against the reflex, so your ROM increases. Plyometrics teaches you how to amplify that reflex, so that you produce massive amounts of force.
LS and PS are hypertrophy-specific versions of these two schools. They enable you to effectively use peak tensions that theoretically go beyond 120% of your training 1rm. With a normal weight. Way, way beyond. Moreover, they would be applied at where the muscle is most sensitive to damage, which is through a more stretched range of motion.
The problem with the stretch reflex is exactly the same problems that are in flexibility training and plyo. The reflex resets itself according to the new range of motion and training weight.
So, if you were to do LS, you have to either go longer and longer, or use heavier weight, or go even deeper. The problem with going longer is that,
eventually, a relaxation response kicks in, meaning however long you go, the tension will either peak or start to decrease. This is why flexibility training works at all. The conditioning response is similar in behaviour to the RBE.
But, I feel that as long as you progressively load and wait until the 5s or post-5s to implement these stretches, you'll always get a pretty strong reflex response before it doesn't become a useful technique. The key thing here is that the stretch reflex effectively increases the training load. If this reflex is high enough that it exceeds the peak tension generated through an eccentric contraction, then you can say that the LS is the "primary disruptor" for that muscle.
If it doesn't, then well, you've done a static hold with a heavy weight at a stretch. That's pretty good at generating strain too.
In theory, pulses stretches with >100% of 1RM is as high as you can possibly go with producing mechanical strain. It would be like suddenly applying a 300-500lbs bench press onto your muscle for 100millieseconds. Pain!
cheers,
Jules
