Well, I'll just toss this out here:
Here I think is the passage in question:
"In particular, attempting to apply research suggesting that maximal isokinetic eccentric training at faster speeds (41) promotes greater muscle growth to a free weight training scenario lacks external validity from a scientific standpoint. Doing so would likely result in eccentric unloading (due to the momentum when attempting to rapidly lower a barbell or dumbbell) or increased injury potential if a spotter were to apply downward force during high-speed eccentric contractions."
Isokinetic training involved maximal efforts at a pre-determined speed. In other works, the machine to which the trainees limb is attached will move at whatever speed (angular velocity) the machine is programmed for. You can do MAXIMAL eccentric efforts at -10 ˚ / second or -300˚ / second.
With free weights, living the same load, if you were to exert MAXIMAL effort during an eccentric - using a load you can and have been lifting and lowering during a set - the weight just goes back up. There is no maximal eccentric here.
I explain this in the 2nd sentence above in a different away: You try to move the eccentric a higher speeds and you'll likely be unloading the muscle. Drop at heavy weight on yourself or try to pull it downward and you're gonna have less loading on the muscles that contract eccentrically in lowering (and concentrically in lifting) the load.
(Hopefully, and TP4U gets at this, the idea increased injury risk with high speed eccentric that are loaded - to mimic an isokinetic scenario - is pretty clear.)
So, for those who understand this (the citation #41 refers to one such isokinetic study), the point about external validity is simply that the mode of training in an isokinetic training is categorically different than that of free weight training.
You can't study bananas and assume your results apply to all fruits.
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So, that ought to explain that paragraph. This is not to invalidate any study using an isokinetic device. The work GREAT for loading. I used one to train folks for my dissertation study. (The loading you can create with an isokinetic device, using e-stim to override neural inhibition during eccentrics is phenomenal.)
I don't think anyone's saying this, but I'm also not suggesting that someone use a super slow negative approach as a mainstay of his / her training to produce growth.
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Summary: Isokinetic ≠ Free weight.
-S