it depends on the athlete, but, in general, most people need "heavy explosive", since most people are weak.. always "as explosive as possible" with heavy weights.. max acceleration is a must, the descent can be controlled, but the transition from ecc to con has to be explosive as possible, accelerating all the way through to the top.. that's how you improve explosive strength to a greater degree than just lifting slow.
There was an interesting test I once read & it corresponds to guys I mentioned who lift light (Bruce Lee, Tyson Gay, Usain Bolt etc). Two groups were divided to perform squats. One group trained at a slow rate, and the other at a fast rate. The fast group took 2 seconds to perform each repetition, 1 second down and 1 second up. The slow group took 3x as long. Both groups were tested in their before and after abilities for broad jumping and the strength of the individual muscles of the legs. The fast group increased their power output in their hips and ankles more so than the slow group, and more significant was the improvement in fast group's broad jumping performance. The slower group did gain more overall strength, but the increase in power output was higher overall in the fast group.
Guys like Bruce Lee, Tyson Gay, Usain Bolt etc train primarily for power NOT mass or incredible strength.
What I'm trying to say is, would you better off going 1 second down and 1 second up (increased power output at the hips) as opposed to going "semi" heavy/heavy (more strength incremental based) lifts to increase your VJ?. So going to heavy seems to decrease your power output when squatting.
It's like when you want to increase the speed of your punches by throwing out weight plates. You have to use a weight that offers resistance but at the same time the speed factor is majorly important. You can't go too heavy because you bury the speed element (power).
right of course, speed is important BUT, slower less athletic individuals won't be able to make long term gains if they restrict their lifts to 1 sec down + 1 sec up.. very powerful individuals naturally, will be able to make long term progress using that rep tempo, they already have that ability to display force quickly.. weaker individuals have "slower rate abilities" naturally, which isn't something you can improve very easily.. instead, weaker individuals want to maximize their strength which will allow for more force to be produced at their "natural rate of MU recruitment".. sure that rate will improve over time slightly by focusing on speed of each lift (but not worrying about 1s down + 1s up), incorporating reactive work, incorporating sprints, and incorporating jumps etc, but it's not going to improve dramatically.
for example, look at how I plant during my RVJ's, and how golden child/t-dub plants during their RVJ's.. the difference in plant speed/force production is amazing, and I actually have a pretty fast plant right now, it has improved over the years, but nothing close to an elite level jumper.. my body needs slightly more time on the ground to produce force, or i'm not going to get up good at all.. if i were to focus completely on minimizing GCT, I would not be able to put out maximal force development for my specific "rate abilities".. this is why depth jumps are properly taught by: "focusing on achieving max height", because if you focus on "the fastest GCT", some individuals will make MORE GAINS than others, ie, the naturally more powerful athletes will achieve better gains than the weaker/slow athletes... but if you cue them properly, they both will make very similar gains, because the force production adaptations achieved will be specific to each athletes rate of force production abilities.. some people need to stay on the ground a little longer, it's just how they are wired.
i'm always trying to improve my speed under the bar, for example I could hit 315 x 5 and then 335 x 1, 315 x 5 being nice speed, 335 being a 7 second grinder... i'm not someone who can achieve REALLY FAST absolute 1rm's, for example, compared to someone who is a "freak oly lifter".. a freak oly lifter will get a true 1RM squat done in <2s, my true 1RM's are done in 5+ seconds... over time I will improve the speed of my 1RM's, but NOT BY A CONSIDERABLE AMOUNT...... though not considerable, it is extremely significant.. for example, if it takes me 5s to perform a true 1RM squat, and after 1 year it now takes 4.7s to achieve a true 1RM squat, those are significant gains in speed for my specific abilities.
if you look at the MSEM blog (maximal strength effort method), we preach speed bigtime on this site.. not restricted to 1s down+1s up though.. really heavy lifting (MSEM) done for singles and MAX SPEED (whatever that max speed may be specific to your abilities), with strict & proper form... that is more effective imo, than restricting to a really fast rep scheme... let's not forget that workup sets should be done with speed also, so you're getting speed work there too, and can definitely move the submaximal weights much faster than when you start hitting 80% through 100% max
also, reactive work, reactive/explosive lifts, can definitely help to improve true 1RM rep speed.
pC
edit: in the end, you have to look at what is going to get you the most progress, and that is usually utilizing intensity ranges around 80-95%, so if I progress faster in those intensity ranges, regardless of the rep speed, improving my 1RM by ~100 lb for example, will allow me to improve greatly the speed of my 1RM minus 100 lb, so very significant adaptations in speed have occurred.
