That is a much more complex question that it appears. We must look at many many factors.
First I will say that ANY form of resistance training “works”; including VERY slow reps.
For example Ellington Darden used a 1 minute chin-up (30 sec up, 30 sec down) and a 1 minute dip (30 sec up, 30 sec down) for arm specialization. Note that he only did one rep per set, this is an important distinction and you will see why a bit later.
Also, my old weightlifitng coach Pierre Roy often used sets of squats with up to a 10 seconds eccentric and 10 seconds concentric for sets of 2-3 reps.
Jay Schroedder (and his fake personna DB Hammer) used long duration isometrics (1-3 minutes per set) as one of his main strength/muscle-building method.
And it works.
Heck, I use such methods myself plenty of times in the past.
Let’s look at why it can work, the potential benefits and drawbacks of very slow work.
I’ll use a bullet point approach to make it easier to understand, but it might get all over the place.
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For hypertrophy doing sets lasting 40-70 seconds is a very powerful stimulus. I have stated this myself on many occasions, in fact I mention it in an article I just published on my website and also in an upcoming T-nation one. The reasons why it works well is that 1) this leads to the higher level of lactic acid production, lactic acid has just found to directly increase muscle growth by inhibiting myostatin and increasing satellite cells (which help with muscle repair) 2) it increases local growth factors (MGF, IGF-1) the most, especially if tension is constant and 3) it leads to a lot of muscle fiber fatigue.
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It has been my experience that as long as you reach 40-70 seconds under tension during a set, and that at the end of the period you significantly fatigued (to failure or close to it) the muscles, it will work. It might not even matter how you reached that TUT, as long as your muscles are contracting hard for 40-70 seconds. In that regard superslow reps will work if the TUT is in the proper range.
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We have known for decades that a slow eccentric works well to stimulate muscle growth. But in my experience, it is not just the slow motion that is key but the fact that the slower eccentric makes it easier to flex/tense the muscle as hard as possible during the eccentric.In that regard a 6 seconds eccentric where you actively flex the target muscle will be a lot more effective than simply going down in 6 seconds, using the minimal muscular effort.
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Recent work by Dr. Shoenfeld has shown that focusing on the mind-muscle connection during a set leads to a greater activation of the muscle you are focusing on during your set and could lead to more hypertrophy. An argument could be made for slower reps in that regard: it’s easier to focus on contracting a muscle when you are lifting more slowly.
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During a slow concentric you minimize acceleration and momentum. In a 'normal" set you are creating momentum during the initial portion of the rep. Even though it’s not a lot (you are not throwing the weights), it is enough to decrease the need to create contractile strength in the later parts of the range of motion. When you are using slow concentric you get rid of that momentum and as a result you must rely more on contractile strength during the whole range of motion. This could lead to a create muscle activation and more muscle fiber fatigue.
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When doing super slow reps the muscle is more easily kept under constant tension. When a muscle is contracting, blood cannot come in or out. As a result lactic acid accumulates inside the muscle (and hydrogen ions) and oxygen cannot come in. Both of these factors increase the release of local growth factors that are very anabolic.
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One last theory is that superslow movements (or very long duration isometrics) could actually stimulate the fast twitch fibers because of the lack of oxygen. If you deprive your muscles of oxygen for long enough they are more likely to rely on FT fibers since the ST fibers are better suited for oxydation utilising oxygen. That is the theory behind very long duration isometrics (up to 3 minutes) like Jay Schroedder uses.
As you can see, slower reps can absolutely work.
But they have some drawbacks…
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Force production is lower. As someone pointed out, when using very slow tempos (especially during the concentric) the weights you can lift will be dramatically lower. This decreases force production and the amont of tension produced. This means that at the onset of the set you will not be recruiting the high threshold fast twitch fibers (that are the strongest and most growth prone).When force production is low, these fibers only kick in when fatigue accumulates and more force needs to be produced to keep going on (to compensate for the fatigue). So it still works for hypertrophy, no doubt, but for strength and power it is far from ideal as you don’t improve your capacity to quickly recruit the FT fibers and make them twitch fast (firing rate).
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This lower force production can counteract the benefit that slow reps can have on increasing contractile strength throughout the whole range of motion. As we saw earlier, if you get right of the momentum by lifting slowly. Let’s look at an example.
If you are doing a curl, when you start from position A from a velocity of zero your muscles must produce more contractile strength to get the weight moving. If, when you reach position B, the movement has speed you don’t have to poduce as much force to keep moving the weight, conservation of movement and all of that. The slower you are moving, the more contractile strength you must produce.
But it only means that you must produce a greater percentage of the force required to move the weight. For example if the weight is 20lbs and you are starting from zero velocity you must produce “all the force” required to overcome the inertia and more 20lbs. But if has some speed already when iot reaches point B you might only need to produce 50% of the force produced at point A to keep that 20lbs moving. The faster you go,the less contraction force you will need to produce.
HOWEVER by going slowly you will not exceed the contraction you would need to overcome the inertia of the 20lbs. So if the slow speed forces you to use a very small weight, you might lose the potnetial benefit of having te rely mostly on contraction at every point in the range of motion.
Being able to keep 90% of 1 dollar still only gives you 90 cents. Whereas keeping 50% of 5 dollars will give you 2.50$.
The point is that there is a point of diminishing returns here; if the weight used is too low you lose the benefits for strength (could still work for size).
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The theory about very long duration work (2-3 minutes) leading to oxygen deprivation and FT fibers recruitment is not well understood, and it only potentially works if the muscles are kept under constant tension (that’s why Schroedder uses isometrics)… the constant tension creates an occlusion effect, preventing oxygen from coming in. Very slow reps could potentially offer the same benefits and might make some sense in theory, but it has not been substentiated. Sure Jay’s athlete are super strong and powerful…but they do a boatload of explosive work, plyometrics, heavy lifting, etc. Soit’s hard to quantify the benefits of the long duration isometrics.
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F = ma, Force equals mass x acceleration. Accelerating during a loaded movement requires more force than simply lifting it. Lifting a weight slowly reduces force production. Fast and slow actions have a different pattern of force production:
Fast = big burst of force at the beginning, relative relaxation afterwards; higher PEAK force but lower force production if you take out the small part of the ROM where you accelerate.
Slow = gradual/constant force production with a much lower peak force but more actual muscle work(if the weight is similar)
I’ll let you make up your own mind considering the info I provided.
I personally believe that for muscle growth there are benefits in slower reps, within reason. Doing up to a 10 seconds eccentric and 5 seconds concentric can work for growth, to improve mind-muscle connection, create more muscle fatigue, increase lactate and growth factors production and make it less mentally boring to reach a TUT of 70 seconds (which would require 20+ regular reps). But IMHO there is a point of diminishing returns when going slower have no added benefits and can have drawbacks because the load has to be decreased too much.