Slow Vs. Fast Reps. Which is better for muscle growth?

There are many things you have to consider when lifting weights. Rep schemes, amount of sets, amount of rest, the weight itself, and even the distance between you and that girl you try to avoid eye contact with through the mirror. With so many things to consider, there is still one possibly important factor that people tend to brush off. How fast should you be lifting? Conventional wisdom is to lift at a moderate tempo. Some say the speed doesn't matter much, and some say that slower is definitely better. 

There are a few reasons why people want to slow down reps. One of the goals is to increase the amount of time your muscles are under load. In the bodybuilding realm, this is known as "time under tension." The belief around these circles is that increasing time under tension, aka TUT, has magical properties to improve growth. Mechanically, it might make sense since placing your fibers under longer tension should elicit greater fatigue, thus greater growth. Slower reps can also restrict blood flow through longer contraction times. Cutting off blood circulation increases metabolite buildup within the working muscle, creating one, a bigger "pump," and two, the occlusion creates greater lactic acid buildup, which has been linked to muscle growth. Going even a bit further with this, the “slow” rep believers emphasizes the importance of slowing down specifically the lowering, negative portion of an exercise. This touches base with more growth theories where this negative, eccentric portion is responsible for greater muscle growth and even strength adaptations. One of which deals with eccentric contractions being responsible for the muscle tears during lifting. Know that sore feeling you get after your workout? Blame eccentric contraction for that one. You can also partly blame eccentrics for activating recovery-induced satellite cells, cells that are pivotal for growth. Eccentric contractions have also been linked to the release of phosphatidic acid, which have also been linked to, you guessed it, muscle growth. Well, it’s one thing to hypothesize the possibility of slower reps and greater time under tension increasing growth, but how does the applied research stack up? 

Studies that look directly into time under tension have shown that, unfortunately, it doesn't provide any significant benefits. And the big reason is, because greater time under tension means you have to use... lighter weights. Let me explain. When you're trying to find a weight that you can perform for 8 reps per set using a conventional lifting speed, it's usually a weight that's around 75% of your 1 rep max, aka the weight that you can lift one time and only one time. If my 1 rep max for an exercise is 100 pounds, I can expect to do 8 reps of 75 pounds using a regular tempo. Now if I try 8 reps of 75 pounds at a slower speed, say 5 seconds up and 5 seconds down, I won't be able to do all 8 reps. So right away I have to drop the weight. And it's at this point where things start to go bad for the slower rep fad. You see, there are two things that generally dictate muscle growth effectiveness. One is making sure that you take your sets and reps as close to "muscular fatigue," aka "failure," as possible. Studies have consistently shown the importance of volitional failure because failure means all muscle fibers are fully used. Slow and conventional rep speeds will be able to achieve failure, but faster speeds have a small advantage. At a faster rep speed, you will not only be able to lift heavier weights, but you will also simultaneously recruit more muscle fibers to exert a greater force. Slower reps also utilizes mainly smaller type 1 fibers, which doesn't help with growth as much as the bigger, type 2 fibers. But even with that mentioned, reaching fatigue is the primary goal, and is more important than the speed in which you get there. Along with reaching muscular fatigue, the other dictating factor is increasing total work volume. Total work volume is the product of the amount of reps, amount of sets, and amount of weight you lift. Studies consistently show that greater total work volume means greater muscle growth. Looking back at our 8 rep example, if I lift 75 pounds for 3 sets of 8 reps, that's a work volume of 1800. Using a slower rep speed, I will probably drop down to 55 pounds for the same 3 sets and 8 reps, totaling at a work volume of 1,320, or 27% less volume than using normal rep speeds. To summarize, the effectiveness of slow rep training might be held back due to the fact that one, although it does eventually recruit all your muscle fibers, it does not recruit as much simultaneously as faster reps would, which is important to muscle growth stimulation, but also important to increasing muscle strength. And two, slower reps ultimately mean lower weights. Lower weights mean lower total volume. Lower total volume means lower gains. But it doesn't mean slower reps are at all useless. As mentioned earlier, there are some merits to the hypothesis of eccentric contraction, but more importantly is the fact that one, you are stronger during the eccentric, lowering phase of a movement. Stronger means lifting heavier weights, which adds more to your total work volume. An eccentric-focused set, or some of you more commonly have heard it being called "negatives," can serve as a finishing set to push your gains a bit farther. Another is that by lifting a slower eccentric phase, you are assuring that you're the one controlling the weight down instead of letting gravity do all the work, keeping more tension in your muscle. Also, a generally slower rep scheme is good for beginners to work on form and prevent injury. Obviously, the faster you go, the less in control you are of the weight. By controlling your speed, you control the weight. 

Now if all of this still makes no sense to you, then the greatest takeaway is to simply just lift and not worry about it too much. Lift heavy, lift often, and lift consistently. IF lifting faster or slower works for you, then as boring as this answer may be, just stick with the speed you are used to. What's your thoughts of rep speeds? Faster or slower? What's better? Please share your thoughts!