The purpose of this study was to provide a systematic review and meta-analysis of the current evidence revolving around the duration of repetitions (the tempo) and the effect that different durations of repetitions has on producing muscle growth.
In the world of building muscles, there are many different variables, all of which can play a significant role in muscle growth. We're talking, the weight you use, the reps you complete, the rest in between sets, how much protein you ate yesterday, did you stimulate the muscle fully, etc. The list goes on, but one variable that has always been up for contention is that of time under tension (TUT).
The theory of time under tension basically has always stated that the longer the muscle is under tension, or actually working (such as during an active repetition), the greater the potential for muscle growth.
Surely, total time under tension, better stated as Volume, is quite important. I.E. The total number of reps x sets x weight used. We've talked about this before - total volume increase is probably a major driver of muscle growth and is likely a primary player in practicing progressive overload. However, this is different.
This TUT argument is acute or the amount of time under tension for single reps and sets.
So basically, these researchers wanted to observe the available evidence and draw conclusions based on this evidence.
Systematic reviews and meta-analysis are the gold standard of research.
This is because it's viewing a combination of all data, relevant to the analysis, rather than simply drawing conclusions from one single study. Doing so makes the conclusions much stronger than saying, "Hey, I read this study, now it's fact."
Unfortunately, research doesn't work like that. A systematic review and meta-analysis pools all data, even ones that oppose the primary idea and then determines how strong the argument is, on either side, based on a collection of data.
That being said, context is still required. For this particular case, specific data on tempo of repetitions is scarce, meaning there isn't a ton. Basically, the more research you have for the analysis, the more data you can have in the pool, and the stronger the conclusion.
A good example is creatine. There are literally hundreds of studies corroborating that creatine improves power output. If however, there were only 5 studies, it would be harder to draw a definitive conclusion. 1000 studies all saying the same thing however, makes the argument stronger. Just keep that in mind.
For studies like this, there isn't actually any testing, but rather what is known as inclusion criteria. This is taking all the studies in the world on a certain subject and then adding filters. This allows researchers to pinpoint specific variables.
Consider for a moment, a meta-analysis of consuming whole eggs and their effect on heart disease. This is a pretty interesting topic, but there are tons of studies that may have evaluated this topic, in different ways. Where as one study may have studied the effect of eggs and heart disease in over weight people, another could study the same outcome in healthy people.
In the former scenario (overweight + eggs) it's hard to draw conclusions because these individuals are already overweight, which increases the chance of heart disease. It's too difficult to say "hey, these people have heart disease because of eggs."
Whereas in the second scenario, it may be clearer to view the effect of eggs on heart disease if the subjects were normal weight in addition to eating eggs and then developed heart disease. Surely, there are other variables, but you can include or discard studies based on the specifics of the studies itself.
Basically, researchers can say "If the study meets these criteria, include the study. If they don't meet these criteria, remove them from the analysis."
Inclusion Criteria (Methods)
Remember, with a systematic review / meta-analysis, there isn't an actual study. Just inclusion criteria. The criteria for this analysis is as follows:
Above is a representation of the filtering process (top to bottom). They start with a ton of studies based on their titles and abstracts and then apply filters, based on their inclusion criteria. If the study matches the criteria, it's added. If not, they toss it. Simple Enough
Here is a table of documents which were included in the meta-analysis. You can see the subjects and the types of rep durations that were studied in each, followed by results of each study.
Using this, you can begin to see the importance of a meta-analysis. Surely, many of these articles show positive or not effect whatsoever of the tempo of repetition. If you were to take one study and read it as gospel, you may have an idea that is directly opposing the outcome of a different paper.
A meta-analysis allows you to view all relevant data and then draw a conclusion rather than doing the same with only one outcome.
In light of brevity, one specific thing you need to know about research like this is effect size. Effect size is basically a way to measure differences between variables and then determine how significant that difference is.
It's basically a scale ranging from 0.00 all the way up to 2 with zero begin very insignificant and 2 being extremely significant. Here's an example of what effect size is actually determining.
Say you have two groups, that are being tested for tempo of lifting and muscle growth. Group 1 practices slow lifting. Group two practices fast lifting. Also for sake of simplicity, we'll measure muscle growth on a scale of 1-10 with 10 being a maximum amount of muscle.
Effect size helps us sort of visualize differences by taking all data in to account. This is important because surely some people who lift slow will see muscle growth and some that lift fast will see muscle growth. Effect size allows us to see which does it better and if there is actually a meaningful difference.
"The ES for the fast/heavy duration was 0.42 ± 0.17 (95 % CI -0.10 to 0.95); the ES for the medium duration was 0.37 ± 0.17 (95 % CI -0.16 to 0.90). There was no significant difference between the fast/ heavy and medium duration categories (P = 0.73)."
The ES for fast/heavy duration was 0.67 ± 0.19 (95 % CI 0.22–1.13); the ES for the fast/light duration was 0.79 ± 0.37 (95 % CI 0.095–1.67); the ES for the medium duration was 0.27 ± 0.20 (95 % CI -0.22 to 0.75); the ES for the slow duration was 0.29 ± 0.27 (95 % CI -0.34 to 0.92). There were no significant differences between any of the tempo categories (Hochberg-adjusted P value = 0.94).
What This Means
Basically, the findings of this meta-analysis show that according to a number of different studies that evaluated rep duration (tempo) as a primary variable of muscle growth, there's no significant difference between a fast or slow tempo, when the primary goal is to increase muscle size.
Important Considerations (Continued)
1. This isn't definitive
Even being a meta-analysis, you must remember that it's difficult to draw a definitive conclusion. There are many different variables that determine muscle growth, some of which we don't even know. Additionally, this is simply a pool of relevant data, carried out by different people with different procedures.
The use of this information should be done so with discretion. This isn't gospel, it's just saying that it seems there is a trend for no differences between rep durations.
2. Primary goal with probably change the meaning of this.
If you depend on improving strength or power, it's likely this information won't apply. When training for power or strength, speed plays a role in momentum and even just overcoming inertia (the amount of force you need to generate to overcome the weight of the object).
For example, to lift a 500 pound barbell, your intention should be to lift fast, even if it doesn't actually occur at the intended speed. I.E. - trying to lift 500 lbs slowly just won't work.
An additional example would be like a power snatch. It just simply won't work if you lift slowly. Just keep that in mind that next time you are considering super slow training.
3. Acute TUT vs. Long Term TUT
As I mentioned, total time under tension or total volume over an extended period of time probably plays a large role in hypertrophy, since you're consistently increasing stress on the muscle.
Acute TUT or the time under tension during a single set may matter, but not in the way you think. It would make sense that a longer set duration would add more volume, as long as that extended duration is due to increasing repetitions. The benefit of extending the duration of single repetitions is a bit unknown, even with this meta-analysis.
Just remember, total time under tension over a long period of time, probably matters and increasing volume is important. Increasing time under tension within single repetitions probably doesn't provide much benefit, and is ill advised if your goal is maximum strength or power.
4. Every dog has it's day
I'm a firm believer in varying training to hit all bases. Surely, you could have workouts which incorporate very slow training. It may provide some additional benefit over that of intentionally fast lifting. Just keep in mind that currently, the evidence doesn't show any direct benefit of using slow training, to a greater extent than other styles. You can use slow training, it just shouldn't be the only thing you do.
Why This Should Matter To You
It should matter because TUT is a controversial subject. Tempo of repetitions has long been discussed to be relevant, so finding strong information with regards to those claims is important. Further, keep in mind that even though these studies don't show much difference, it's likely that speed will play a role in performance outcomes such as strength and power (i.e. if you want strength and power improvements, you should at least intend to lift quickly and explosively, even if the amount of weight your using doesn't actually allow it).
1. There is probably no significant difference between fast or slow rep speed in terms of directly increasing muscle size.
2. A faster speed however will likely allow you to increase weight used, which may provide benefit (increased volume) as opposed to super slow training.
3. Intended outcome should drive speed. If you want to increase strength and power, training with a slow tempo just won't work.
4. Every training variable has it's place. There's certainly nothing wrong with training slowly, yet, it shouldn't be the only way you train (just as with every other training variable).