Most people stretch as part of their warm-up because they want to increase their range of motion, improve flexibility and reduce the risk of injury.
However, they don’t give a lot of thought to the potential effect that stretching can have on gains in muscle size and strength.
How does stretching before you lift weights, or between sets, affect muscle growth? Does it speed it up, slow it down, or have no effect at all?
To answer that question, a team of Brazilian scientists set up a very simple experiment .
They rounded up a group of men, and got them to train their legs twice a week on the leg extension machine.
With leg extensions, you normally work both legs together. But in this study, the men trained one leg at a time. This meant that each leg could be exercised under slightly different conditions.
It’s an elegant way to design a study, mainly because it takes genetics, lifestyle and diet out of the equation .
When you use different groups of people to compare training protocols, the food they eat, the life they live, and the genes they were born with can skew the results.
But with the “unilateral exercise model” as it’s known, nutrition, lifestyle and genetics are identical for both legs.
The men taking part in the study did four sets to failure, using a weight that was around 80% of their one-rep max.
The only difference was that one of the legs was stretched for a total of 50 seconds immediately prior to the first set. This allowed the researchers to see how stretching affected gains in muscle mass and strength.
Is Stretching Before Lifting Bad for Muscle Growth?
After 10 weeks, the muscles in both legs were bigger than they were at the start of the study. But the leg that had been stretched grew much more slowly. In fact, muscle hypertrophy in the unstretched leg was over 70% greater compared to the stretched leg.
Interestingly, not everyone responded to stretching in the same way.
- In four subjects, stretching led to a decrease in the rate of muscle hypertrophy.
- Three subjects posted very similar gains in both the stretched and unstretched legs.
- In one individual, muscles in the stretched leg actually grew slightly faster.
So what’s going on? Why did stretching put the brakes on muscle growth?
The difference in gains between the stretched and unstretched legs appears to have been caused by stretching reducing the amount of work the men were able to do.
In the first five weeks of the study, they averaged 37 reps across all four sets in the unstretched leg. But in the stretched leg, that number was just 30 reps.
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It was much the same story from weeks 6 to 10, where stretching cut the total number of reps from 46 to 39. In fact, total training volume (defined in this study as weight multiplied by reps) in the stretched leg was down by around 20%.
Static Stretching and Repetition Strength
This isn’t an isolated finding that contradicts a sea of existing data on the subject, and there are plenty of studies out there showing much the same thing.
And we’re not just talking about a rep or two. Static stretching has been shown to cut repetition strength, which refers to the number of reps you’re able to do with a given weight, by somewhere in the region of 20% [3, 4].
There are plenty of theories as to why stretching affects repetition strength in this way, from changes in the nervous system (meaning that fewer muscle fibers are called into action during an exercise, and a certain portion of the muscle never gets trained) to alterations in the “stiffness” of muscles and tendons, leading to a reduction in the transmission of force between muscle and bone .
Whatever the reason, the takeaway is clear:
If you want to build muscle mass as fast as humanly possible, you should avoid stretching before you lift weights as part of your warm-up.
Or should you?
Why Stretching Isn’t Killing Your Gains
For one, not all forms of stretching have been shown to harm your performance, especially when the length of each stretch is relatively short.
In one study, a single stretch lasting 20 seconds had no adverse effect on repetition strength, while a 40-second stretch did .
In another, 90 seconds of static stretching (3 x 30 seconds, separated by 30 seconds of rest) had no significant effect on repetition strength in the bench press and leg extension .
Brazilian scientists found that even 80 seconds of static stretching prior to lifting weights failed to put the brakes on muscle growth .
The training routine consisted of 4 sets of 8-12 reps of seated leg curls, performed twice a week, with at least 48 hours between workouts.
After 8 weeks of training, muscle hypertrophy in the hamstrings was almost identical in the stretching and no-stretching groups.
Much of the research that shows stretching harms performance involves a large amount of pre-exercise stretching, far more than most people are doing in the gym as part of their warm-up .
In one study, stretching the biceps for six minutes immediately prior to a strength test reduced strength by around 5% .
In another, thirty minutes of stretching led to a 30% drop in strength immediately following the stretch . After five minutes, strength was still down by around 20%. Even one hour later, full strength had still not returned.
But who’s stretching a single muscle group for 30 minutes, or even six minutes, as part of a warm-up before they lift weights? I’m not, and you’re probably not either.
Does Stretching Between Sets Build Muscle?
More interesting still, there is research to show that stretching between sets – which is essentially multiple bouts of pre-exercise stretching – leads to a faster rate of growth than just sitting around doing nothing .
In fact, subjects who stretched the muscles they were working between each set (that is, they stretched their chest between sets of the bench press, their lats between sets of seated rows, their quads between sets of squats, their hamstrings between sets of leg curls and so on), saw gains in muscle thickness that were were roughly 50% greater than their counterparts in the passive rest group.
Expansion of the fascia, the network of connective tissue that surrounds muscle cells, is often cited as one of the main mechanisms behind stretch-induced hypertrophy.
However, there’s no evidence to show that the muscle growth seen in this study had anything to do with changes in fascia.
The main stimulus for muscle hypertrophy is mechanical tension, and this tension can be produced by actively contracting your muscles or passive resistance to stretch. In fact, research shows that stretching can trigger muscle protein synthesis, albeit to a smaller degree than muscular contractions .
Although a fifty percent increase in the rate of muscle growth sounds impressive, it’s worth pointing out that it was measured in untrained subjects, doing a relatively small number of hard sets each week.
Given that there’s an upper limit on the amount of stimulation your muscles can respond to in any given workout, stretching between sets isn’t necessarily going to deliver the same results if you’re doing a higher volume of training.
Nor is someone with several years of serious lifting behind them likely to see the same kind of results.
Thirty seconds of interset stretching may well have been below the threshold required to reduce repetition strength. But we don’t know for sure, as the researchers didn’t measure training volume, so we don’t know if stretching between sets had an adverse effect on performance.
What’s more, certain exercises, such as Romanian deadlifts or dumbbell flyes, tend to generate high levels of muscle activation at long muscle lengths. Including these types of exercises in your training program may well replicate the effects of interset stretching.
Finally, there aren’t many studies to look at how interset stretching affects gains in muscle size, and the years to come may see this paper buried in an avalanche of research showing the exact opposite.
Still, it’s an interesting finding, and offers some support for the claim that stretching between sets can help your gains, an idea that’s been doing the rounds in bodybuilding circles for many years.
Here’s what Arnold Schwarzenegger had to say on the subject in his Encyclopedia of Modern Bodybuilding:
I believe it is also essential to do certain kinds of stretching during your training. Just as I recommend flexing and posing the muscles between sets, I also believe in stretching certain muscles between one set and the next. The lats, for example, benefit from careful stretching interspersed with various chinning and pulldown movements.”Arnold Schwarzenegger
Does Stretching Reduce Muscle Soreness?
It was once believed that stretching after a workout would help reduce delayed-onset muscle soreness, or DOMS for short.
The idea is that stretching increases the delivery of essential nutrients and decreases the build-up of lactic acid, which causes soreness and fatigue in the muscles.
However, there’s very little evidence to show that post-workout stretching has any kind of meaningful impact on DOMS.
A team of Danish researchers, for example, found that combining pre- and post-workout stretching had no measurable effect on muscle soreness.
Publishing their findings in the Scandinavian Journal of Medicine and Science in Sports, the researchers persuaded seven healthy women to take part in two experiments .
During the first experiment, the women trained their quadriceps to exhaustion on a knee extension machine. Ratings of muscle pain were taken for the next seven days.
In experiment two, the women performed the same type of exercise. This time, however, they spent 90 seconds stretching before and after exercise. Again, muscle pain in the quadriceps was assessed for seven days.
Contrary to popular belief, the results showed that stretching had no effect on muscle soreness, which reached a peak two days after exercise.
This isn’t the only study to highlight the fact that stretching doesn’t seem to do much as far as muscle soreness is concerned.
When a group of New Zealand scientists reviewed several muscle soreness studies, they found that post-workout stretching led to an average reduction in post-exercise soreness of just 2% — an effect that’s likely to be of “no practical significance” for most people .
Do a large amount of stretching for a single muscle group, and then attempt some kind of maximal effort in a task involving that exact same muscle group. The chances are high that you’ll perform worse compared to the same task done without stretching.
But I don’t know anyone who is doing that, and the stretching protocols used in many studies are very different from what most people are doing in the gym.
The extent to which stretching affects performance depends on the amount and type of stretching you’re doing, how much of a gap you leave between stretching and hitting the weights, and the type of training that follows the stretch (i.e. strength/power training vs bodybuilding-style workouts).
In short, stretching before you lift weights isn’t automatically going to kill your gains, and relatively short periods of stretching between sets may end up helping rather than hurting muscle growth.
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