Stretching is what you do after a workout to lengthen your muscles, increase flexibility, and help your body transition into a more relaxed state. These are the most common reasons we stretch but there’s something you might not know about stretching. There’s some evidence that vigorous stretching might boost muscle hypertrophy as well. The idea that stretching helps muscles hypertrophy sounds a bit foreign. After all, muscles grow in size when you work your muscles against resistance, not when you stretch them out. But, surprisingly, some research suggests that elongating your muscles, under certain conditions, can help them grow.
Passive Stretching and Muscle Hypertrophy
Stretching to boost muscle hypertrophy is a novel idea, but where did it come from? Some of the earlier evidence that stretching a muscle might aid hypertrophy was a study carried out on quails. In the study, researchers hung weights from the wings of quail for a full month, as they gradually increased the amount of weight that hung from their wings. By the end of the month, the muscles stretched in response to the weights had grown by 318%! When they looked at the individual muscle fibers of the birds, the number of fibers had also increased.
Well, quails are one thing, but what about humans? Can passive, static stretching boost muscle hypertrophy in humans as well? In a study published in Strength and Conditioning Research, researchers asked participants to take part in a single leg stretching program. The participants used a leg press machine loaded with 20% of maximal voluntary isometric contraction. The load was increased by 5% per week over the course of the 6-week study. The plantar flexors were also stretched to peak dorsiflexion for 3 minutes after which the participants got a dose of whey protein to supply the amino acid building blocks muscles need for growth.
The results? The length of the fascicles in the medial and lateral gastric muscles both increased in response to loaded stretching. The enlargement was greatest at the junction of the muscle and tendon. There were also documented increases in muscle thickness, but no change in maximal force development.
What’s the Mechanism?
You might wonder how passive stretching aids in muscle growth. Rodent models show that passive stretching increases the activity of a protein kinase called p70S6K. This enzyme adds a phosphate group to a specific ribosomal protein called S6. That’s important since this is part of the mTOR pathway, a pathway that turns on muscle protein synthesis for muscle repair and development.
We know that for muscles to grow, they have to synthesize new muscle proteins and that muscle protein synthesis drives the hypertrophy and strength adaptations we get after a workout. Your muscles are always tearing apart muscle proteins and synthesizing new ones. But, for muscles to grow, net muscle protein synthesis must exceed net protein breakdown. The mTOR pathway has a major impact on the rate of muscle protein synthesis. In fact, it’s the main regulator of the synthesis of new muscle proteins. mTOR senses a cell’s nutritional and energy status and if there’s plenty of extra fuel and nutritional building blocks around, it turns on muscle protein synthesis.
Studies show that high levels of branched-chain amino acids, including valine, isoleucine, and leucine, are strong activators of mTOR. That’s why it’s so important to get enough protein and calories if you strength train. If your body is in an energy deficit, anabolic pathways, like mTOR, won’t turn on. Instead, your body will enter a catabolic state.
So, if passive stretching activates mTOR, it’s easy to see how it might boost muscle growth. In fact, research suggests that active muscle contractions AND passive stretch turn on anabolic pathways and the two together ramp up these pathways even more.
You might ask whether stretching helps with strength gains as well. A recent found that vigorous stretching even in the absence of weight training boosted strength by 20% over a 3 to 8-week period. So, stretching may help with gains in muscle size AND strength.
Impact on Fascia: Another Way Stretching May Help
Some experts believe that fascia, the tough connective tissue that surrounds and encases muscles, limits a muscle’s ability to expand in size. The thinking goes like this. By tugging on the fascia through vigorous stretching, we can loosen it a bit so that the underlying muscle can more easily expand and grow. To do this, proponents of fascial stretching recommend stretching the muscles you just worked after each set and holding the stretch for 20 to 30 seconds before releasing.
The idea behind stretching after a resistance set is that you have a muscle pump and this makes the stretch more effective. Here’s an example. Do a set of, for example, chest flies. After completing the set, hold the final fly at the bottom of the movement for 20 seconds to stretch the muscles and fascia. Although stretching in this manner might increase flexibility and conceivably boost muscle hypertrophy via other mechanisms, there are no documented studies showing it impacts the fascia that covers the muscle. Fascia is pretty tough material and would likely only respond to intense stretching or manipulation. Any benefits might come from other factors like an increase in blood flow rather than a loosening of the fascia itself.
The Bottom Line
What can we conclude? There is some evidence that passive stretching may boost muscle hypertrophy, partially by activating the anabolic mTOR pathway. But, research is limited. However, there are few downsides to adding it to your routine, but don’t count on it to have a huge impact on muscle growth. You’ll get the most bang for your training buck by sticking to the basics – using progressive overload, consuming enough calories, choosing unprocessed foods, getting adequate sleep, resting sufficiently between training sessions, etc.
Yet, stretching does have benefits. It increases flexibility and boosts blood flow to the muscles. On the downside, there’s little evidence that it prevents injury or reduces post-workout soreness. (DOMS) So, make stretching a part of your routine, but don’t count on it to have a major impact on muscle growth. More research is needed. Instead, stick to the tried and true basics!
BodyBuilding.com. “Ask The Muscle Prof: What Training Technique Produces The Craziest Muscle Growth?”
Front Physiol. 2017; 8: 788.
Journal of Applied Physiology May 2009 vol. 106 no. 5 1720-1729.
Physiological Reviews, Volume 84, 209-238.