Will stretching lead to bigger, stronger muscles? Original paper

In this 8-week randomized controlled trial, a static stretching program and a resistance training program had similar effects on strength and muscle mass in recreationally active participants.

The effect of static stretching on maximal strength, muscle mass, and flexibility of the pectoralis major (the largest chest muscle), compared to resistance exercise.

Strength was measured unilaterally (i.e., the left and right pectoralis major muscles were assessed individually) using an isometric maximum strength test, muscle mass was measured using ultrasound, and flexibility was measured using the shoulder pass through test (shown below).

81 recreationally active participants (average age of 24; 64% men, 36% women).

Potential participants were excluded from the study if they reported performing daily stretching for the chest/shoulder.

In this 8-week randomized controlled trial, the participants were assigned to a static stretching, resistance training, or control group. In the stretching group, the participants performed 15 minutes of chest stretching 4 times per week. The intensity of the stretch was the maximum amount of discomfort the participant could tolerate.

How the chest was stretched

In the resistance training group, the participants performed 3 workouts per week, which consisted of 5 sets of the machine chest fly using a load equivalent to the participant’s 10–12-repetition maximum. There was no intervention in the control group.

Compared to the control group, strength and muscle mass increased in the stretching and resistance training groups, with no differences between the stretching and resistance training groups.

Flexibility increased in the stretching group compared to the resistance training and control groups. There was no difference in terms of flexibility between the resistance training and control group.

The findings of the summarized study align with those of the body of evidence on stretching in general. The performance of approximately 3 stretching sessions per week for 6 weeks increases flexibility and strength, with a much greater effect on flexibility than strength.^[1]

With respect to muscle hypertrophy, it seems that only high-intensity stretching interventions increase muscle mass, and the magnitude of effect is small.^[2] Specifically, interventions that involve stretching exercises performed to “the point of discomfort”, “maximum tolerable discomfort”, or what’s maximally tolerable “after the onset of pain” generally report an increase in muscle mass.^[2] On the other hand, interventions that involve stretching exercises performed to an intensity that doesn’t produce any pain or discomfort typically don’t increase muscle mass.

For muscle growth to occur, mechanical tension is needed.^[3] In this process, the mechanosensors in skeletal muscle cells detect an external physical stimulus (tension), which is then translated into biochemical signals. These signals regulate anabolic/catabolic pathways and the expression of genes that ultimately control the net protein synthesis rate. Although resistance training is typically used to induce mechanical tension, stretching is another viable option, but it’s a bit trickier to design a protocol that provides a sufficient stimulus to promote a robust increase in muscle mass.

In the vast majority of studies, stretching intensity is regulated by having the participants report the level of pain they’re experiencing on a numerical scale. However, one study involving 30 seconds of static stretching failed to find a correlation between objectively measured stretching intensity and the level of pain reported by the participants.^[4] This finding casts doubt on the idea that someone would be able to repeatedly apply sufficient tension (i.e., enough to promote hypertrophy) on the muscle solely through their perception of pain, especially because the perception of pain is sensitive to various factors (e.g., it can be exacerbated by anxiety), so a person’s pain threshold is likely to fluctuate from day to day.^[4]

To address these concerns, the summarized study used a stretching device to manage intensity. The device was strapped to the participant’s elbow to pull them into the stretched position, and the strap was attached to a force transducer to measure the amount of tension being applied to the muscle every 10 seconds. As shown in the graphic below, the amount of tension being applied continuously decreased (presumably because the individual adapted to the position, so it was less challenging), leading the research staff to “retighten” the straps on the device to adjust the range of motion and increase the amount of tension back to a sufficient level.

Measurement of tension during 15 minutes of stretching

When determining how the results of the summarized study might apply to the general population, it’s important to consider how unique and impractical the intervention was, as well as the population studied. The participants were recreationally active, which meant that they engaged in some form of physical activity at least twice per week. They did not necessarily have experience with resistance training.

People who are resistance trained are less responsive to resistance training interventions than those who are recreationally active or untrained because they are closer to their theoretical limit for muscle growth.^[5].^[6] So although the stretching intervention in the summarized study was sufficient to increase muscle mass in the particular population studied, it’s unclear whether the same results would have been found in resistance-trained participants.

In partial support of the idea that the same results would not have been found in resistance-trained participants, a 2023 meta-analysis reported that the effect of stretching on strength decreases with increasing training status, with no significant effect reported in either recreationally active or trained participants.^[1] However, this finding was based on a subgroup analysis that included any type of stretching intervention.

The results of a few studies demonstrate that stretching can promote increases in the strength and size of the calf muscles of recreationally active or trained participants if it’s high volume (1–2 hours per day) and intense (i.e., a pain level of 7–8 on a scale of 1–10).^[3]

The summarized study is notable for a variety of reasons. First, it included a resistance training comparator group, as opposed to only a control group that did not undergo an intervention, which is how most previous studies were designed. In addition, it examined the pectoralis major, whereas most previous studies have examined changes in the lower body, typically the calf muscles. Furthermore, it was the first study to look at changes in muscle mass in the pectoralis major, instead of just strength and flexibility.^[7][8]

The findings of the summarized study are captivating because they suggest that high-intensity static stretching can promote muscular adaptations on par with that of resistance training. However, the impracticality of the intervention must be considered: a unique stretching device was used, stretching intensity was objectively measured to ensure that tension was sufficiently high throughout the stretching period, and assistance from the research staff was required to adjust the device when tension dropped to an inadequate level. Furthermore, as noted by the researchers in the discussion section, considering the relationship between the amount of time invested and the muscular adaptations produced, resistance training appears to be more efficient than stretching.

Overall, although the evidence supports that stretching can promote gains in strength and muscle mass, it requires a particularly strenuous and time-consuming intervention, especially for physically active people. Furthermore, there is currently insufficient evidence to determine whether such an intervention increases strength and muscle mass in people with substantial resistance training experience. For these reasons, anyone looking for an efficient way to increase strength and muscle mass should stick to resistance training. Future research should investigate whether there’s any unique benefit to combining resistance training with an intense stretching intervention on muscular adaptations, compared to resistance training alone.

A potential limitation of this study is that all of the participants were instructed to maintain their regular exercise routine during the study. As such, the results could have been influenced by differences between groups in the exercise they performed outside of the study.

Another potential limitation is that some participants were not willing to undergo the stretching intervention, which prevented complete randomization.