Does stretching between sets make muscles grow more?

Short answer

No. Of 4 trials that measured muscle size, only one, with 29 people, showed clearly more growth. Stretching the muscle you just worked even cut the total weight lifted by 22%.

Does stretching between sets make muscles grow more?
'Stretch during rest → extra stimulus → more muscle' — does this formula hold?
A popular trick says that stretching the muscle you just worked during your rest between sets makes it grow more. Same time, extra stimulus. We checked trials that compared it with simply resting.
The first study found more growth
Outer thigh: 17.0% vs 7.3%
29 people lifted 2 times a week for 8 weeks; one group did 30-second static stretches between sets. The outer thigh muscle grew 17.0% thicker in the stretching group and 7.3% in the resting group. The sum of four sites was 10.5% vs 6.7%.
But other trials found no difference
Not in arms, not in a one-leg comparison
When 22 people compared the same approach for 8 weeks, there was no difference in arm muscle thickness or strength. In a 5-week trial where 16 people stretched only one leg between sets, the front thigh grew similarly: 8.0% and 7.3%.
Loaded stretching helped only one muscle
One of three calf muscles grew 0.7 mm more
21 people trained for 8 weeks, holding a 20-second stretch with the same weight right after each set on one calf only. Only the deep calf muscle grew 0.7 mm more; the outer calf differed by a small 0.4 mm and the inner calf not at all. Of 4 trials that measured muscle size, only the one with 29 people showed clearly more growth.
The cost: you lift less
Stretching the working muscle cut volume by 22%
When 12 people stretched the front of the thigh between sets of leg extensions, they lifted 4,855 kg in total, about 22% less than the 6,206 kg without stretching. Stretching the opposite muscle, the back of the thigh, did not reduce it. In the 22-person, 8-week trial, the stretching group also did less total work.
What you gain is flexibility
Same strength, joints that move further
In an 8-week trial of 16 people, both groups gained the same strength, and both gained flexibility, but in different joints. In the one-leg trial, the stretched leg's knee bent further. In a study of 14 people who stretched for just 20 seconds, the number of reps did not drop.
Bottom line: stretching between sets doesn't build more muscle
If you want to, keep it short or stretch the opposite muscle
In 3 of 4 trials that measured muscle size, stretching between sets did not clearly add growth. Stretching the muscle you just worked cut the weight lifted by 22%. If flexibility is the goal, keeping it under 20 seconds or stretching the opposite muscle works.