Can stretching before training decrease muscle growth? A 2017 study used a within-subject design to answer this question. The subject performed unilateral knee extensions for 4 sets to failure, twice per week for 10 weeks. For each subject, one leg performed those knee extensions without prior stretching, and one leg performed those knee extensions following some pretty intense static stretching before the first set: 2 sets of 25 seconds, at a discomfort level of 8 (on a 0-10 scale).
Strength gains were similar between conditions, but the legs in the non-stretching condition experienced larger increases in vastus lateralis cross-sectional area: 12.7 ± 7.2% v. 7.4 ± 3.7%. In fact, all but one of the subjects experienced more quad growth in their non-stretching leg. This difference in hypertrophy may have been driven by the effects of stretching on performance: subjects averaged almost seven more reps per session with their non-stretching leg.
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The bottom line: The biggest takeaway ISN’T that stretching is bad. The biggest takeaway is that any stretching you do pre-training should not be intense enough or take place close enough to your working sets to negatively affect performance. If you do some light stretching, or if you stretch early in your warm-up (so performance has time to rebound before you start your work sets), you should be fine.