Mixed

Chronic static stretching exercises produce an overall unclear effect on skeletal muscle hypertrophy in healthy individuals, with a trivial point estimate and a prediction interval ranging from trivial negative to moderate positive effects.

Do not expect static stretching to build significant muscle on its own. While it improves flexibility, its impact on muscle size is negligible for most people. If you do stretch, ensure you are doing it for long durations (e.g., >60s per exercise) and high frequency, but prioritize resistance training for hypertrophy.