The evidence-led answer
NST verdict
No. The "muscle bound" idea is a myth. Strength training done through a full range of motion with real load improves flexibility about as much as stretching does. A large review of 55 studies found resistance training increased range of motion, with the biggest gains in people who started out sedentary, and found no difference between lifting and stretching for flexibility. Two conditions attach: you have to train through a full range, so partial reps will not do it, and you need external load. In that same review, bodyweight only resistance training did not significantly improve range of motion. If you lift through full ranges with proper weight, you probably do not need separate stretching just to keep or gently improve your everyday flexibility.
Does lifting make you stiff?
No. Done properly, it does the opposite. A systematic review and pooled analysis of 55 studies found that resistance training increased range of motion, a moderate effect overall, with sedentary and untrained people gaining the most. Age, how long people trained and how often did not change the result. Training a muscle through its full range under load, including the stretched position at the bottom of a rep, lengthens it and opens up the joint rather than tightening it. The picture of a big lifter unable to scratch their own back is not what the data shows.
Strength training versus stretching for flexibility
They come out roughly even. The 55 study review found no significant difference between resistance training and stretching for range of motion, and no extra benefit from doing both over stretching alone. A second pooled analysis, of 11 randomised trials, reached the same "no difference" conclusion. And a controlled trial had untrained adults do either full range resistance training or static stretching of the same muscle groups for five weeks: hamstring and hip flexibility improved equally in both groups, and the resistance training group also got stronger.
The two conditions
The flexibility benefit is not automatic. Two things have to be true. First, full range: the gains come from training through a complete range of motion, so half reps and short partials should not be expected to build flexibility. Second, load: in the big review, resistance training using only body mass did not significantly improve range of motion. It was the versions with external load, or a genuine loaded stretch at the lengthened position, that produced the effect.
Do you still need to stretch?
For everyday flexibility, if you already lift through full ranges with proper load, probably not, on this evidence. Keep stretching if any of these apply: you enjoy it, your sport demands extreme range like splits or deep overhead positions, it is part of a warm up you like, or a physiotherapist has prescribed it. Stretching is not harmful or pointless. It is just not the only route to being flexible.
What this means in practice
Do not avoid strength training for fear it will make you stiff. Lift through full ranges of motion with a load you can control, let the muscle stretch under tension at the bottom of each rep, and your flexibility will hold or improve. If you lift that way, a separate stretching routine is optional for general flexibility. Do it if you like it or need it for a specific goal, skip it without guilt if you do not. If a particular joint is tight or painful, that is worth getting assessed rather than just stretched at.
Practical meaning
Do not avoid strength training for fear of getting stiff. Lift through full ranges with a load you can control, letting the muscle stretch under tension at the bottom of each rep, and your flexibility will hold or improve. If you lift that way, a separate stretching routine is optional for everyday flexibility; keep it if you enjoy it or need it for a sport, skip it without guilt otherwise. A tight or painful joint is worth getting assessed rather than just stretched.
Limitations and uncertainty
Both pooled analyses report high heterogeneity in study designs, protocols and populations, though the direction is consistent. The supporting trial is small (25 people) and short (5 weeks). The 'must train through a full range' condition is reasoned, since the pooled studies mostly used full range protocols and there is no clean partial versus full subgroup, while 'you need external load, bodyweight only did not work' is a direct subgroup finding. This covers everyday flexibility, not extreme sport specific mobility or rehabilitation.
