Growing up I would hear things like “lifting weights will make you muscle bound and slow” or “going to the gym makes you tight and increases the chance of injuries”, l now know comments like this are not true. If you train correctly, strength training will increase your speed and can even enhance flexibility, not hamper it.
As early as 1956, Massey and Chaudet demonstrated that heavy resistance training does not cause a decrease in flexibility (Fleck). In 1987 others showed that weight training actually increased the range of motion of the ankle, trunk, and shoulder joint after 11 weeks of resistance training without any additional flexibility training.
The type of training program used and the initial flexibility level of an athlete influence the gains you can have in flexibility from resistance training. If an athlete comes in and is extremely tight then incorporating a flexibility program in conjunction with a resistance training program is in order. In this case resistance training alone may not get the athlete flexible enough to perform certain movements safely or perform better in their chosen sport. The type of program will also dictate the amount of flexibility gained from resistance training with Olympic lifters displaying greater ranges of motion than say a powerlifter. This makes sense when you look at the two sports and see the degree of flexibility that is required to perform a snatch in comparison to a bench press. There is an optimal amount of flexibility needed for every given sport, you don’t always need or want maximal flexibility.
Muscle hypertrophy may decrease flexibility in some instances. This could be the case if someone had an extremely large chest and tried to touch their elbows in front of them. While I do believe hypertrophy may affect flexibility I think that most times it is because no attention is paid to maintaining or increasing flexibility in this group of athletes more so than hypertrophy being the only cause. Imbalance between the agonist and antagonist is also another reason for this decrease in flexibility in my opinion so it is important to look at program design to maintain or optimize flexibility.
If you are looking to maintain or increase flexibility with strength training alone, than training through the full range of motion is extremely important. This seems like common sense but what most people I see in a public gym consider full range of motion is a joke.
Resistance training and flexibility training complement one another. Charles Poliquin has said that stretching is the yin to strength trainings yang. Flexibility training may increase functional range of motion while strength and power training helps to improve the control of that functional range of motion.
So generally strength training will either improve or show no change in flexibility. If you want to enhance flexibility you have to train it like everything else. As muscle hypertrophy increases you may have to pay attention to flexibility training. Balance between the strength of agonist and antagonist has to be monitored and using a full range of motion will help improve gains in flexibility and performance. If you want the best results combine a strength training program with a flexibility program.