Loss of flexibility as a result of strength training is a very common myth!
Once upon a time athletes were advised not to do strength training because it would make them stiff and reduce their flexibility. We now know this to not be true. Strength training does NOT lead to shortening of tendons and ligaments, nor does it lead to loss of flexibility. Research has shown that weight lifting when using full range of motion, actually increases flexibility just as well as, or even better than, static stretches.