Flexibility and strength seem to have an interesting dichotomy. The prevailing belief way back when (I recall hearing this a lot in the 80's) was that those that are more muscular will always struggle with flexibility. So this was almost always followed up with some advice about not overtraining through weight workouts for certain athletes that need to be flexible (in my case it was as a hockey goalie, and in fact I was pulled from being a goalie because I was considered too muscular to be able to flex as much as needed)
However, current research has shown that stronger muscles actually assist flexibility. The stronger a muscle is the farther you can stretch it before the stretch reflex kicks in. When you stretch, the nervous system activates the myotatic reflex. Also called the stretch reflex, this mechanism is your body's natural defense against muscle tears. It is essentially reigning your muscles in when it feels they have gone too far. The good news is that you can train this reflex to fire much later, thus improving your pre-tensile range of motion (i.e. how far you can stretch before your muscles tense up. This equals deeper splits). Research has shown that connective tissues (tendons, ligaments and fascia) also lengthen over many applications of stretches, which can delay the onset of age-related reductions in flexibility.
But with all that said the amount of muscle specifically around the joints can hinder stretching, but with time and training even that can be overcome. Luckily for females, they also come with a greater amount of a chemical called Relaxin which decreases the resistance of the ligaments to stretching, and helps them make faster flexibility gains than guys (grrr) They also tend to have greater range of motion in most joints, in particular the ball joints like the hips, shoulders, and ankles.
So the good news is that even though she may be stockier it does not mean she cannot achieve high flexibility. And while the muscle density might slow things down, the higher strength can offset that to some degree.
I always love getting something I can actually answer. I have been studying this a lot recently as I started taking Taekwondo and am working on my own flexibility, much to Midget's amusement. She really seems to love dropping, cold, into full splits right in front of me as I am trying to stretch.
. By the way, our first 2 hour class went very well. In usual fashion, on the way home, she informed me of how many days until she could go back. I swear she would live there if they rented out rooms!
