12 month old suffers from leg spasms

My friends' 12 month old has had leg spasms for several months.  She sits down, crosses one leg over the other, always the same leg, and usually cries/moans in what appears to be pain.  She has had an MRI, and it came out clear.  They thought it could be seizure related, but it wasn't.  When she is having the spasm her leg is very stiff and you cannot move it easily.  Some days she does it all the time, and then other days it's not as often.  Any suggestions as to what it could be?

Thanks so much!