Dear Patient: Thank you for writing to us.
Are you saying that your niece has a tight leg muscle which is causing difficulty in moving it. You have not described which muscle is affected. Is it the whole leg? Or one muscle.
Please resend the question to us with pictures and a more complete history. I believe that she has a muscle disorder like Muscular dystrophy and you should see a Doctor to get it diagnosed with a biopsy.
This is a very provisional diagnosis with no facts and insufficient history. The diagnosis could be completely wrong.
Hope this helps you. All the best.