What Causes Weakening Of Hamstrings In A Polio Patient?

a patient with polio in his left leg-calf muscle affected severely hamstrings and quadriceps weak but less affected than calf has broken his right foot due to foot fall and buckling of his left leg.
right leg hamstrings are weakened over the years. patient's age is 53. his bone density has dropped in past few years. not tested for bone density but doctor-orthopaedist believes so. right foot phalanges and fibula have broken. same phalanges had broken last year due to a fall and now the opposite end have broken in this fall. he uses walker to commute even in the house since the fall and even a chair. today he started complaining left(polio affected) knee ache and is unable to stand due to pain in muscle behind the knee. patient has a history of fischer's due to heavy pain killers. please suggest how this could be happening