Aching muscles are a symptom of arthritis, it is caused by the fact that you are trying to compensate for the condition by unconsiously adjusting posture whilst moving ariubd, ie if you have pain in the hip you would walk, trying to shift your weight around so as not to use the painful bit. If you whatch peaople walking, you can notice some waggling or rocking from side to side with each step, this is charicteristic of someone suffering with arthritis in the hips and would likely put more strain in the leg muscles.
Hth
Pete