A physical therapist says weakness in this one muscle could be contributing to knee and back pain and there’s a simple way to check

Woman performing a side plank with a leg lift on an exercise mat in a home setting, looking down at a laptop on the floor.

Many people assume that knee pain is caused by weakness in the knee joint, but that’s not always the case. Often, the real culprit is a weak supporting muscle elsewhere in the body.

According to physical therapist Tom Williams at Activ8rehab, the culprit could be your gluteus medius—one of the three muscles in your buttocks—located on the outside of your pelvis.