This muscle or tendon feels maximum stress when supporting body weight with the knees flexed 90 degrees or more, but flexing the quadriceps with the leg straight doesn't involve it at all. It can become weak when pedaling a bicycle on hills. It seems like all the tendons from the quadriceps are supposed to cross the knee joint, but this is an inch or two above the patella toward the medial side so I can't identify it. It feels like it attaches to the top of the large knob at the end of the femur.
The vastus medialis is made up of two parts.