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.
- It has a long part called the VML - vastus medialis longus
And it has a more distal part closer to the knee which you appear to be describing called the VMO - vastus medialis obliquus.
(Whether or not they are actually one muscle or separate is not fully known.)
This VMO video explains why the muscle may not work at full extension.
This video shows two exercises to help strengthen the VMO.