The injection should be intra-muscular, the needle being boldly plunged into a muscular mass, such as that of the deltoid or the gluteal region.
If you are sitting in a chair and you squeeze your gluteal muscles together and hold them, you have also performed an isometric contraction.
Gluteal muscles are also strengthened by half squats and hamstring curls.
This large nerve exits the pelvis through the greater sciatic foramen to enter the gluteal region - more info.
It works the quadriceps in conjunction with the gluteal muscles, and produces compression force, which are safer for the knee than shearing forces.