The Video shows me doing a muscle test on the Psoas (hip flexor) on Donna to see how it is firing, if the leg can not hold it's self in that position then the Psoas is not doing it's own function.
When you have pain else where in the body and previous injuries, say to the lower leg/knees/back/shoulder/arm, I take it that your hip flexor (psoas) is not doing it's correct function of flexing the hip and another muscle has taken over it's job, which puts your body into a stress state.
When I have activated the Psoas it then holds strong in the muscle test, which takes far more effort from me than before, the hip flexor now is being taught to remember it can do the hip flexion job and keep doing so. You can see the big difference in the test and how simple it is to change the dynamics of the body.
The Psoas muscle is quite an important muscle to have functioning correctly in our bodies, we can not do anything without hip flexion happening in any form of movement. So the better it is performing the easier our movement can be from exploding outwards with power instead of the common pattern of implosion due to stressed out lives/jobs/injury trauma.
The hip flexors partner is the butt (glutes) both these muscles should be driving our movement and power outwards, when they are not functioning other muscles take on their jobs which other muscles are not designed to do and means the body starts to implode in on it's self, which means pain and stress in the body, survival mode instead of live and thrive mode.
Just my musings.....