Mr. V wrote:
https://www.youtube.com/watch?v=GMDaY5sQ0Qk
I watched the first part of this about quad strength. His demo was pointless.
The "quad" is four muscles, three of which cross only the knee joint. The way he was pushing that guy's leg, the three Vasti would not have the leverage to resist it without very strong help from the hip flexors.
The only part of the quad that flexes the hip is the R. femoris. That's a bi-articulate muscle that crosses both the knee and the hip. Like the hamstrings, it is alternately short on one end and long on the other, for example when the leg is fully extended behind in running, it is contracted on the bottom and extended on the top. Then when the top contracts, flexing the leg at the hip, the hamstring flexes the knee, lengthening the R. femoris at that joint and allowing the top part to continue to contract with force. A long muscle with plenty of room to contract is strong, a short muscle that's already contracted is weak.
In the video, the subject's R. femoris is shortened both at the top and the bottom, so of course it will not have much strength. But that's not its job. It's not built to flex strongly at the hip while the knee remains mostly extended. If the guy's leg had been bent, it would have been different.