Yes! This. A lot of runners, including myself, have/had weak muscles like the poster described. I myself would emphasize the how widespread weak hip flexor muscles are. Unlike, say, hamstrings or biceps, which have a lot of common gym machines to work, hip flexor isolation exercises are hard to come by. There are many suggestions out there, but I think the best one--though I don't have empirical Scientific data for this, it's just my caring opinion--is something I devised. A few years ago, me and my running coach were thinking about this and he suggested a bunch of free-standing motions, one which utilized a bench. Somewhere along the line, I came up with the idea to add significant weight resistance to this motion.
Basically, if you can get an ankle strap, you can bring it to a gym that has a weight stack and a pulley attachment that slides down to floor level (usually, those structures with pullup bars in the middle and weight stacks--usually used by people for arm exercises--will do. This is what I use specifically).
Then, you essentially do like a sprint driving motion, raising the foot and getting lift with the leg that has the ankle strap attached to the pulley.
I like to pretend I'm Usain Bolt or something when I'm doing this (hehe). Your typical weight stack will likely provide for your entire lifetime of hip flexor strength growth. I've always wondered what Usain Bolt could do for this exercise (how much weight?).
But anyway, that poster has it spot on. If you have knee problems, the most essential and simplest explanation is that the knee is bearing undue weight and force/compensation owing to some imbalance or inadequacy from a root function--that is, the upper leg/hips--"higher up" or more "basic." That's a pretty good heuristic to use for treating any training injuries in which the suspected culprit is muscular imbalance/weaknesses. A good place to start. After that, then, there are muscles of the lower leg and foot which are often neglected--and of course, always try to see a Physical Therapist through your Health Insurance provider, and don't substitute Letsrun forum advice for seeing a Doctor!