Why do some runners have poor knee lift?

Need to focus more on hill sprints and building leg strength. Things that also help with this is doing tempo/threshold work on the treadmill as cadence is often slightly lower on the treadmill, after a few months it will translate outside to a more efficient stride. I'm assuming she has a pretty high cadence relative to other girls as well.

Basically the more force you generate, the longer your stride will be (within reason) and the greater the knee lift. All things which will help biomechanics and being more efficient.

Her cadence was ridiculous a year ago. She was easily 210+ steps/minute. It's probably about 190 now.

We do drills daily and 20 second hill sprints a couple days a week. But I just don't see much of a change in her running form.

Going down from 210 to 190 should have been a pretty large change in running form in itself. It's not something that will fix overnight but month by month should get better.

Garrett Heath ran pretty fast with that style, if it ain't broke don't fix it. Interesting that you mentioned skiing cause Garrett Heath had a background in cross country skiing.

You don’t need to over emphasize form, you need focus on fitness.

Your form will improve with fitness.
You may have gotten in her head and now she’s over striding and braking a bit with each step.

Michael Johnson and Centro have “poor” knee lift.

Force is part of it but the biomechanics of running are complicated and specific to the individual. I would discourage excessive cuing for higher knee lift during training and instead work on improving strength and power in her hip flexors and extensors and ensuring her abdominal muscles are also strong.

Hill sprints, Lydiard hill drills, and resistance training in the gym are ways to go. Try to not obsess over one aspect of biomechanics and instead work on existing training methods that are known to improve overall biomechanics when running.

I've always thought that "high knees" was a poor verbal cue for what runners want to be doing. But to answer your question about the biomechanical cause of poor knee lift.

Weak quads, weak hip flexors, running with the pelvis tilted back (butt back) instead of tilted slightly forward.

What are the fixes? Well, does it really need to be fixed? I can't see her run so I don't know, but if it does need to be fixed here are some thoughts.

Reebok bench (8") heel drops progressing to heel drops while holding a dumbbell

Air squats, skater squats, jump squats (all done without weights)

fast 1 leg hops done in the Bulgarian squat position

Prime times

High knee drills

A skips

Lunges

Carioke

Low (8-16") box jumps

1 leg hops 10x each leg

Pogos (like jumping rope, but getting a little higher and/or hopping forward a little)

Very steep hill "sprints" of 15 sec. She should look ahead (not down) and focus on powering up with each step, NOT on getting up the hill as fast as possible. There is a tendency to hunch over and shuffle with tiny steps up very steep hills. Don't do that. This is a drill... the objective is not to get up the hill as fast as possible. The objective is to build leg power.

Have you ever seen someone who can't do a squat? She literally can't get into position like what a baseball catcher does. Impossible for her to get her butt down.

She's not some out of shape hobby jogger. She's almost 15 years old with PR's of 5:10 1500m/11:01 3000m/19:08 5K.

She's got a monster breakthrough coming if I can somehow figure out where the weakness is and how to address it.

I think you just found the problem. This is either very weak squat muscles or a secondary cause might be muscle tightness, but it's probably weak squat muscles. The drills to strengthen the squat muscle groups will also help alleviate any tightness.

The squat muscles are mostly quads and glutes, but the entire lower leg and the core muscles are also used so the air squat is a pretty good starter exercise for all of them.

One place to start would be with assisted squats using a chair and an assistant to provide support/lift throughout the movement. The first video shows how. Avoid going low at first.

NOTE: She can add books to the chair at first so she won't go so low and getting up will be easier. She can use a resistance band/TRX strap mounted to a door frame to assist her in getting up. (I do one leg squats use these same techniques.) As she gets stronger, she can use the back of a high back chair for security and balance.

EDITED TO ADD: Once she can do assisted air squats, she should do 3 sets of 5 reps. Then, build to 3 sets of 10 reps. Then, gradually remove the assistance. There is no need to go lower than thigh parallel to the ground for her. Going way down puts more stress on the knees and is a risk for runners who aren't sprinters.

If she can sit down and get up from a chair then she can squat. If she can walk up stairs, 2 stairs at a time, then she can lift her knees. Maybe start with these or variations of these to help with her mobility and strength.

190 is much more consistent with what I see written. Daniels seems to feel that would be a good cadence. Assuming anatomic blocks are not an issue, this is why do drills high knees, lift weights, and run fast (strides, speed, intervals, hills) I do think slow mo on your smart phone can be instructive to athlete.

This is why I'm asking for help - she literally can not get up from a full squatting position. There's some major muscle weakness or flexibility issue.

I know it sounds crazy because any of us would just stand up from a squat but she can't do it.

So.... unless she intends on going for distances <800m, don't worry about it

Yea I agree. Look at this way OP, I think you identified the issue, barring any anatomic bony blocks. Im weak and old and can do any number of squats. She has great turnover, 15 is definitely OK for girls to start weights. Sounds like weakness. Unless you meant shecan't actually squat down, which may be flexibility.

If you really serious and want to take subjectivity and online posters out of the equation, get a good sports med doc, will who will likely send you to PT for eval. Its easy if you are near a big hosp or academic med center.

Wow. Have you gotten an evaluation, pt, physio. I'd find the best sports person in your area and pay for a few sessions, not just for sports performance but for long term health.