We're not designed to move in isolation (single joint) but rather in multi-joint and in multi-plains. If you're pulling a 500 deadlift chances are you're pulling with your hips and back which usually means you're not recruiting your hamstrings making it more hip dominant than knee dominant. Make sense?
Not to me. Do you mean he's pulling the deadlift with "hips and back" or the GH raise? If the former, how else are you supposed to pull it? How can a deadlift be "knee dominant"? Your knee joint barely articulates... and what do you mean "pulling with your back"? Aren't back muscles mostly locked in isometric contraction during a deadlift? It's mainly a hip exercise, and your hamstrings extend the hips as well as flexing the knees.
Maybe you aren't implying he's somehow deadlifting wrong, or that there's some other way to deadlift, but this is how this reads to me...
When you have a weak link in your posterior chain (hamstrings, glutes, low back) it can create all kinds of problems, injury speaking not to mention doing a relatively simple bodyweight exercise such as GH raise. Your weak link is your hamstrings and should be worked simultaneously with glutes and low back.
I can agree this is probably true... however the reason he can't do a GH raise seems obvious to me, yet this quote isn't very clear. Hamstrings have two functions, extending the hips and flexing the knees. In a deadlift, they extend the hips. In a GH raise, they flex the knees.
So yes he needs to strengthen hamstrings, but not indiscriminately. Romanian deadlifts, squat, reverse hyper, and regular deadlifts are all hamstrings exercises, and all "work simultaneously with glutes and low back," but they emphasize the hip extension aspect, not the knee flexion aspect, so I don't think they will help.
If anything leg curls SHOULD help, but if he wants to get better at the glute ham raise, he should just do the glute ham raise... this will really work the knee-flexion capacity of his hamstrings.
