Q:Is having an imbalance or greater strength in the posterior chain really a big problem? I ask because I have a client who can lift much heavier weight in the stiff legged RDL, then they can squat. Due to lumbar issues, he generally does unilateral work for the quads, because heavy spine loading has caused some injury in the past due to insufficient hip mobility.
A: No, it actually isn't a bad thing but it also is not a good thing. Most often we see greater imbalances in the anterior to posterior relationship.Too much hamstring strength can actually increase the chances of PCL strain or injury since there needs to be a favorable quad to hamstring ratio.
I'm not too concerned with him not doing back squats. There not essential especially with a lumbar injury. You can load someone pretty heavy with dumbbell squats. What are his loading parameters like? Remember even with rehab work we still need to achieve overload.
I remember when I was working with a track athlete with lumbar stenosis( a narrowing of the spinal column). She couldn't back squat and her trainer had her doing a ton of posterior chain work(which he should of). In order to optimize her performance I actually had her do more single leg work and ,get this, more tib anterior work.
Now I'd pick a split squat over some ankle dorsiflexion any day but due to her hamstring, glute and calf strength she actually was limited in any anterior strength aspect. Just another area to look into.