I have recently started squatting ass to the grass and feel like I keep pulling a muscle in my lower back. The next day I experience minor back spasms. As I start the movement I focus on keeping chest up, shoulders back, and lower back arched but when I am at rock bottom my lower back seems to round. Is this a weak abs or weak glutes kind of thing or could it be related to flexibility?
I suspect adhesions/spasm in the piriformis or QL muscles.
You're going down too low. Go down until you cannot keep proper form in your back anymore, then reverse the movement.
Arse to the grass is good if you have the lower back and hammy flexibility for it, but stopping a few inches higher isn't going to stop the exercise from being effective.
Just make sure that your leg muscles are all turned on at the bottom (flex everything as hard as you can) and you'll be stimulating the max amount of fibres. That will be enough to spark mega growth.
And keep that arch in your back!
Heck of a jump.
More likely is that your back is rounding too much, causing some compression where it shouldn't be in the spine and making the fibers fire all wacky in your back.
Major points of flexibility in the full squat:
Calves - both gastroc and soleus
Hamstrings - you should be able to touch the finger tips of both hands together around your foot doing a modified hurdler stretch with little to no back rounding. (not exactly the same for everyone, that's just a point of reference to start with)
Hip area - all the glutes, piriformis, adductors, abductors
Hip flexors are a big one
Chest/shoulders - tightness here can cause you to have to round a little in the upper part of your erectors and lose balance a little
I personally have to stretch for about an hour to get loose enough for a full squat workout, but I spend all day sitting in class and at a computer. You may need less.
Thanks for the advice. I'll spend a little more time on stretching and keep working on maintaining a proper arch at the bottom.
Generally this is caused by tight glutes, not tight hamstrings. Due to the concurrent flexion at both the hip and knee joints the hamstrings maintain a fairly constant length during a squat.
The glutes however lengthen quite substantially and if they're too tight you won't be able to maintain that arch at the bottom.
Could also be you're too fat, a bit of a belly can do the same thing.