Weak back? Have trouble getting back up from a squat?

Stiffness goes along with weakness.  As the core muscles weaken from lack of use, sitting or muscular imbalance, the lower back can arch forward or slump backward creating more stiffness, discomfort and eventually pain. 

Stretching the hips and thighs while strengthening the gluts and hamstrings (the back muscles of the body) help balance these muscles. This equals more freedom of motion and more flexibility. This create more correct posture.  \ 

HIP ROLLS UP is a good exercise for  this:

Start by laying on your back with your knees bent; hip width apart, feet flat on the floor.  INHALE to prepare, and then EXHALE, scooping your stomach muscles in and up while tilting the pelvis and lower back - back towards the floor. 

Continue this motion slowing rolling your hips up in the air. First peel off the tailbone and lower back, then middle back until you are resting on your upper back.  Push your feet and heels into the floor feeling the hamstrings and glut muscles working. Also, push into your big toe joints feeling your inner thighs engaging. Slide your ribs down toward your hips so you aren't arching your back; your torso should be 'flat' from your chest to your knees. (See the photo).  

To return, INHALE to prepare, then EXHALE slowly and roll back down, melting the ribs towards the floor first, then the middle back, lower back and finally the pelvis and hips to the mat.  Repeat 5-7 times.  

See how smoothly and with control you can roll the hips up and roll down, trying to 'peel' each vertebra of your spine off the mat and replace them onto the mat one at a time.