Lower back exercises should be done at the end of your training routine due to the fact that they need to assist the prime movers during other exercises. For example, in a traditional squat movement the lower back muscles are firing to help balance the position of the body relative to the Olympic bar which is resting across the upper traps along your shoulders. The prime movers of the squat are the leg muscles while the lower back muscles are the assisters. The squat can be a very fatiguing movement for the legs but also for the lower back. This is why you would not want to fatigue the lower back muscles before you perform the squat.
The appropriate time to perform lower back exercise would be at the very end of your workout. Because the lower back does assist in so many movements, it is not necessary to train it in the same manner you train other body parts. A few safe and effective movements are enough lower back exercise to keep the muscles up to task as an essential part of the core.
Here are a few examples that can be done on a daily basis. Included are a few stretches that are also a good idea to make a part of your daily routine.
Prone position, arms overhead and legs extended.
Lifting opposite arm and leg, then switch.
Holding each lift for a one count pause.
Face hovering off the surface of the floor, shoulders drawn down, and abdominals engaged.
Do an alternating count of 5 to 10 reps per side.
LOW BACK EXTENTION
Can be done on a hyperextension bench.
Arm position-down at the sides, crossed at chest, or hands behind head
Extend body up to a straight line from hips to head and back down.
Repeat 5-10 reps, moving slowly, keeping abdominals engaged
On all fours start with back flat
Flex spine up into a cat stretch, tucking chin and tail
Hold for one breath and then articulate back to a flat back
Repeat 5 full reps