The Lower Back
Your lower back contains some of the most important muscles of your core. Below are some great beginner exercises for your lower back.
In order to do back extensions correctly, keep legs on the floor and lift your chest off the floor. You can either leave your hands on the floor for support, or in front or behind the head. It's best to engage your abs before you lift your chest off the floor. This will allow you to focus on the lower back muscles.
Back Extensions on the Ball
This exercise can give you greater range of motion.
Reverse Hyperextension on the Ball
Roll forward on the ball until you're on your forearms. Keep the legs straight and lift them straight up, just until your body is in a straight line. In other words, don't take the legs way past the body, but keep them to hip level.
For this exercise, it's very important to brace your abs, which helps to protect your lower back as you strengthen it. Keep your hands behind your head as you tip from the hips, keeping the back flat and knees slightly bent, until your torso is parallel to the floor.
Hold a broomstick or other straight stick behind your back, one hand behind yoru head and the other behind your lower back. Keep the stick in contact with your head and between the shoulder blades and your tailbone. Carefully hinge forward and try to keep the stick in contact with all three points the entire time.
The key to this exercise is to keep the back flat and the shoulders back as you tip from the hips, knees slightly bent. Second, keep the weight really close to your legs. That will allow you to work your lower back without too much strain.
Lift the oppossite arm and leg until your entire body is in a straight line. It looks easier than it is. This is a great exercise for overall core stability as well as strengthening the lower back.
This is a gentle, simple exercise that is great for strengthening the lower back, glutes, and hamstrings. It's also a great way to work core stabalization. Keep your feet under your knees, and lift your hips until your body is in the straight line. Repeat that or hold the position for 30 or more seconds.