Back Exercise #3: The Sideplank
The sideplank exercise is a great exercise to build the strength of your obliques and other trunk muscles.  It also helps to strengthen your hips which is also useful for preventing excessive back motion.  In addition to the benefits of strengthening your core, it’s also great in that it does not compress your spine.
Compression is bad!
The main reasons that situps can be bad for you is that when you flex your spine, it puts a lot of compression which make your more susceptible to bulging discs and increased wear and tear of your spinal column.  Be wary of these strengthening exercises which may seem great, but are not only hard but put unnecessary stress on your spine:
Russian twists
Double leg lifts (as mentioned in a previous post)

The Modified Sideplank
Begin by trying the modified sideplank. This is done with your knees bent, which decreased the lever arm (makes it easier).  Try to place your shoulders, hips, and knees inline.  Prop yourself up onto your elbow and retract your shoulders.  This is important so that your shoulder is in a more stable position. You should not have pain in the shoulders, but they may get tired (and that’s ok)
Try by holding the upright position for 10 seconds with your body straight.
Rest for 3 seconds
And then immediately pop back up into the correct position again and hold for another 10 seconds
Repeat for a total of 10 sets, or 100 seconds.
Advanced: after mastering this, you can perform these extra movements to increase the difficulty.  These variations are great for biasing your hips and making your spine stabilize.

a) perform clamshells while holding the sideplank

Modified sideplank with clamshell
Modified sideplank with clamshell

b) perform the straight leg raise

Modified sideplank with leg raise
Modified sideplank with leg raise
c) do a simulated running motion with your top leg. This will make you more unstable; your shoulder and your core will have to focus more on keeping your body still. (not shown)

The Full Sideplank
Do the same as above, but with your legs straight.  Again, there should be no pain with this exercise.
You can also perform the advanced variations as above, but with the full legs extended sideplank.
Basic Variations: same as above a, b, and c.
Other variations: if you are feeling unstable, you can place your feet one in front of the other to help.

You can also try to “thread the needle” by having your top hand rotate down and underneath your body, and then rotate back up and point to the sky. This will work your shoulders and your trunk even more!

Sideplank with thread-the-needle
Sideplank with thread-the-needle