This is one of the best core strength exercises. Check out the video.
Core strength is important. No matter if you want to crawl through the mud or just make it through a day at the office. Core strength is critical to so much we do. Here is one of my favourites.
Check out this Youtube Video:
You start with a side plank and then add a leg raise to place more load on the glutes. A great exercise for hips and core that I use all the time.
If this causes pain in your shoulder, check out this modification.