Even when you are in the army, a good army crawl is a hard thing to do. You're asking your body to make use of a whole lot of muscle groups that have to carry a good portion of the body's weight in slow motion.
What's involved: Primarily core, abs, lower back, quads then shoulders, front hip flexors, triceps, chest, intercostal muscles.
Powered by: Front hip flexors, primarily, shoulders and triceps.
Tip: Perform this in a back & forth motion, as shown on the video, balancing on the tips of your feet so that you challenge your core and further improve balance and posture.