This exercise is excellent for working several muscle groups. It targets the shoulders, glutes (butt muscles), core (trunk musculature- abdominals, low back) and hips.  It can be performed at all fitness levels, however if you are a beginner see tip below to slowly work up to the advanced version.

• Start in a full push-up position, abdominals pulled in tight.
• Pull left knee in toward chest; hold for 1 count.
• Keeping left knee bent, rotate left leg out to side; hold for 1 count.
• Extend leg back to starting position without lowering, and repeat; 5 to 10 repetitions with left leg.
• Rest for 30 seconds and repeat on right side.
• Do 1 to 3 sets of 5-10 repetitions per side.
• Work up to 15-20 repetitions per side.

 
Beginner Tip:  Remain in plank position without pulling knee in; hold for 30 to 60 seconds. Gradually work up to performing exercise above.