I posted the plank a few weeks ago and this exercise is a progression on the plank which is great for core strength, and upper body strength. These are also especially good for those struggling with pushups as it helps build the stability and control needed to perform pushups correctly.
Primary: Pectorals, Rectus abdominus, Obliques
Secondary: Triceps, Deltoids, TVA, Serratus Anterior
To start, begin in a pushup position. You should be prone and holding yourself up with both hands on the floor and on your toes. The feet should be about hip width apart and the hands should be about shoulder width apart. Like the plank, make sure that the shoulder blades are pinched together and the back is slightly arched.
Make sure to brace the abdominal area and slowly raise one arm off of the ground. Keep the arm straight and raise up until the arm is parallel to the ground. Make sure not to sway or shift the weight too much, ideally, the rest of the body should be stable while lifting the arm. Hold the arm in the air for 2-10 seconds, lower and lift the opposite arm. Alternate raising the arms for about 12 repetitions on each side.