How to do Plank arm lifts
Begin in a high plank position on your hands and feet with the hands directly under the shoulders. Be on your toes with the feet hip distance apart. Have your spine in a straight line from head-to-toe.
Lift one arm straight forward with the thumb up until it is in line with the rest of your body. Be sure to maintain a neutral spine and avoid any twisting in the rest of your body.
Then, lower that hand back down to the ground.
Repeat for the desired repetitions, alternating arms.