The wide-stance plank with diagonal arm lift improves muscular endurance and core strength. The movement also improves balance and stability of the core.
Position yourself on your elbows and toes, with your body forming a straight line from your shoulders to your ankles. Brace your abs, squeeze your butt, and spread your feet just wider than shoulder-width apart.
Without moving your body, lift one arm off the floor and reach diagonally to that side. Hold for a second, then return to your starting position. Repeat on the other side.
Do not allow your torso or hips to rotate. Stay rigid.
Do not hike your hips high or sag the hips your low during the plank.
Squeeze your core and your glutes during the entire movement.