Planks and Wall Sits.
Get into the top of a pushup position. Keep the arms straight, but not locked. Keep the hips, heels, and shoulders in line with one another. Breathe deeply and hold for as long as you can.
Find a wall that you can sit against. Squat down until your thighs are parallel with the floor. Breathe deeply and hold for as long as you can.
Your muscles will burn!!!