Using stairs is an opportunity for a posture exercise break. The key to maximum core muscle benefit is keeping upright, with head over hips.
Don’t lean forward. Keep your chest up and over your hips to avoid training your head to poke forward over your chest.
When climbing stairs, hold your pelvis in a very slight tuck as you push your chest up This forces you to hold your shoulders back and over your hips.
Hold an imaginary plumb line (or a tie or necklace) from your neck, and keep it against your body as you go up the stairs. This forces you to keep your body erect, and to train your muscles to work your body in a more bio-mechanically effective pattern.
You’ll be amazed how being posture aware makes your core work stronger.
Remember. When climbing stairs, don’t bend at the waist or collapse forward. Keep your body erect and stand tall as you climb.