The towel plank and knee-in increases strength throughout the core, specifically the lower abdominals and obliques. This exercise also improves strength in the quads, hamstrings, and glutes.
Start in pushup position with your hands shoulder-width apart and each of your feet placed on a small towel. Brace your abs and squeeze your glutes. Your body should form a straight line from your shoulders to your ankles.
Bring one knee in toward your chest by sliding your foot along the floor, making sure your foot stays on the towel.
Return your leg to the starting position. Next, bring your other knee in toward your chest, making sure your foot stays on the towel. Return your leg back to the starting position, and continue alternating legs.
Avoid dropping your hips as you bring your knees to your chest.
Don't raise your hips too high while holding the pushup position.
Try to prevent your upper back from rounding at any point.