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This is an exercise suitable to strengthen the abs; the advantage of the soft-ball is that it spares your waist.

Lie down to the ground, place the soft-ball under your waist, bend your legs and lift them up. 

Place your folded hands on the back of your head. Bring your knees to your chest then let them go back slowly, hold them, don’t put them down onto the ground, then repeat the exercise. Inhale when you bring your knees to your chest and exhale when you let them go back.