The Laerdal Little Junior compression spring serves as a critical internal mechanical component for child-sized CPR training manikins. Housed directly beneath the chest plate, this calibrated steel spring provides the precise physical resistance required to simulate realistic child chest compliance during basic life support (BLS) training. Replacing a fatigued or broken spring restores proper mechanical recoil, ensuring students receive accurate tactile feedback and develop correct muscle memory for chest compression depth and release velocity.