Complex sentence: Core muscles are also involved in the Valsalva maneuver, where the thorax tightens while the breath is held to assist, often involuntarily, in activities such as lifting, pushing, excretion and birthing.
Split into simpler sentences: Core muscles are also involved in the Valsalva maneuver, where the thorax tightens while the breath is held. This action can assist, often involuntarily, in lifting, pushing, excretion and birthing.