Complex sentence: In children, a tethered cord can force the spinal cord to stretch as they grow, in adults the spinal cord stretches in the course of normal activity, usually leading to progressive spinal cord damage if untreated.
Split into simpler sentences: In children, a tethered cord can force the spinal cord to stretch as they grow. In adults the spinal cord stretches in the course of normal activity, usually leading to progressive spinal cord damage if untreated.