Planks are considered more of a muscular endurance exercise rather than cardio. Muscular endurance refers to the ability of a muscle or group of muscles to sustain repeated contractions over an extended period without fatigue. During planks, you hold a static position for an extended period, which increases muscular endurance in not only your core but also your shoulders, arms, and legs depending on variations. While planks can slightly elevate your heart rate due to engaging multiple muscles at once, it does not provide cardiovascular benefits comparable to activities like running or cycling.