Back in the late 1990s, “Core Stability” started getting popular. Studies found that people with back injuries and chronic lower back pain had wonky timing in their trunk muscles, basically, their muscle coordination was off. This got researchers thinking -maybe strengthening certain muscles, like the transversus abdominis (TrA), could help fix this and prevent pain and injury. They came up with all these exercises: the "tummy tuck", “trunk bracing” and the “core stability” craze grew.