Why they matter more than biceps or quads — and why your 30s and 40s are the critical years to build them We tend to focus our training on the muscles we can see — the ones that look good in the mirror. But research shows the muscles that truly define your quality of life and longevity in later years are often the ones we rarely think about. These four key areas act as powerful biological markers, predicting everything from disease risk to mobility and independence. And cruci