We often picture aging as wrinkled skin, gray hair, or slower steps. But the real decline that steals our vitality starts deeper — in our muscles. Specifically, in the fast-twitch muscle fibers that give us power, speed, and explosive strength. Youth is speed. The spring in your step when you were younger, the quick reaction to catch yourself from falling, the ability to sprint for the bus or jump out of the way — these weren't just "being young." They were the signature of healthy, abundant...

By your 40s and 50s, something strange starts happening. You notice it in the mirror first. Your arms look a little softer than they used to. The waistline becomes more stubborn. Your energy isn't quite what it once was, and recovery from workouts seems to take longer. You may find yourself sleeping a little worse, feeling a little slower, or wondering why activities that once felt effortless now seem to require more effort. Many people simply assume this is what aging looks like, and to some...