I was lying on my side at physical therapy, halfway through a set of leg raises to strengthen a butt muscle gone soft, when a therapist attending to a 41-year-old woman with knee pain said something that made my ears perk up.

“After they turn 40, they all come in here with lower-body problems,” she said, referring to us, the patients.

Until that moment, I hadn’t considered the possibility that a perplexing parade of injuries I’d experienced since my 44th birthday — to my foot, my lower back, my hamstring, hip and elbow — was connected to my age. I do the kind of physical activity — high-intensity yoga, running and rowing — that should theoretically keep my whole body strong. Yet doctors and physical therapists kept telling me that certain muscles were very weak, causing my other joints and muscles to overwork, leading to injury.

I stumbled into my 40s largely ignorant of the changes to come, not just to my muscles but also to my hormone levels. We may be prepared for wrinkles and gray hair, but the decline in strength and increased risk of injury, even among very active people at this stage in life, is little recognized and far too infrequently discussed.

New research shows that the bodies of men and women may age in waves, with one significant acceleration in our mid-40s (and another in our early 60s). Acknowledging our 40s as a turning point can help demystify this era, allowing us to see it for what it is: a crucial time to counter some aspects of aging and the more punishing health problems that could lie ahead.

Fortunately, there’s good science on how to do that, specifically through strength training and hormone therapy, the latter more often recommended for women than men. Rarely are people in their 40s getting these messages.