Blame it on a job change, a chronic health issue, or simply a loss of motivation: whatever took you away from your regular exercise routine has led to a sedentary lifestyle. But don't assume you can jump back into the same exercise regimen you followed when you were younger. "Your body has aged, and things have changed," says Dr. Clare Safran-Norton, clinical supervisor of rehabilitation services at Harvard-affiliated Brigham and Women's Hospital.

What's different

Age-related physical changes aren't always obvious. "We lose muscle mass and strength as we get older, and the muscles become less flexible and less hydrated," says Dr. Safran-Norton. Arthritis weakens joints. And vision changes, neurological disease, joint pain, or problems inside the ear can throw off your balance.

Meanwhile, underlying conditions that become more common with age, such as high blood pressure or cardiovascular disease, may put you at risk for severe health consequences if you suddenly start exercising like you did when you were 20.

Increased risks

Jumping back into a workout without accounting for physical changes sets you up for injury. "Lifting weights that are too heavy or taking an exercise class that's too strenuous often causes trouble, and it's usually a muscle tear or a strain," says Dr. Safran-Norton.

Other risks include

• rotator cuff tears, from arthritic shoulders with bone spurs that can rub on a tendon

• falls that result from poor balance, and broken bones from the falls

• heart attack or stroke from an intense workout, particularly if you have undiagnosed high blood pressure or heart problems.

You might also fail to recognize symptoms of heart disease if you are taking medications that mask them, such as beta blockers. If you are taking a beta blocker and start exercising again, you can't rely on heart rate to determine overexertion. You should stop your workout immediately if you experience any symptoms that could indicate heart disease.

Don't feel self-conscious if you're not the fittest person in the room, if you need to modify a particular exercise, or if you need to take breaks. Since you're older now, also be wiser — about your fitness.