To begin building muscle, you don't need a gym membership—or even a set of hand weights. Here are a few classic exercises you can do almost anywhere. With each, strive for three sets of eight to 12 repetitions, resting 30 to 60 seconds between sets; as the exercises become easier, mix up your routine to challenge your muscles in different ways. (For more moves, check out acefitness.org/exerciselibrary).
Squats activate all four muscles of the quadriceps, which are important for stability as you age. Make sure your knees don't go past your toes as you squat.