When done correctly, yoga and weightlifting can actually strengthen your bones.
Your skeleton acts not only as a scaffolding for your muscles to push and pull against, enabling you to keep moving through space, but also as a mineral storage repository. Physical activity and sound nutrition can keep your bones as strong as possible for a lifetime, but it is possible to compromise bone density through inadequate exercise. Participating in exercises such as yoga and weightlifting will not promote notable bone loss, though it may help improve bones under certain circumstances.
The Bone Bank: Deposits and Withdrawal
The human skeleton is composed predominantly of hydroxyapatite, a sturdy mix of calcium, magnesium, phosphorus and other minerals bound together by collagen. These minerals are also necessary for other body functions such as muscular contraction and nerve conduction, so if levels of calcium in your blood are low, for example, your body will withdraw calcium from your bones. Weight-bearing physical activity such as walking, dancing, or lifting weights stimulates bone growth and repair, causing minerals to be added to your skeletal "bank account." Sedentary lifestyles rob bone from your reserves; the bottom line is simply use it or lose it.
Yoga: The (Mostly) Good News
Unlike activities such as running or walking, most forms of yoga do not involve repeated impact stresses on your bones, which may minimize its effectiveness for building and maintaining bone density. Many yoga postures are also done seated or reclined on the floor and can be considered essentially non-weight bearing stretching exercises. But some yoga poses are quite challenging, requiring you to push and pull on your skeleton using your muscles which stimulates local bone growth. A "Yoga for Osteoporosis" pilot study in progress by the Columbia College of Physicians has actually shown improvements in bone density among menopausal participants.
Weightlifting: Also (Mostly) Good News
Weightlifting improves your bone health throughout life, according to BMC Medicine. But it is important to bear in mind that the best results have come from lifting fairly heavy amounts of weight; cranking out 30 repetitions of biceps curls with a one pound dumbbell will not yield the same benefits as lifting 15 pounds twice. The American College of Sports Medicine recommends that you strength train two times per week on non-consecutive days. Choose at least eight different exercises, targeting all of the major muscle groups in your body, and use a weight that causes you to fatigue after one set of eight to 12 repetitions.
Practicing yoga in isolation from any other form of physical activity could lead to small losses in bone mass over many years. Lifting insufficient amounts of weight during strength training activity leads to minimal or no increase in bone density. To keep a healthy skeleton, the best approach is to engage in a wide variety of weight-bearing physical activity, including all components of fitness; regular cardiorespiratory endurance exercise combined with flexibility, strength, and balance training.