Strengthening is extremely important for improving bone density.

Bone mass peaks in our 20s.  In the mid 30s, bone breakdown starts happening. This is when bones lose more calcium than they can replace. Loss of bone density peaks after menopause. Women who are thin and small framed are at a higher risk. So are women who smoke or drink excessively and women who lead sedentary lives

Bone density

Women Exercising — Image by © Brooke Fasani Auchincloss/Corbis

Bones are living tissue. Weight-bearing exercises cause new bone tissue to form, and this makes bones stronger. Muscles and bones are connected .by tendons.  Bones and muscles both become stronger when muscles push and tug against bones during physical activity.

Weight bearing exercises can be done with own body weight, external free weights (like dumbbells or resistance tubes) or machines.

Examples of own body weight workouts are
Wide squats
Zumba / high impact dancing
High impact aerobics
Tennis/badminton/ football

Examples of low impact strengthening
Cardio machines- treadmill/ stationary bikes/ elliptical
Low impact aerobics

Examples of Machine workouts
Strength training machines in gyms

Chandra Gopalan
Director, Contours India