The Healthy People 2010 public health initiative (along with similar efforts by the U.S. Surgeon General, the Centers for Disease Control, and the American College of Sports Medicine) recommends the following amounts of physical activity for adults (Pate et al. 1995):
- Adults should engage in moderate-intensity physical activities for at least 30 minutes on five or more days of the week, or
- Adults should engage in vigorous-intensity physical activity three or more days per week for 20 or more minutes per occasion.
These guidelines fit within the scope of recommendations for improving your bone health and lowering your fracture risk. However, since they are rather general, I will add some detail to these recommendations based on research specific to bone health and summarized in the following paragraphs. Note that selection of exercise type should focus on weight-bearing activities.
Aerobic exercise studies suggest that exercise must be of moderate to high intensity to be effective. As discussed earlier, lower intensity exercise such as leisurely walking or easy cycling is unlikely to benefit bone. More vigorous aerobic exercise, such as very fast walking, jogging, running, or dancing (particularly when some form of stepping routine is included), that is weight bearing and is performed at a moderate to vigorous pace for 30 to 60 minutes three to five days per week should improve bone health.
Resistance exercise, too, must be slightly rigorous to affect bone. Low-intensity resistance training performed with light weight and for many repetitions generally is of no benefit. This type of exercise is often touted as “sculpting” or “toning” exercise and simply doesn’t place enough force on the bones. Low-weight, high-repetition resistance exercise may be a good starting point for people who have never performed resistance exercise. (Note: Low weight is weight you could comfortably lift 15 to 20 times (15- to 20RM, or less than 60 percent of 1RM), and high repetition refers to three to five sets of 15 to 20 repetitions per set.) Beginning at this level, novices can become familiar with resistance training and start to build a base of strength from which they can progress to heavier weights (weight that cannot be lifted more than 8 to 12 times (8- to 12RM, or 70 to 85 percent of 1RM) lifted less often (two or three sets of 8 to 12 repetitions per set). We’ll look at an example of this type of program in chapter 6. Most studies suggest that resistance exercise must be performed at least two times per week to be effective, although one study showed positive effects from one day a week of resistance exercise. For those who are new to this type of training, one day a week may be a good starting point for becoming familiar with the activity, ideally progressing to two or three days per week.
The appropriate overload for jump training has been studied far less. Most studies have women perform a variety of jumping routines, including simply jumping straight up and down. When the height of the jump (jumping onto and off of small steps) or the weight of the person jumping is increased (jumping while wearing a weighted vest), the jump produces more force. Our studies have shown that younger women who jump wearing a weighted vest and onto boxes increase their hip bone mass, but that older women who jump without a vest maintain their hip bone mass over time. Generally, 50 to 100 jumps in place done three to five days per week are recommended as a suitable routine based on the research. Jumps are usually done in multiple sets of 10.
You may also wonder how long you have to exercise before your bones start to benefit. Unlike other physiological systems such as the heart and muscles, which respond to exercise rather quickly, the skeleton takes its time. Because the process of bone building is slow, it takes at least six months before we can measure the effect of exercise. Rest assured that the bone-building process begins as soon as you start to exercise regularly, but it takes a while for us to be able to detect that change through bone mass (density) measurement. All exercise research studies are a minimum of six months in length and usually last for one year. This means that you must be patient yet persistent, and most of all, you need to find an exercise that you will stick to for a lifetime. We know that bone benefits from exercise are lost when someone stops training, so any exercise you do for your bones must be something you can commit to long term. Fortunately, you have many choices and are likely to find and begin such a program!