The loss of muscle mass and bone density with age - and how to stop it!

As we get older our muscle mass begins to decline and there is a potential loss of bone density. Bone density is the amount of bone tissue within a volume of bone and a decrease in bone density can increase the chances of injury and osteoporosis (bones become more fragile with more chance of fracturing).

Reduction of muscle mass

The reduction in muscle mass makes it more difficult to perform activities such as running, swimming and cycling powerfully. Although running and cycling (approx 20 miles per week enhances bone density) can preserve leg muscle tissue they do not have any effect on the upper body.

Some studies have shown that runners who log 40-75 miles per week may actually have decreased bone densities in their upper spines, shoulders and ribs, compared to sedentary individuals.

Effects of ageing on bone denisty and muscle mass

Therefore based on the effects of ageing on bone and muscle loss many experts recommend older athletes include resistance training alongside their aerobic sessions. This is because weight training will maintain bone and muscle mass.

A study at East Tennessee State University compared aerobic only training with aerobic plus strength training on the skeletal and muscular health of 43 healthy individuals over the age of 55.

The subjects were split into the following two groups:

  • 23 of the subjects performed three 30 minute aerobic sessions per week, including walking vigorously on a treadmill, stair climbing or cycling. Heart rates were between 65-85% of maximum during all of the workouts.
  • The other 20 subjects performed 15 minutes of aerobic activity with 15 minute strength training three times a week. They worked all major muscle groups at a resistance of 50-65% of 1RM (the greatest amount of weight which could be lifted successfully for one repetition).

After four months of training bone density (averaged over the whole body) and lean muscle mass were tested. It was found that the aerobic plus strength group increased muscle mass and bone density but the aerobic only group did not improve.

In addition, the density of the 'femoral neck' (a part of the thigh bone which links the bone with the actual hip joint socket) advanced for strength trained athletes but stayed constant in the aerobic group. This is particularly important for older individuals, since the femoral neck is a frequent site of fractures.

Neither group was able to increase the density of the lumbar vertebrae (lower back bones) and each group improved the ability to do sit ups and push up by similar amounts.

Although weight training is sometimes viewed as 'risky' for older athletes, none of the weight trainers were injured during the four month study.

Overall, a programme of aerobic activity plus strength training was better than aerobic exercise alone in terms of improving the skeletal and muscular systems. As the researchers put it, 'We recommend that healthy people over the age of 55 years enrol in a combination of aerobic and weightlifting exercises'.

('Are Aerobic Exercises as Beneficial on the Musculoskeletal System as Weight-Lifting Exercises in Subjects 55 Years of Age and Older?' Journal of Aging-and-Physical-Activity, vol. 1, October 1993).

The loss of muscle and how to stop it

Muscle reaches its maximum size by about the age of 25 in most people. There is then about a 10% decrease between the age of 25 and 45 with a 45% shrinkage over the next 30 years. But why does so much muscle tissue disappear, and why does the degeneration accelerate after the age of 50?

New research from Sweden has the answer. The main reason for the reduction is that the total number of cells in any particular muscle stays pretty constant until the age of 30 but then begins a steady decline.

The fall off is slow at first but increases dramatically after the age of 50. For example, if one of your muscles consisted of 100 cells (fibres) when you were 30, the muscle would probably still contain 90-95 fibres 20 years later, but the 'fibre count' would plummet to only 50-55 when you became 50.

Two types of muscle fibres in the body

There are two types of muscle fibres in your body, which are type I, slow twitch fibres, which contract slowly but have great endurance and type II, fast twitch fibres, which contract quickly but have low endurance capacities.

A decrease in the size of type II fibres plays a key role in the muscle shrinking process, with individual fast twitch fibres shrivelling by about 25-30% between the ages of 20 and 80.

However, the type 1 ('slow-twitch') muscle cells, either remain unchanged in size or can expand by up to 20% in individuals who remain very physically active as they get older. This therefore compensates for the loss of size of type II fibres.

But what causes the fairly dramatic loss in muscle cell numbers? Over time (especially after the age of 50) 'motor nerve cells' in the spinal cord begin to deteriorate at a steady rate. The motor nerve cells are normally in close contact with muscle cells and tell the muscle fibres when to contract during physical activity..

This connection between motor nerves and their associated muscle cells is also necessary to keep the muscle fibres alive. Therefore as motor nerve cells die, the muscle cells to which they are attached also bite the dust.

Resistance training: Positive effects

Fortunately, there's a positive side to the story. People who participate in resistance training don't necessarily halt the fibre death process, but they can stop and even reverse the reduction of size in the type II fibres.

Although the number of muscle cells declines, type II (and sometimes even type I) fibres may get larger as a result of strength training, leading to a potential advancement, instead of a loss, of total muscle tissue in the body.

Since the overall process of muscle atrophy (reduction) picks up steam after the age of 50, strength training for people over 50 is especially critical.

Fortunately, it's never too late. Research demonstrates that even individuals over the age of 80 can improve their muscles by participating in regular strength training workouts.

('Ageing and Human Muscle: Observations from Sweden': Canadian Journal of Applied Physiology, vol. 18(1),pp2-18, 1993)

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