The following article, based upon work recently reported in the British 
Medical Journal, corroborates what I pointed out some years ago in one of my 
books about the benefits of impact training, because far too many 
well-meaning health and fitness professionals have been claiming that impact 
loading is universally harmful to the joints.  Such findings should also be 
viewed in the light of observations that osteoporosis, arthritis, spinal 
deterioration and other types of joint demise are very common among sedentary 
folk who spend minimal time in impact activities or any other types of 
physical activity, for that matter.  

For those who may be interested, I have quoted the relevant section from my 
book, plus other references.

Dr Mel C Siff
Denver, USA

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High Impact Activity Protects against Future Fractures

<http://uk.news.yahoo.com/010121/103/ax96u.html>

High impact physical exercise is far more effective than moderate or low 
impact activities at protecting people of both sexes from hip fracture in 
later life, according to a study in this week's British Medical Journal.

Dr Nicholas Wareham and colleagues from the Institute of Public Health in 
Cambridge identified 2,296 men and 2,914 women who had had a heel ultrasound 
measurement taken (a method used to predict their risk of hip fracture) as 
part of the European Prospective Investigation of Cancer.

Both men and women who reported participating in high impact physical 
activity - including jogging, tennis, badminton, and step aerobics - had a 
significantly higher ultrasound measurement than those who reported no 
activity of this type, the researchers found. This finding, they say, could 
be translated into a 33 per cent reduction in risk of hip fracture in men and 
a 12 per cent reduction in women. .......

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MYTH: All Impact Loading Training should be Avoided

From: Siff MC, “Facts and Fallacies of Fitness” (Ch 5, p57)

<http://24.16.71.95/SPORTSCI/JANUARY/textbooks_by_m_c_siff.htm>
<http://groups.yahoo.com/group/Supertraining/files/ >

These most interesting remarks are what Dr Mark Swanepoel, my former 
colleague from the University of Witwatersrand in South Africa, sent to me 
about the well-meaning, but misleading avoidance of impact loading and its 
implications for joint integrity. They are highly relevant, not only to the 
use of many endurance training machines, but also to the entire popular 
fitness philosophy of avoiding all ballistic or impulsive exercise.

“I am suspicious of exercise machines that control both displacement and load 
simultaneously.  They must be physiologically appropriate for a very small 
sector of the population, if anyone. Unless performances on various exercise 
machines become recognized competitive events, machines that do not offer the 
athlete some freedom in the speed of muscle usage and contraction versus 
displacement, cannot possibly be a good way to prepare the body for 
competition.  

Dr Seedhom of Leeds University and his postgraduate students such as Drs Tony 
Swann and Nelson Chen have investigated joint useage and degradation.  Their 
work, taken as a whole together with that of such workers as Drs Bullough and 
Goodfellow of Oxford, and Kempson,  demonstrates conclusively that joints 
subjected to ‘heavy impacts’, such as the ankle, are relatively free from 
osteoarthrosis in old age, and those that are subjected to much lower loading 
experience a greater incidence of cartilage fibrillation and osteoarthrosis.  
In fact, as one progresses up the lower limb, from the ankle, to the knee, on 
to the hip, and then to the lumbar apophyseal joints, so the extent of 
fibrillation increases at any given age.  The reason appears to be that the 
cartilage of joints subjected to regular ‘peaky’ loading with relatively 
high joint contact stresses, is much stiffer and better able to endure the 
odd exceptional load, than softer cartilage that is lowly loaded.  

There are now many new exercise machines on the market that are advertised as 
being ‘low’ or even ‘zero’ impact machines, including one horrific device 
that subjects the lower limbs to a centripetal acceleration about a fixed 
horizontal axis through the hips, with the knees locked straight.  My problem 
is that joint cartilage and muscles subjected to such activities will 
certainly not adapt appropriately for normal walking, running and stair 
climbing, and that people using such unphysiological exercise devices may be 
letting themselves in for serious joint trouble later in life.

The catch is that because the market for such devices is relatively modern, 
we have no studies of the long-term effects of using them. Should 
biomechanists be setting up some sort of body that investigates exercise 
machines, and award their ‘mark of approval’ to decent ones, while 
withholding their blessing from the bad?  Should biomechanists not try to 
establish the long term effects of various exercise machines using 
experimental and control groups?

Having seen ankle, knee, and lumbar  apophyseal joints myself, I fully 
support in essence the hypothesis advanced by Bullough and Goodfellow, and 
later by Seedhom  independently, i.e. that joint cartilage subjected to 
regular  repetitive loading due to vigorous exercise is healthy and remains  
so, while cartilage that is only heavily loaded now and then softens, (i.e. 
proteoglycan production decreases), the collagen network loses its cohesion, 
and the cartilage then becomes damaged due to the  inevitable odd heavy load. 
 

Healthy cartilage is cartilage that is subjected to repetitive, physiological 
loading regularly, and this  includes full proper joint motion during 
exercise.  Of course, impact loading should be built up gradually, but there 
is nothing bad about  impact loading per se - cartilage ‘loves’ to be loaded 
properly, and  it is the cartilage of the ankle that is least subject to  
fibrillation.

Zero impact machines that hold joints immobile while subjecting them to 
compression, and variations on this theme - are bound to be very bad  for the 
health of chondrocytes and cartilage metabolism.  Soft,  irregularly loaded 
cartilage, is cartilage that eventually  deteriorates.  Walking and running 
are healthy exercises for joints,  provided that footwear is not worn and a 
suitable running surface is  present, or that footwear is very carefully 
chosen so as not to alter the natural loading of the foot significantly.”

References that support the above essay are the following:

Seedhom B & Wright V  Is repetitive loading a cause of osteoarthrosis? J 
Orthop Rheum 1988, 1: 79-87

Seedhom B & Swann AC   Biomechanics of the osteoarthritic knee.   Pendragon 
Papers No. 1, Proc of  workshop at the Duke of Cornwall  Dept of Rheum, Royal 
Cornwall Hospital, Truro, Cornwall, Oct 1985

Seedhom B, Takeda T, Tsubuku M & Wright V  Mechanical factors and 
patello-femoral osteoathritis.  Ann Rhem Dis 1977, 38: 307-316

Bullough P, Goodfellow J & O'Connor J   The relationship between degenerative 
changes and load-bearing in the human hip.  1973

Meachim G & Fergie I  Morphological patterns of articular cartilage 
fibrillation.  J Path  115: 231-240

Swann AC   The effect of mechanical stress on the stiffness of articular 
cartilage and its role in the aetiology of osteoarthrosis.   PhD thesis, 
School of Medicine, Univ of Leeds, UK  1988

Kempson G Mechanical properties of articular cartilage and  their 
relationship to matrix degradation with age.  Ann Rheum Dis 1975, 34,  Suppl 
2: 111-113

Kempson G, Freeman M & Swanson S The determination of a  creep modulus for 
articular cartilage from indentation tests on the human femoral head.  J 
Biomech 1971, 4: 239-250

Swanepoel MW, Adams L & Smeathers J   Human lumbar apophyseal joint damage 
and intervertebral disc degeneration.  Ann Rheum Dis 1995, 54: 182-188.

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Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/