PDA

View Full Version : PUZZLES & PARADOXES C



Dr M. C. Siff
09-04-1995, 12:11 AM
Here are a few more P&Ps (which I pose as teaching propositions to
my students) for anyone who is interested in commenting.


PUZZLES AND PARADOXES

14. The concept of isokinetic exercise and testing is misleading and
inaccurate.

Isokinetic testing, research and exercise is very popular in fields
such as sports physiotherapy, but too infrequently do the users
question the concept that it is impossible for any mechanical device
to facilitate isokinetic exercise. Isokinetic (constant 'velocity')
exercise is impossible for many reasons, the prime one being that all
human motion begins from rest and returns to rest. In other words,
there always HAS to be acceleration and deceleration on every so-
called isokinetic dynamometer.

This the more honest machine manufacturers readily acknowledge and
they advise that one concentrates on the 'constant velocity' range of
the device. However, injuries are very commonly associated with
changes of state, such as acceleration or deceleration. We also know
that isokinetic action is a rarity in any athletic activities (even
swimming), so why does so much analysis of joint action rely on quasi-
isokinetic devices which are very costly and highly specific?
Another point - do the users all know if the constant velocity is
constant speed, constant linear velocity,
constant angular velocity, constant velocity of muscle contraction or
what? Discuss the problems associated with the use of isokinetic
(really, quasi-isokinetic) testing and exercise. Pay particular
attention to its specificity of action and the effect of the
acceleration and rate of force development (or change) phases that
generally are ignored.

15. The concept of isotonic muscle action is misleading and misused.

Isotonic (same tension) muscle action is
regarded as synonymous with 'dynamic' muscle action and casually
applied in numerous texts as such. However, it is extremely rare
that constant tension is ever observed in an active muscle. The only
time when constant tension probably exists (before fatigue affects
it) is during an isometric contraction or over a very small range of
quasi-isometric action. The Russians prefer to use the term
'auxotonic' and some Western workers suggest the use of the term
'allodynamic' to describe the real situation of dynamic muscle action
(i.e. changing muscle length and tension). Others consider it all a
matter of unproductive semantics. What do you think?

16. Abnormal foot pronation or supination may be misidentified as
the primary causes of lower extremity running injuries.

Many authorities label
overpronation or oversupination etc as the major causes of many
running injuries to the leg. However, running and all forms of gait,
for that matter, involve the action of several interacting joints, so
that it is impossible during dynamic gait to have pure pronation iof
the foot. Depending on the runner's particular style, the action
rather may be described as inversion or eversion (where these terms
are NOT synonyms for pronation or supination!). Even this may be a
serious oversimplification, since the athlete may also exhibit varus
or valgus, plus other idiosyncrasies of joint action of the hips,
toes and other joints. Do you consider it useful and informative to
label leg injuries as a simple result of overpronation or
oversupination?

Dr Mel C Siff
Mech Eng Dept
University of Witwatersrand
South Africa

++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++
Dr Mel C Siff
School of Mechanical Eng
University of the Witwatersrand
South Africa