On 4/3/00, [log in to unmask] writes:
<< Just because a 2-joint muscle is active, does not mean it is active at
both of the joints to the same degree- for instance, at one time, the
hamstrings may act to stabilize the knee joint while extending the hip.
The quadriceps cannot be considered a complete group in all actions, because
only rectus femoris is the 2 joint muscle. Vasti act quite differently in
normal gait, much less in more dynamic and demanding sport activities.>>
***It was not clear if this comment was made to point out some deficiency or
to reinforce some item in my original "Muscle Action" letter. Do let me
know, so that I can comment appropriately. Anyway, whatever the objective of
that brief comment, it raises some interesting further points.
The observation that "the hamstrings may act to stabilize the knee joint
while extending the hip" emphasises the important point that analysis of
movement in terms of an isolated joint at a time can skew our understanding
of the motor situation, because it neglects the fact that several actions of
stabilisation and mobilisation are happening concurrently and sequentially.
That is why a systems approach often is preferable to an analysis of
individual elements in that system.
The idea of completeness is never a sound one in analysing any movement,
since any movement or process is complete unto itself. In other words, in
pathological and non-pathological movement, the nervous system will select a
motor strategy that is optimal for the motor problem that has to be solved.
Thus, the degree of to which any muscle group is involved during any given
action will change to meet this criterion for optimisation.
Even then, this contribution will vary according to one's change of posture,
pace of movement, environmental conditions and fatigue, so that it is
impossible to talk about real determinism or fixed contributions by any
muscles, as is often implied by testing with isokinetic dynamometers. Muscle
involvement is situation dependent and situation specific, so that all
methods of analysis or testing motor output have to state clearly the testing
conditions and the validity of any extrapolation made from such tests.
It is well known that the contribution to multi-joint movement by different
muscles changes from instant to instant as (1) all joint angles change and
(2) accelerations about different fulcra alter (according to that last
article that I submitted).
Classical movement analysis regards movement simply as the result of single
or multi-joint leverage systems acting about a fixed fulcrum (only item 1),
while the revised method of analysis takes into account the effect of
momentum transfer from parts of the body that do not necessarily involve the
joints that are spanned by the muscles that are being considered as primary
or synergistic movers (relies on items 1 and 2).
To examine what the above implies in terms of the sporting world and
rehabilitation, carry out the analysis of a throwing action or serving action
(in tennis) in two different ways:
* Model the situation as a sequence of actions taking place about individual
joints as they play out certain roles from beginning to end of movement.
Analyse which muscles are directly involved with each joint as each joint
angle changes, clearly identifying certain muscles as movers, stabilisers and
various other synergists. This tends to be the standard approach adopted in
all "muscle testing" manuals.
* Model the situation as a complex system in which specific joint actions
are modified and augmented by momentum created by distant limbs, so that
movement about any joint is the result of the usual leverage system acting
about specific fulcra plus acceleration contributed by more remote actions
elsewhere in the body or in external objects interfacing with the body. The
reference by Zajac & Gordon given in my previous letter discusses this type
of analysis in great detail.
This suggests a modification to the standard muscle testing strategies which
complements the usual isolated joint testing with additional multi-joint
tests carried out in different types of movement pattern and situations.
This might be facilitated in the clinical setting, for example, by the use
of standard biomechanical computer packages involving video analysis of
prescribed movement scenarios worked out specially to suit the clinician,
such as the systems produced by Ariel Dynamics (http://www.arielnet.com/).
Mel Siff
Dr Mel C Siff
Denver, USA
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