The classification of certain muscles as prime movers for a given joint
action may be misleading.


Previously we examined the issue of whether or not it was useful and
accurate to categorise some muscles as agonists and others as
antagonists during a given joint action.  This paradox asks us to
re-examine the classification of muscles as prime movers, assistant
movers, synergists, shunt muscles and so forth.


This type of classification scheme has been used for many years and
has always attracted its supporters, detractors and modifiers (e.g.
see Basmajian 'Muscles Alive' for his discussion of this topic).  It
has sufficed to assist us in appreciating which muscles appear to be
necessary to train or rehabilitate, as well as to select exercises
and design exercise machines.  However, it may be misleading us into
regarding one muscle group as being far more fundamental than another
in controlling an entire joint action. 

Classification as a matter of organisation and convenience might not
necessarily be entirely valuable - in fact, it might be keeping us in a state
analogous to that persisting in classical physics when particles and
waves were separate entities and absolutes reigned supreme.

For example, are we justified in referring to one specific muscle
group as the prime mover from the beginning to the end of a given
movement.   Take the case of lateral raising of the arm (upper
extremity) during 'side raises with dumbbells'.  Here we have an
integrated interplay between the deltoids and supraspinatus muscles,
with both groups being active to differing extents from start to
finish of the movement.  Initially, the mechanical advantage offered
by the supraspinatus militates that this muscle should contract more
powerfully than the deltoids (let us ignore the role played by any
'stabilisers' at this stage).

Later, the deltoids tend to dominate (at least in the normal subject)
over the supraspinatus.  However, the different components of the
trapezius and rhomboid muscles also play valuable roles, such as
rotating and fixating the scapula to allow a greater range of shoulder
abduction.  During the lowering or eccentric phase, we have a similar
situation occurring approximately in reverse, with the supraspinatus
and deltoids acting eccentrically instead of concentrically.

The question is this:  Which muscle should be regarded as the prime
mover in this joint movement?  The above picture is certainly
oversimplified, but it serves to illustrate the point that many muscles
are involved in managing any movement from start to finish and that
it might be misleading to single out one muscle group as being the
prime mover at all stages during the action.

Maybe we need to talk about primary, secondary and tertiary (and
other order) movers.  Maybe we need to recognise initiatory muscles
(for the starting phase of the movement), accelerative muscles (for
movement after the initial inertia of the primary phase has been
overcome), terminating muscles (invoked to slow down or stop the
action) and modifying muscles (which change the initial direction of the

Similar comments may well apply to so-called 'stabilisers' of the
prime or other movers.  Some muscles may stabilise the earlier phases
of movement than others, and may even act as movers elsewhere during
the same movement.

Many such speculations may be raised, but they all tend to make us
ask the question:  Are we justified in regarding any specific muscles
as the exclusive prime movers during a given joint action?  Similar
questions apply to our classifying stabilisers, shunts, synergists,
assitant movers exclusively as one or other type of muscle.

What do you think?


The role played by abdominal strength in preventing back pain and disability
may be inaccurate and misleading.

The role of increasing the strength of the abdominal musculature to
prevent or minimise back pain and disability is legendary.  How often
do we read that back problems are caused by an imbalance in strength
between the trunk flexors and extensors?  This belief is not confined
simply to non-academic fitness professionals, but also to many physical
therapists, chiropractors, orthopedists, sports medical doctors and
similarly qualified therapeutic specialists. 

Certainly, most of these professionals stress the importance of correct
lifting skills and 'ergonomically correct' beds, chairs etc, but many still
prescribe an over-abundance of trunk flexion at the expense of even
greater emphasis on the trunk extensor and various stabilising
muscles (of the pelvis, hip etc).


It seems as if many back problems will be resolved if only the
average person does plenty of abdominal exercises.  Back schools,
and entire training programs have addressed back problems in much the
same way for many years.  A few folk have latched onto the fact that
the abdominal musculature is recruited most powerfully from a trunk
position of about minus 15 degrees (extension), as a few of us have
been stressing for decades now. 

So, we are now inundated with situps lying supine over Swiss or
Physio balls (decades ago, the old-style'Roman chair was used and
still ofers some real advantages over 'supine ball crunches').  The
techniques appear to have changed, but the objectives and principles
hardly seem to have progressed.  The underlying philosophy still
appears to be correction of assumed imbalances between abdominals and
erector spinae (with a little recognition given to stretching and
twisting).  Abdominal strengthening still seems to be more important
than a more integrated approach focusing more on the primacy of the
back extensors.

To this we must add the plague of abdominal devices advertised daily
on TV infomercials.  These follow the same rationale - weak abs mean
back injury - strong abs mean no back problems.


Let us examine the apparent logic behind this ab strengthening idea. 
Somewhere, it was stressed that weak back muscles lead to inefficient
pelvic posture, lumbar spinal 'lordosis' and the host of back
problems which afflict modern Western humans.

Is this really logical?  The back is kept in its appropriate series
of complementary curvatures by action of the erector spinae group of
muscles, since the upright posture of standing, walking, running and
so on is the result of back extension processes.  The muscles around
the front of the trunk are there to curve the lumbar spine forwards
and downwards, not backwards and upright.  If the abdominals contract
actively, they tend to disrupt the upright posture or neutral
lumbar curvature, so why do they need to be dynamically strong?

Of course, we know that the abdominal musculature plays a vital role
in assisting in raising the intra-abdominal pressure during the
Valsalva manoeuvre elicited during heavy lifting, jumping or sudden
transitions in alignment of the spine - but this role is passive and
does not rely on the flexing mode of action of these muscles.

In fact, the abdominal musculature (as monitored by EMG) is rather
quiescent during erect or slouched standing, sitting and walking, so
in what respect will stronger abdominals improve spinal function -
except during heavy lifting when one is carrying out the much
maligned act of breath holding?


I often recommend a simple visualisation exercise with my students -
I ask them to perform mental surgery on any muscle group to see
what the result will be.  If we do this to the abdominals during
normal daily activities which do not involve heavy lifting, we will
probably conclude that the erectors, no longer being opposed
statically by the abs, will try to pull the trunk strongly backwards.

This implies that the ab group seem to be there simply to act as
anatgonists under certain extreme conditions of trunk extension, but
not a s a primary stabiliser.  The sheer inactivity of abs (except
for mild activation elicited by one's breathing) suggests that they
play a minimal role in spinal posture unless heavy lifting, jumping
and other very athletic movements are involved.  Why then, is there
this preoccupation with abdominal strength for the average person,
who suffers a greater incidence of spinal unhappiness than
competitive athletes?

During heavy lifting, of course, the scene is very different,
because the breath holding action promotes abdominal bulging and
strong recruitment of the abdominal group.  During the activities of
the average person, even lifting is rarely done with such large loads
that the Valsalva manoeuvre is strongly involved, so where is there a
serious role for so much abdominal strengthening in John Doe's life?


Possibly the far more rational approach (which is indeed promoted by
physical therapists and others) is the teaching of correcty postural
and movement skills in all daily and sporting tasks.  Has it ever
been shown conclusively that abdominal strengthening in the average
human definitely prevents or minimises the incidence of back
problems as we are often being led to believe?  On the other hand,
there seems to be more than adequate evidence experimentally and
experientially that learning of optimal postural skills (in lifting,
sitting etc) is very effective in preventing back problems.


If one examines the exercises being prescribed to strengthen the ab
group, the number of repetitions and loads commonly being used
suggest that they will produce far more muscle endurance than
strength (which increases in response to near maximal or maximal
loading).  In other words, the very 'protocols' being offered to
'strengthen' the abs are doing little to achieve this, so that this
paradox becomes even more worrisome.


Maybe this paradox suggests that a great deal of the so-called
'abdominal strengthening to protect the back' philosophy needs to be
scrapped and replaced by a far greater emphasis on strengthening the
spinal erectors and improving movement and stabilising techniques in
daily life.

What do you think?