MUSCLE INVOLVEMENT IN JOINT MOVEMENT
Standard anatomical textbook approaches describing the action of certain
muscle groups in controlling isolated joint actions, such as flexion,
extension and rotation, frequently are used to identify which muscles should
be trained to enhance performance in sport. Virtually every bodybuilding and
sports training publication invokes this approach in describing how a given
exercise or machine 'works' a given muscle group, as do most of the clinical
texts on muscle testing and rehabilitation.
The appropriateness of this tradition, however, recently has been questioned
on the basis of biomechanical analysis of multi-articular joint actions
(Zajac & Gordon, 1989). This classical method of functional anatomy defines
a given muscle, for instance, as a flexor or extensor, on the basis of the
torque that it produces around a single joint, but the nature of the body as
a linked system of many joints means that muscles which do not span other
joints can still produce acceleration about those joints.
The anatomical approach implies that complex multi-articular movement is
simply the linear superimposition of the actions of the individual joints
which are involved in that movement. However, the mechanical systems of the
body are nonlinear and superposition does not apply, since there is no simple
relationship between velocity, angle and torque about a single joint in a
complex sporting movement. Besides the fact that a single muscle group can
simultaneously perform several different stabilising and moving actions about
one joint, there is also a fundamental difference between the dynamics of
single and multiple joint movements, namely that forces on one segment can be
caused by motion of other segments. In the case of uniarticular muscles or
even biarticular muscles (like the biceps or triceps), where only one of the
joints is constrained to move, the standard approach is acceptable, but not
if several joints are free to move concurrently.
Because joint acceleration and individual joint torque are linearly related,
Zajac and Gordon (1989) consider it more accurate to rephrase a statement
such as "muscle X flexes joint A" as "muscle X acts to accelerate joint A
into flexion". Superficially, this may seem a matter of trivial semantics,
but the fact that muscles certainly do act to accelerate all joints has
profound implications for the analysis of movement. For instance, muscles
which cross the ankle joint can extend and flex the knee joint much more than
they do the ankle.
Biomechanical analysis reveals that multiarticular muscles may even
accelerate a spanned joint in a direction opposite to that of the joint to
which it is applying torque.
In the apparently simple action of standing, soleus, usually labelled as an
extensor of the ankle, accelerates the knee (which it does not span) into
extension twice as much as it acts to accelerate the ankle (which it spans)
into extension for positions near upright posture (Zajac & Gordon, 1989). In
work derived from "Lombard's Paradox" ('Antagonist muscles can act in the
same contraction mode as their agonists'), Andrews (1985, 1987) found that
the rectus femoris of the quadriceps and all the hamstrings act in three
different ways during cycling, emphasizing that biarticular muscles are
considered enigmatic.
This paradox originally became apparent when it was noticed that in actions
such as cycling and squatting, extension of the knee and the hip occurs
simultaneously, so that the quadriceps and hamstrings are both operating
concentrically at the same time. Theoretically, according to the concept of
concurrent muscle antagonism, the hamstrings should contract eccentrically
while the quadriceps are contracting concentrically, and vice versa, since
they are regarded as opposing muscles.
Others have shown that a muscle which is capable of carrying out several
different joint actions, does not necessarily do so in every movement
(Andrews, 1982, 1985). For instance, gluteus maximus, which can extend and
abduct the hip, will not necessarily accelerate the hip simultaneously into
extension and abduction, but its extensor torque may even accelerate the hip
into adduction (Mansour & Pereira, 1987).
Gastrocnemius, which is generally recognised as a flexor of the knee and an
extensor of the ankle, actually can carry out the following complex tasks
:
(a) flex the knee and extend the ankle
(b) flex the knee and flex the ankle
(c) extend the knee and extend the ankle
During the standing press, which used to be part of Olympic Weightlifting,
the back bending action of the trunk is due not only to a Newton III reaction
to the overhead pressing action, but also due to acceleration caused by the
thrusting backwards of the triceps muscle which crosses the shoulder joint,
as well as the elbow joint. This same action of the triceps also occurs
during several gymnastic moves on the parallel, horizontal and uneven bars.
This back extending action of the triceps is counteracted by the expected
trunk flexing action of rectus abdominis and the hip exension action of the
hip flexors, accompanied by acceleration of the trunk by the hip flexors.
Appreciation of this frequently ignored type of action by many multiarticular
muscles enables us to select and use resistance training exercises far more
effectively to meet an athlete's specific sporting needs and to offer
superior rehabilitation of the injured athlete.
Finally, because of this multiplicity of actions associated with
multiarticular complex movement, Zajac and Gordon stress a point made by
Basmajian (1978), namely that it may be more useful to examine muscle action
in terms of synergism rather than agonism and antagonism. This is especially
important, since a generalised approach to understanding human movement on
the basis of breaking down all movement into a series of single joint actions
fails to take into account that muscle action is task dependent.
Andrews J G (1982) On the relationship between resultant joint torques and
muscular activity Med Sci Sports Exerc 14: 361-367
Andrews J G (1985) A general method for determining the functional role of a
muscle J Biomech Eng 107: 348-353
Andrews J G (1987) The functional role of the hamstrings and quadriceps
during cycling: Lombard's paradox revisited J Biomech 20: 565-575
Basmajian J (1978) Muscles Alive Williams & Wilkins Co, Baltimore
Mansour J M & Pereira J M (1987) Quantitative functional anatomy of the
lower limb with application to human gait J Biomech 20: 51-58
Zajac F E & Gordon M F (1989) Determining muscle's force and action in
multi-articular movement Exerc Sport Sci Revs 17: 187-230
-------------------------------------------
Dr Mel C Siff
Denver, USA
mcsiff@aol.com
On 4/3/00, JILL.KISON@cuw.edu 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.>>
Mel Siff:
***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 (/).
Mel Siff
Dr Mel C Siff
Denver, USA
mcsiff@aol.com