The use of the terms, "agonist" and "antagonist" is fraught with
misunderstanding and misapplication. Although "agonists" traditionally refer
to the "prime movers" in a given joint action and the "antagonists"are
understood to be those muscles which act in the opposite direction to the
agonists, it is incorrect to presume that, when a given joint is moving, the
antagonists are always opposing the agonists. As I pointed out in other
letters on concontraction and ballistic action, antagonists sometimes are not
antagonists at all - they may simply be inactive spectators during much of
the joint action. That is a major reason why Basmajian, for one, suggests
that all muscles should rather be thought of as "synergists", or co-workers
who cooperate in specific patterns, as the movement need arises.
Thus, while the triceps may be the anatomical or STRUCTURAL antagonists to
the agonistic biceps, they are not necessarily the FUNCTIONAL antagonists to
every action involving the biceps or elbow flexors, in general. Similarly,
the hamstrings do not necessarily act antagonistically to the quadriceps in
all actions involving the knee joint, especially during explosive movemenst
such as running, jumping and kicking. Any marked concurrent contraction of
the hamstrings while the quadriceps are contracting powerfully in these
situations would rapidly slow down the knee extension or lead to hamstring
injury. The antagonists come into meaningful play only when the end stage
movement of the joint has to be halted, not throughout the movement. That is
the nature of ballistic movement.
Here is some more information on agonists and antagonists:
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CLASSIFICATION OF MUSCLE ACTIONS
[Siff MC "Supertraining" 2000 Ch 1.8 ]
In producing the various directions of movement, many muscles are
simultaneously involved in controlling the action from its beginning to its
end. This orchestration of diverse contributing muscles has led to different
roles being allocated to each muscle group. One of the typical classification
schemes recognizes the following traditional roles:
* Agonists: Muscles acting as the so-called prime movers of the action.
Prime movers produce the most significant contribution to the movement, while
assistant movers play a more secondary role in assisting this prime action.
Some authorities recognise emergency muscles as a special class of assistant
movers which come into play only when maximal force must be generated during
a particular movement. In physiotherapy, the phenomenon of assistant movers
being recruited by significant increases in loading is referred to as
overflow. The value of training periodically against near maximal or maximal
resistance then becomes obvious.
* Antagonists: Muscles acting in direct opposition to the agonists. As
emphasized by Basmajian (1978), this action does not necessarily accompany
every agonistic contraction, but only under certain circumstances. Central
nervous processes are sufficiently refined to control agonist activity
without routine reliance on opposition by antagonists. After all, in
engineering situations it is rarely necessary to use two motors in opposition
to offer sophisticated control of any movement. All that is necessary is a
single sufficiently powerful motor with rapid feedback processes to ensure
accuracy, control, appropriate force and appropriate speed throughout the
defined movement range.
Moreover, it is important to note that reciprocal inhibition of the
antagonist usually occurs whenever an agonist is strongly activated (see
3.5.3). In other words, the antagonist invariably relaxes when the agonist
contracts, except when the action is extremely rapid and some 'antagonists'
come into play to prevent joint damage due to the large momentum of the
moving limb. The inappropriate activation of 'antagonists' to oppose
strongly contracting agonists, however, is recognised as one of the causes of
musculoskeletal injury. For instance, the hamstrings can rupture if they
contract to oppose the quadriceps while an athlete is sprinting or kicking.
Russian researchers have shown that antagonist inhibition occurs most readily
during rhythmic activity associated with motor learning.
So-called 'antagonists' may act continuously or in periodic spurts throughout
a movement. Moreover, the presence of antagonist activity may be due to
motor learning and not inherent reflexes. Sometimes, the existence of
antagonistic activity is a sign of unskilled movement or nervous abnormality.
Clearly, the automatic labelling of specific muscle actions as antagonistic
needs to be tempered with caution, unless confirmed by careful
electromyography (recording of electrical signals from the muscles).
* Stabilisers: Muscles stabilising or supporting a body segment statically
or dynamically while other muscles carry out a movement involving other
joints. During static stabilisation, the muscles either contract
isometrically or quasi-isometrically (very slowly). An example is the action
of the spinal erectors (erector spinae) during pushups. They maintain an
isometric contraction to prevent the spine from hyperextending (sagging)
while the pectoral and anterior deltoid muscles produce the up and down
movement of the upper body relative to the ground. During dynamic
stabilisation, the muscles are in continuous contraction while simultaneously
carrying out a mobilising role. For instance, during running and walking,
the quadriceps contract not only to extend the knee, but also to stabilise it
during the ground contact phase of each stride. Stabilising muscles often
augment the action of ligaments, which are passive stabilisers (unlike
muscles, which are active stabilisers).
In keeping with the concept of emergency muscles described earlier, one may
also recognise the existence of assistant or emergency stabilisers which are
recruited to assist the 'prime' stabilisers during very intensive effort by
the prime and assistant movers. This principle is used in rehabilitation by
physiotherapists to recruit 'lazy' muscles.
* Neutralisers: Muscles counteracting the unwanted actions of other
muscles by tending to produce opposite movements. An example is offered by
situps, in which the internal and external obliques contract simultaneously
to produce trunk flexion. Singly, these muscles tend to produce lateral
flexion and rotation of the trunk. Simultaneously contracting, they oppose
one another to cancel these movements and thereby assist the rectus abdominis
muscles in producing pure trunk flexion. Similarly, the anterior and
posterior heads of the deltoid muscles neutralise one another's tendencies to
elicit medial/lateral rotation and horizontal flexion/ extension, thereby
assisting the medial head in producing pure abduction (raising of the arm
laterally).
Variations on these categories exist, such as the division of muscles into
two classes: synergists, which cooperate to produce a movement, and
antagonists which act in opposition to the direction of movement. However,
it must be emphasized that all muscles operate synergistically in the normal
person; they all co-operate by contributing forces in the most appropriate
directions to produce the desired motion, although not necessarily with
maximum efficiency. It is preferable to remember that all muscles interact
to produce the two fundamental characteristics associated with all human
movement: stability and mobility (see 1.7.4). To do this, the same muscle
may act as a mover at one time and a stabiliser at another. It is
inappropriate to identify a muscle as one of the above types under all
circumstances. Accurately speaking, a muscle may only be identified as
carrying out a specific role in a specific situation. Thus, it is imprecise
to refer to specific muscles as being agonists, antagonists, stabilisers,
synergists and so forth: it is more accurate to state that each muscle plays
a specific role at a given moment (or during a certain movement phase) in a
given situation.
Muscles are also categorised as being tonic (postural or anti-gravity) to
offer stability and resist gravity, or phasic (dynamic) to provide movement.
Tonic muscles are usually penniform, contain a higher proportion of slow
twitch muscle fibres, generally cross only one joint, lie deeper below the
surface and perform extensor actions including abduction or lateral rotation.
Phasic muscles usually occur more superficially, contain more fast twitch
muscle fibres, often cross more than one joint and perform flexor functions,
including adduction and medial rotation.
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Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/
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