Mel Siff Defining Different Muscle Actions and Contractions

Now and again list members ask for some basic definitions that are used
regularly in strength and fitness training. Here is a collection of these on
the different classes of muscle action from my “Supertraining” book

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Types of Muscle Contraction

[Siff M C "Supertraining" 2000 Ch 1: 51-52]

Traditionally, the following types of muscle “contraction” beginning with the
prefix ‘-iso’ (meaning ‘the same’) are defined: isotonic (constant muscle
tension), isometric (constant muscle length), isokinetic (constant velocity
of motion) and isoinertial (constant load). In addition, movement may occur
under concentric (so-called “muscle shortening”) and eccentric (so-called
“muscle lengthening”) conditions. Before these terms are unquestioningly
applied to exercise, it is important to examine their validity.

In all of the above cases, it is more accurate to speak about muscle
“contraction” (action) taking place under various movement conditions. It
is well known that a muscle can only contract or relax relative to its
resting or inactivated state, so that it is a misnomer to refer to eccentric
muscle contraction as a “contraction” in which a muscle contracts and
lengthens simultaneously. Actually, this means that a muscle which has
contracted under concentric or isometric conditions is simply returning under
eccentric conditions to its original resting length. To avoid confusion like
this, it is preferable to define muscle action as follows:

* Concentric - Action in which the proximal and distal muscle attachments
move towards one another
* Eccentric - Action in which the proximal and distal muscle attachments
move away from one another
* Isometric - Action in which the proximal and distal muscle attachments do
not move relative to one another

“Isometric” literally means ’same length’, a state which occurs only in a
relaxed muscle. Actually, it is not muscle length, but joint angle which
remains constant. Contraction means ’shortening’, so that isometric
contraction, like all other forms of muscle contraction, involves internal
movement processes which shorten the muscle fibres. Isometric contraction
may be defined more accurately to mean muscle contraction which occurs when
there is no external movement or change in joint angle (or distance between
origin and insertion). It occurs when the force produced by a muscle exactly
balances the resistance imposed upon it and no movement results.

Although not incorrect, the term “isometric” may be replaced by the simple
word static, without sacrificing any scientific rigour. It is interesting to
note that, during isometric contraction, mechanical work, some of which is
absorbed by the tendinous tissue, is generated by the shortening of muscle
fibres (Masamitsu et al, 1998).

The term “isotonic”, however, should be avoided under most circumstances,
since it is virtually impossible for muscle tension to remain the same while
joint movement occurs over any extended range. Constancy is possible only
over a very small range under very slow or quasi-isometric (almost isometric)
conditions of movement for a limited time (since fatigue rapidly decreases
tension). Naturally, constant tone also exists when a muscle is relaxed, a
state known as resting tonus. Whenever movement occurs, muscle tension
increases or decreases, since acceleration or deceleration is always involved
and one of the stretch reflexes may be activated.
European and Russian scientists often prefer to use the term “auxotonic”,
which refers to muscle contraction involving changes in muscle tension and
length. Other authors use the term “allodynamic”, from the Greek ‘allos’
meaning ‘other’ or ‘not the same’. Both terms are more accurate than
“isotonic” in this context.

Isotonic action is most likely to occur under static conditions, in which
case we have isotonic isometric action. Even then, as is the case with all
muscle activation, there is rise time of tension build up, an intermediate
phase of maximal tension and a final decay time of tension decrease. For any
prolonged action, the tension changes irregularly over a range of values.
If the load is near maximal, the muscles are unable to sustain the same
level of static muscle tension for more than a few seconds and the situation
rapidly becomes “anisotonic isometric”. In general, the term “isotonic”
should be reserved for the highly limited, short-movement range situations in
which muscle tension definitely remains approximately constant.

The word “isokinetic” is encountered in two contexts: firstly, some textbooks
regard it as a specific type of muscle contraction, and secondly, so-called
isokinetic rehabilitation and testing machines are often used by physical
therapists.

The term “isokinetic contraction” is inappropriately applied in most cases,
since it is impossible to produce a full-range muscle contraction at constant
velocity. To produce any movement from rest, Newton’s first two Laws of
Motion reveal that acceleration must be involved, so that constant velocity
cannot exist in a muscle which contracts from rest and returns to that state.
Constant velocity can occur only over a part of the range of action.

Similarly, it is biomechanically impossible to design a purely isokinetic
machine, since the user has to start a given limb from rest and push against
the machine until it can constrain the motion to approximately constant
angular velocity over part of its range. The resistance offered by these
devices increases in response to increases in the force produced by the
muscles, thereby limiting the velocity of movement to roughly isokinetic
conditions over part of their range. They are designed in this way since
some authorities maintain that strength is best developed if muscle tension
is kept at a maximum at every point throughout the range, a proposition which
has neither been proved nor universally accepted with reference to all types
of strength.

Moreover, research has shown that torque (and force) produced under
isokinetic conditions is usually much lower than that produced isometrically
at the same joint angle (see Figs 2.8, 2.9). In other words, it is
impossible to use isokinetic machines to develop maximal strength throughout
the range of joint movement.
The presence of any acceleration or deceleration always reveals the absence
of full-range constant velocity. Isokinetic machines should more accurately
be referred to as “quasi-isokinetic” (or pseudo-isokinetic) machines.

One of the few occasions when isokinetic action takes place is during
isometric contraction. In this case, the velocity of limb movement is
constant and equal to zero. Approximately isokinetic action also occurs
during very brief mid-range movement phases in swimming and aquarobics, with
water resistance serving to limit increases in velocity to a certain extent.
However, even if a machine manages to constrain an external movement to take
place at constant velocity, the underlying muscle contraction is not
occurring at constant velocity.

Two remaining terms applied to dynamic muscle action need elaboration.
“Concentric contraction” refers to muscle action which produces a force to
overcome the load being acted upon. For this reason, Russian scientists
call it “overcoming” contraction. The work done during concentric
contraction is referred to as positive. “Eccentric contraction” refers to
muscle action in which the muscle force yields to the imposed load. Thus, in
Russia, it is referred to as “yielding” or succumbing contraction. The work
done during eccentric contraction is called “negative”.

Concentric contraction occurs, for example, during the upward thrust in the
bench press or squat, while eccentric contraction occurs during the downward
phase. Apparently, more post-exercise soreness (DOMS - Delayed Onset Muscle
Soreness) is produced by eccentric contraction than the other types of muscle
contraction. However, it should be noted that adaptation processes minimise
the occurrence of DOMS in the musculoskeletal systems of well-conditioned
athletes. Apparently, microtrauma of connective tissue plays a significant
role in the DOMS phenomenon, but the relationship between the intensity and
volume of eccentric muscle activity, biochemical changes, the influence of
adaptation processes and the extent of DOMS is still poorly understood.

A little appreciated fact concerning eccentric muscle contraction is that the
muscle tension over any full range movement (from starting position through a
full cycle back to the starting position) is lower during the eccentric phase
than the isometric or concentric phases, yet eccentric activity is generally
identified as being the major cause of muscle soreness. Certainly, muscle
tension of 30-40% greater than concentric or isometric contraction can be
produced by maximal eccentric muscle contraction, as when an athlete lowers a
supramaximal load in a squat or bench press (but can never raise the same
load), but this degree of tension is not produced during the eccentric phase
of normal sporting movements. Clearly, it would be foolhardy to assume that
our current understanding of all aspects of muscle contraction is adequate
for offering optimal physical conditioning or rehabilitation…..
Dr Mel Siff
Author of Supertraining + Facts and Fallacies of Fitness
http://www.melsiff.com