Mel Siff Defining Different Muscle Actions and Contractions

Published: 23rd June 2009
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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


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


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

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