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





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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


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