Date: Tue, 26 Sep 2000 14:41:27 EDT From: Mcsiff@aol.com Subject: Strength_List: Muscle Lengthening It is often stated that a muscle lengthens, but all shortening and lengthening takes place relative to some starting position. In functional anatomy, the length of a contracting muscle is invariably measured relative to its resting, unactivated length. During concentric action, the joint angle decreases while the muscle contracts; during eccentric action, the joint angle increases while the muscle contracts. During isometric action, the joint angle remains the same, even though the muscle is contracting. Isokinetic action does not take place under any natural daily life conditions, but is the result of movement that is controlled by a special type of accommodating resistance machine (such an hydraulic clutch or feedback-controlled motor). Even then, the process of muscle contraction is exactly the same as action during eccentric, concentric and isometric conditions. Any differences in movement are a result of various neural processes, not different types of muscle action. So, we see that no muscle can voluntarily lengthen relative to its RESTING length. Of course, if a joint has not quite reached its greatest possible angle in any given plane (such as the elbow joint at rest), the maximum length of a muscle may be reached by means of passive loading or the action of antagonistic muscles. This will cause slight lengthening, but no further lengthening is possible under eccentric action. It is a common misconception that a muscle lengthens under conditions of eccentric action. What actually happens is that the relevant muscle is still contracting while the joint angle is increasing, so it appears as if the muscle is contracting and lengthening at the same time. The muscle is simply returning actively to its original resting length under eccentric conditions. In typical stretching or yogic manouevres, the muscles is being passively stretched to its anatomical limits. Bear in mind that the situation for rotational displacement (e.g. of the trunk and neck) is very different from flexion-extension, abduction-adduction and so forth, because the limits are not necessarily determined by one bone comprising a joint butting up against another bone. In such cases we indeed may have passive action taking a muscle beyond its resting length. Dr Mel C Siff Denver, USA Date: Wed, 27 Sep 2000 22:11:51 EDT From: Mcsiff@aol.com Subject: Strength_List: Muscle Lengthening & Definitions Since my recent letter on "Muscle Lengthening", I realised that an addendum was necessary concerning my general remark that concentric action usually refers to movement in which the joint angle decreases and eccentric action to movement in which joint angle increases. Most folk will realise that this generalisation, of course, does not apply in all cases. For example, the joint angle increases during elbow extension and decreases during elbow flexion, if we use the same convention for starting position of the joint. All of this takes us back to the earliest use of the terminology, "concentric" and "eccentric" in kinesiology, words which were chosen to avoid this type of problem. "Concentric" means "towards a centre" and "eccentric" means "out from a centre" and some scientists rightly felt that these terms were still rather inexact, because there is no such thing as the muscle or the joint moving towards or away from a centre. So, one scientist more recently suggested that the terms be made more consi stent and contained the root "...metric", just like "isometric". He proposed that "concentric" be replaced by "miometric" (decreasing length) and "eccentric" by "pliometric" (increasing length), with "isometric" remaining as it is. What happened is that someone else, reputed to be American coach Fred Wilt, witnessed the Russians in the early 1970s using 'shock method' rebound training which involves forceful, rapid termination of eccentric action, so he decided to call this sort of training , "plyometric". That rapidly caused a lot of confusion about the intended meaning of the word "pliometric" and led to the word "plyometrics" flourishing and the word "pliometrics" hardly ever being used in its original sense. Possibly the best solution to this problem of describing joint movement consistently is to observe the following convention: Concentric action - Any action which moves the distal and proximal attachments of the muscle closer to one another. Eccentric action - Any action which moves the distal and proximal attachments of the muscle away from one another. Note that I intentionally have avoided use of the terms "origin" and "insertion" of the muscles (or, rather, the tendons of the muscles), because their application is also fraught with contradiction. For instance, if we try to compare muscle action during situps and leg raises, the origin and insertion change places. Dr Mel C Siff Denver, USA mcsiff@aol.com