Dr Mel Siff Blog

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
an adjacent bone. In such cases we indeed may have passive action taking a
muscle beyond its resting length.

Dr Mel C Siff

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