Biomechanics of Muscle Shortening Causes

Biomechanics: causes of muscle shortening

Dr. Mauro Lastrico, physiotherapist.

Extracted from:
"Musculoskeletal Biomechanics and Mézières Method"
Author: Dr. Mauro Lastrico
Marrapese Publisher

Physics teaches that any material deformed with force/time variables will undergo permanent residual variations based on its elasticity coefficient.
The numerical value representing the ideal elasticity coefficient is equal to 1.
Materials with a coefficient equal to 1 fully restitute the accumulated deforming energy, returning exactly to the initial state. From physics we also know that such materials do not exist in nature and the value equal to 1 is an ideal reference.

In muscle there are two different elastic materials: the contractile part of actin and myosin and the connective part of membranes and tendons.
Regarding the contractile part of the muscle, it can only contract and relax, its elasticity coefficient is very high and it is more implicated in muscle tone elevations than in permanent structural modifications.
The connective components, on the other hand, having a lower elasticity coefficient, (see the plastic materials deformation curve) can remain shortened or lengthened in proportion to the force, duration, and frequency of the stimulus applied to them.

Going more specifically, muscles are composed of muscle bundles covered by a dense network of connective tissue (perimysium). The muscle bundles can be subdivided into many smaller filaments called muscle fibers. These fibers are in turn covered by another connective tissue called endomysium, and their size varies from 10 to 100µ.
Each muscle fiber is also subdivided into many small myofibrils of 1 or 2 µ, also covered by a connective tissue membrane called sarcolemma.
Myofibrils are composed of contiguous or joint filaments, depending on the action performed (contraction or relaxation), of actin and myosin.
In the muscle, therefore, there are two different elastic materials: the contractile part of actin and myosin and the connective part of membranes and tendons.
Schematically, the connective elastic elements of the muscle are divided into two:
* Elastic elements in series
* Elastic elements in parallel
The elastic elements in series (Es) consist of tendons and their prolongations inside the muscle belly. Their action is to "absorb" during muscle contraction the produced stresses, both when a muscle shortens and when it stretches. Moreover, the presence of protective structures such as Golgi tendon organs prevents lesions of the Es, inducing muscle relaxation when tension becomes excessive to avoid complete stretching. A further advantage offered by these structures is to restitute the accumulated energy, like a spring, based on their elasticity.
The elastic elements in parallel (Ep) consist of the sarcolemma (connective membrane covering myofibrils), other connective membranes, and interposed connective tissue. Their action is to "dampen" the stresses produced by stretches, reducing resistances. These elements, together with neuromuscular spindles and Golgi tendon organs, perform an external and internal protection action of the muscle itself.
Using a simplification of the muscle fiber behavior on a mathematical model we will have:

where the black vertical lines represent the insertions, the blue parts the contractile components of actin and myosin and the red parts the connective components arranged in parallel. The connective components arranged in series have not been represented because they do not undergo modifications during the active phase.
During the contraction phase with insertions approaching, the contractile components of actin and myosin actively deform in compression pulling:
* the connective components arranged in series, which behave elastically absorbing tensions and distributing the displacement to the joint ends (not shown in the figure);
* the connective components arranged in parallel (shown in red), which undergo compressive deformation.
Depending on the force/time of contraction, at the moment of release:
* the contractile part (having a very high elasticity coefficient) can return to the starting conditions or leave a compressive deformation that manifests as an increase in basal tone;
* the connective component in parallel (having a lower elasticity coefficient) will have undergone residual compressive modifications;
* since the global length of the muscle is shorter, being constrained by the length of the elements in parallel, the elements in series will also be compressed by the released contractile part and, therefore, having to occupy the same space, they will have to shorten permanently.
At the end of contraction, therefore, all the connective components will have undergone a compressive modification and their sum determines the residual shortening of the muscle.
The muscle therefore acts as a compressive force and is not able, autonomously, to move its insertions apart.
Even in eccentric contractions the contractile portion works anyway in compression.
When sitting down, for example, the quadriceps modulates the speed of descent and at the arrival point it is overall longer than at the start.
The overall length is however within the range of the maximum physiological or relative length of the muscle itself.
The modulation of the speed of descent occurs through a sum of moments of contraction and relaxation of the contractile portions. We could therefore define eccentric contraction as a sum of pulsing isometric contractions, performed at different intensities, over a given time.
This type of contraction, also occurring at an overall muscle length that is shorter than its maximum physiological or relative length, determines the active compression activation of the contractile portion and the passive compression of the connective portion of the muscle fiber.
Conclusions
Muscle contractions with insertion approximation and isometrics not at maximum physiological or relative stretching, depending on the force/time of contraction, will produce a loss of muscle length due to the connective component and an increase in basal tone due to the contractile portion.
At the skeletal level, the consequence will be that the bones on which the muscles insert will progressively undergo vectorial traction forces, such as to modify their physiological sequentiality.
At the muscular level, the progressive shortening of the connective component and the increase in basal tone of the contractile part, determine the increase of the muscle resistant force, but at the same time reduce the capacity of Work (force times displacement) and power (Work produced in unit time).