OF ISOMETRIC, CONCENTRIC, AND ECCENTRIC MUSCLE WORK...
The sarcomere is the smallest contractile unit of the striated muscle. Contractile elements are capable of developing or producing force also addressed as strength. Since "strength" as a muscular fitness component, I will use the term force rather than strength while addressing the product of sarcomere work.
Isometric muscle work, concentric muscle work, and eccentric muscle work, are different dynamics that muscles can produce force dependent on their length. As result, the sarcomere develops different extents of force dependent on the muscle force production dynamic (isometric vs concentric vs eccentric).
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Eccentric muscle force production results in the most muscle force production, while concentric muscle force production results in the least force produced, leaving isometric to take second place. My question is, can the sarcomere and how it produces force, explain why these muscle force production dynamics do not produce the same amount off force???
I could be "that guy" that writes all this just to end up telling you that the answer is "no" and then explain why. But...the answer is actually "yes", which makes for a better professional blog post (in my opinion). Yes, the cross bridges created between myosin and actin in the sarcomere, hold the key to understanding muscle fiber force production, and force production per contraction dynamic (isometric, concentric, and eccentric).
After the creation of a cross bridge where myosin physically connects to a binding site on the actin filament, the filaments "slide" relatively one to each other. Yet, as the myosin and actin are engaged in the relative movement of sliding, the myosin's arm literally and physically connects the myosin's shaft/tail part with the actin filament (resembling a bridge, hence the term "cross bridge").
As the myosin arm "bridges" the physical distance between the two filaments, tension is created during the sliding and is dependent on the relative distance between the filaments. Accordingly, as myosin and actin move (slide) away from each other, the sarcomere is elongating, indicating of eccentric muscle force production (contraction). As this happens, three "ranges" can be identified related to the length of the myosin arm.
First range - the myosin's arm length is short-medium and produces a growing extent of force from short to medium myosin arm length. Within this range, the sarcomere cannot develop maximal force since the tension (a mechanical force) is not optimized. Second range - the myosin's arm length is medium-long and produces a growing extent of force from medium to long myosin arm length.
There is an optimal myosin arm length that will result in maximal sarcomere force production. Up to this myosin arm length, the sarcomere is developing growing extents of force, until it reaches maximal force production. Since mechanical tension increases as length is increased, more tension is stored in the arm as it lengthens, explaining why eccentric muscle force production generates the most muscle force compared to isometric and concentric work.
Third range - the myosin's arm is hyper-extended beyond it optimal force producing length, causing a decrease in sarcomere force production, and if hyper-extended too much, could result in injury to the sarcomere, and possible dysfunction as the result of the injury. The same three ranges explain force production during concentric muscle work.
During concentric muscle force production, the sarcomere shortens, causing myosin and actin to move towards each other. The more the filaments get closer, the less tension is stored in the myosin's arm, resulting in less muscle production. Beyond the original location where myosin originally created the cross bridge to begin with, the myosin's arm will be hyper-extended to the other direction (opposite of the filament movement direction during eccentric muscle contraction), increasing the chances of sarcomere injury.
During isometric muscle work, the sarcomere develops force without change to it length. While the sarcomere does not lengthen nor shorten, force is still produced, yet remains concentrated at first locally, then slowly is distributed to proximal areas. This results in a localized increase in mechanical pressure, since the force produced has nowhere to go. I
n addition, the localized force application decreases the chances of force reaching the tendon and being applied to the bone. Muscle force not applied to the tendon, and thus not applied to the bone, does not contribute to movement production. This aligns with the fact that isometric muscle work does not cause any changes to the angle of the joint, whereas eccentric and concentric muscle force production are dynamic work types, causing force to reach the tendon, and thus the bone.
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In summary, the following myosin arm length to force production relationships exists:
Greater myosin arm length (up to optimal length) - greatest sarcomere force production; Characteristic of eccentric muscle contraction
Myosin arm length at point of cross bridge establishment - second greatest sarcomere force production; Characteristic of isometric muscle contraction
Shortest myosin arm length (less than optimal length) - least sarcomere force production; Characteristic of concentric muscle contraction
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