When is a contraction not a shortening? Let’s talk about muscle contractions.
1. The definition of a contraction is
• the process of becoming smaller.
“the general contraction of the industry did further damage to morale”
synonyms: • shrinking, shrinkage, decline, decrease, diminution, dwindling
• “the contraction of the industry”
• the process in which a muscle becomes or is made shorter and tighter.
“neurons control the contraction of muscles”
synonyms: • tightening, tensing, flexing
• “the contraction of muscles”
• a shortening of the uterine muscles occurring at intervals before and during childbirth.
synonyms: • labor pains, labor;
• “my contractions started at midnight” (Source: Google “definition of contraction)
But when it comes to muscle we have three distinct types of “contractions” concentric, isometric and eccentric. Only concentric contractions would fit the definition above of shortening. Isometric contractions keep a constant muscle length, whilst eccentric contractions are “oxymoronic” and actually have muscle lengthening, not shortening.
This issue was discussed in a paper by Faulkner “Terminology for contractions of muscles during shortening, while isometric, and during lengthening” he points out the contraction when pertaining to muscle does not indicate shortening but instead proposes the following definitions: The verb “to contract” and the nouns “contraction” and “contractility” need to be defined correctly in terms appropriate with long-term usage as “specifically for muscle, to undergo activation and generate force. Ref
So simply put a muscle contraction is when a muscle undergoes activation and generates force.
He goes on to propose that the terms concentric isometric and eccentric are also confusing and should be perhaps be replaced with the adjectives that provide the greatest clarity “shortening,” “isometric,” and “lengthening”.
It seems that these definitions have not been universally adopted and muscle contractions are still commonly referred to by concentric, isometric and eccentric.
The molecular magic of a muscle contraction.
Concentric and isometric muscle contractions can easily be explained by the sliding filament hypothesis which will be familiar to many who have studied muscle physiology. It was put forward by Huxley in the late 1950s and the involves the now well recognized, Z bands, sarcomeres, H bands,I zones, actin and myosin filaments. Ref (See diagram below) Ref
Interesting Tidbit: It is possible to have a concentric contraction of muscle even when you are dead and the sliding filament hypothesis still applies. This is of course known as rigor mortis, which develops several hours after death. It is interesting to note that once a muscle has undergone a shortening contraction, energy is required in the form of ATP to allow relaxation (normally created aerobically) obviously after death this is not possible and hence rigor mortis contractions can last for several days. the term “a stiff” indicating a dead body has it’s origin in rigor mortis. Ref
The big problem though is that the sliding filament hypothesis can not explain eccentric contraction and many of the features that they possess such as high force generation with low energy expenditure or the ability to generate a force whilst lengthening against a load. In recent year a large protein molecule Titin which connects myosin to the Z band and to actin has been shown to be highly elastic and have the capability of bonding to actin during the presence of calcium release during eccentric contraction. Work by Herzog has shown that Titin has both elastic and visor elastic properties when muscle is stretched passively, but when stretched actively as in the case of eccentric contractions Titin binds at the Z band end with Actin and acts like a rigid sprain creating high resistant forces in the muscle. This is a proposed mechanism for the high force generation and low energy expenditure of eccentric contractions.Ref
For those who have further interest in this hypothesis and the action of Titin during eccentric contraction, there is an interesting lecture by Herzog here “A new paradigm of muscle contraction.” Thanks to @sportstherapy56 Ian Brown PT, for pointing me to this great reference. Video link
Interesting tidbit number two: “Titin is the short name for an extremely massive protein molecule. The full-length scientific name of this protein molecule contains 189,819 characters and is considered (by some) as the longest word in not just English, but any language.” Ref
Click here for a link to Titin in it’s full glory. Ref
Interesting tidbit number three: a sarcomere is on average 3 microns (micrometers long) that is 3 millionths of a meter. The average human hair is 90 microns hence a sarcomere is 30 times smaller than a human hair. These are “magic” molecular force generating machines. (Authors comment)
What have I learnt and why do we still teach and learn muscle actions?
So what have I learnt in my four day journey of trying to understand eccentric contractions? I learnt there is still much to be discovered about what happens at the molecular level during muscle contractions, especially where eccentric contractions are concerned.
Muscle contractions are fascinating, I wonder why we learn the “action” of muscles as though this reflects some sort of essential function. We seem to focus on concentric muscle contractions which reflect muscle action, yet muscle function involves all three types of contraction. I would propose that we would be better suited to learn the function of muscles and their synergy and role with other muscles. Eccentric contractions are just as common as concentric contractions, we call certain muscles extensors or flexors when they are able to control the opposite movement via an eccentric contraction. A prime example would be the quadriceps muscles during squatting or the hamstrings during walking and running.
There is far more to muscle than shortening.
Ancora Imparo (yet I am still learning) Michelangelo
To be continued…………………………. Thanks for reading. 👍