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Can muscle be a reaction force ?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by mike weber, Jan 14, 2013.

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    So where to start, and how to start without this going down the toilet ;)

    Dennis mentioned MERF´S some new thing he has come up with about muscle engine reaction forces.

    Anyways it got me thinking can muscle be a reaction force?

    1st I thought no as muscle contraction will accelerate.

    then I thought that maybe eccentric contraction might be a reaction force, using the ideas that by modulating stiffness the CNS would be able to increased the resistance to motion and thus a reaction force.

    Concentric contractions would of course provide acceleration and thus not be a reaction force, but then I was thinking in terms of deceleration and acceleration as being both types of acceleration and thus could not be viewed as a reaction force.

    But all this got me thinking

    ps happy new year all :D
  2. efuller

    efuller MVP

    The term reaction force does not necessarily have to be used. Where we commonly use it is ground reaction force. The force we are describing in this instance is a force from the ground applied to the foot. The force of gravity pulls the whole body down and we don't accelerate downward because the ground applies a force that is equal and opposite to the force of gravity. When we jump the force from the ground applied to the foot is greater than the force of gravity. So, you don't have to include the term reaction, but you do have to identify the force that you want to analyze.

    The term probably comes from Newton's 3rd law. For every action there is an equal and opposite reaction. So, when a muscle contracts there is a force applied by the muscles attachment to the bone. The equal and opposite reaction is the force applied by the bone to the muscle. Note that those forces are applied to different objects. Additionally, there is stuff happening at the other end of the muscle where the origin of the muscle applies a force to its bony attachment and there is also an equal and opposite reaction from the bone applied to the muscle. So, as you analyze forces, you don't care whether it is the origin or insertion, you only have to carefully identify the force that you want to analyze.

    In terms of accelerations caused by muscles. Let's take the Achilles causing ankle plantar flexion. The muscle contracts and the tendon applies an upward force on the calcaneus at the attachment of the tendon. The calcaneus should accelerate upward if there are no other forces acting on the foot unit (talus and calcaneus etc.) The bottom of the tibia is in the way of that upward acceleration and applies a downward force on the top of the talus. The force from the tendon and the tibia create a force couple that will create a plantar flexion moment applied to the foot. This plantar flexion moment will cause an angular acceleration, and if unopposed the foot will accelerate in the angular motion of plantar flexion.

    So, you can be perfectly happy analyzing forces without using the term reaction.:D

    The term MERF reminds me of the book the Peter Principle. The main point of the book was that everyone rises to their level of incompetence. One of the things that book identified as people rising to their level of incompetence was the use of acronyms to make the speaker seem to know more than he actually does. Now, I don't mind acronyms as long as they are identified (e.g. ROM = range of motion or Range of motion (ROM)). Throwing a couple extra terms into the acronym certainly adds to the confusion. Muscle forces or Muscle moments would be a simpler term.

  3. HansMassage

    HansMassage Active Member

    "The bottom of the tibia is in the way of that upward acceleration and applies a downward force on the top of the talus. The force from the tendon and the tibia create a force couple that will create a plantar flexion moment applied to the foot. This plantar flexion moment will cause an angular acceleration, and if unopposed the foot will accelerate in the angular motion of plantar flexion."
    So If I understand correctly the muscle reaction force is between the tibia and the talus and the angular moment will create an application force to the ground by plantar flexion.
  4. David Smith

    David Smith Well-Known Member


    What Eric has said is spot on but if you want to use the term reaction force then you can understand it as the force opposite to the force of consideration which can be termed the applied force. So, for example, the force applied to a force plate by the foot is the force of consideration since this is the one you can measure, record and analyse, the reaction force would be the force applied to the foot which characteristics can be deduced from the applied force.

  5. BEN-HUR

    BEN-HUR Well-Known Member

    Appropriate terminology of which professions/practitioners (across the board) are in agreeance (which apparently isn't a proper word :eek:) with regarding the context of terms used (particularly biomechanical related topics) is very important on topics such as this. I sometimes think over scenarios such as this where terminology comes into my head which are not usually (traditionally) used in the context of my thinking (if that makes sense)... sometimes it can be a case of... "for want for a better word/term..." when delving into some uncharted perspectives on topics.

    Anyway, last week I was doing some plyometric work in a weighted running vest (like this one)... the following few days I had a closer affinity to this question... "Can muscle be a reaction force?" (for want of a better term... putting aside the likes of... i.e. eccentric loading forces :D).

    Could muscles involvement in stress fractures be seen as a "reaction force"?
    Last edited: Jan 15, 2013
  6. efuller

    efuller MVP

    Part of my original point was that calling the force a reaction force does not really matter. You identify the force you are concearned with and the name doesn't really matter. However, clearly describing the force does matter. For example, the force from the ground applied to the plantar surface of the foot. You identify what structure the force is applied to and what structure is applying the force. This way you can correctly draw the vector when you draw a free body diagram.

  7. efuller

    efuller MVP

    Again, it doesn't matter what label you put one the force, it only matters that you describe the force correctly. If you were worried about a second metatarsal stress fracture then you would be concearned about the force from the ground applied to the metatarsal head. You also might want to be concearned about the duration of time this force was applied and how many repetitions there were as well as the magnitude of the force. Generally, the magnitude of the force from the ground applied to the second metatarsal head will be higher when the tension in the Achilles tendon is higer. If you know that, then you don't need to classify the force as a reactive force, you just need to know it's there.

  8. Eric has quite nicely pointed out why muscle tension force should not be called a Muscle Engine Reaction Force (MERF). Why? Because the term MERF is not necessary, is confusing and is meaningless. In fact MERF is basically a made-up term that has been dreamed up by an individual who thinks that putting new names and acronyms to terms that have had perfectly understandable definitions and widely accepted names for generations will somehow make him appear to be more intelligent.

    It was in 1685 when Italian anatomist and mathematician, Giovanni Borelli, the "Father of Biomechanics", had his book on animal biomechanics first published (posthumously) De Motu Animalium. For those that are interested in the history of biomechanics, Bruce Martin has a nice review article on the subject, A Genealogy of Biomechanics. Over 325 years ago, Borelli very clearly described the tension forces exerted by muscles on their origins and insertions and these concepts are still used today by the International Biomechanics Community (see plate from his book below). These muscle tension forces may be generated either actively or passively and will produce the joint moments that either accelerate, decelerate or stabilise joints of the body.

    In other words, we don't need to add the term "Engine" or "Reaction Force" to describe the mechanical effects of the tension forces produced by the passive elements and active elements within the muscles of the bodies of animals. Doing so is not only unneccessary, but also complicates a basic principle of muscles that is quite straightforward to mechanically describe and understand otherwise.
  9. BEN-HUR

    BEN-HUR Well-Known Member

    Yes Eric, I'm aware of the point & agree - yet the question was asked ("Can muscle be a reaction force?"). Sometimes it is interesting to ask such questions (sometimes it can be fun)... yet I should have known via the first post that this question had some baggage attached which appears to be stemming from another thread (which I'm not aware of). Despite this baggage (which I don't want to get mixed up in), it can be productive casting out questions & thoughts on such topics (hence my above question).

    As stated in my previous post, correct terminology (particularly in biomechanics) is very important & I personally wouldn't use "reaction force" (or "Muscle Engine Reaction Force [MERF]") to describe the force generated by muscles (in fact never heard of "MERF" until this thread).

    Yes (some may also include the frequency of the force which was a topic of discussion about 18 months ago I think). However, in the context of my question...
    ... a "second metatarsal stress fracture" is of little relevance (i.e. force of muscles on the 2nd Met.)... but your point is a good one in the case of GRFs for the "second metatarsal stress fracture" (i.e. nature of ground reaction forces directed to 2nd met.).
  10. David Smith

    David Smith Well-Known Member


    Just to be really pedantic,:boxing: in answer to your question

    "Anyways it got me thinking can muscle be a reaction force?"

    The answer is NO! A muscle is not a force. A muscle action can produce a force but then only if it has a force to act against. ;);)

  11. Thanks Dave:D
  12. drsha

    drsha Banned

    Down the toilet it went.

    Watch out Mike....Remember what happened to RI when he tried to take my side.

    Dr Weber is a therapist and so unlike some others posting here, his perspective is open to thinking that muscles can be trained to perform tasks and provide moments and forces in different manners after the foot is structurally supported in an orthotic shell. If not, what purpose would training have in general?
    When was the last time one of you trained an inhibited PL back to performing other than HD and Stanley?

    I'll explain how MERF's came about to those more interested in showing off their ability to abuse, deride and be mean to fellow scientists not in their bed.

    While participating on The Arena, I learned the term Orthotic Reactive Force or ORF's. I had never heard it before. I assume one of you made it up as an acronym.
    For me, it became easier to from then on to teach physics to the masses by discussing ORF's.
    As I have long been involved in muscle engine training but never could come up with a viable teaching method, I decided to expand ORF's and attach a sister acronym that dealt with muscle engine training that improved the performance of a given muscle group and called it Muscle Engine Reactive Forces or MERF's.

    MY RF is the same as yours!!!!

    So should we change ORF's to Orthotic Reaction Forces and then call its inventor all the things you are calling me or do we grow up and act like adults when it comes to me?

    Go tell a ballerina that has been trained to be on point utilizing motor control of PL that MERF's are ........................confusing and meaningless but that medial skives are not.


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