Muscle physics confusion

[eudovcic]

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In EK they state "A muscle uses leverage by applying a force to a bone at its insertion point and rotating the bone in some fashion about the joint. This is a likely MCAT topic because it applies the physics concept of leverage to a biological system. It may seem strange, but most lever systems of the body typically act to increase the required force of a muscle contraction. In other words, a greater force than mg is required to lift a mass m. This is done in order to reduce the bulk of the body and increase the range of movement. If the muscle has a shorter lever arm, it is closer to the body, and thus creates less bulk."


If, for example, you consider the arm as the lever system then according to EK the length of the arm from the insertion point functions to increase the required force as well as reduce bulk and increase ROM. From torque=LF, decreasing lever arm (L) would indicate that a greater force (F) is necessary to produce a comparable torque. I tried to imagine an outstretched arm holding a cell phone, as I lift the phone upward my arm bends at the elbow decreasing the L as it rises therefore requiring a greater force to continue lifting the phone. Are they simply saying that the act of raising my arm (which subsequently reduces lever arm) reduces bulk? Or are they saying if I compared two people with different arm lengths then the person with the shorter lever arm would have less bulk (which makes sense)---wouldn't the person with the shorter arm then have reduced ROM?

Thanks

 

[mehc012]

In EK they state "A muscle uses leverage by applying a force to a bone at its insertion point and rotating the bone in some fashion about the joint. This is a likely MCAT topic because it applies the physics concept of leverage to a biological system. It may seem strange, but most lever systems of the body typically act to increase the required force of a muscle contraction. In other words, a greater force than mg is required to lift a mass m. This is done in order to reduce the bulk of the body and increase the range of movement. If the muscle has a shorter lever arm, it is closer to the body, and thus creates less bulk."


If, for example, you consider the arm as the lever system then according to EK the length of the arm from the insertion point functions to increase the required force as well as reduce bulk and increase ROM. From torque=LF, decreasing lever arm (L) would indicate that a greater force (F) is necessary to produce a comparable torque. I tried to imagine an outstretched arm holding a cell phone, as I lift the phone upward my arm bends at the elbow decreasing the L as it rises therefore requiring a greater force to continue lifting the phone. Are they simply saying that the act of raising my arm (which subsequently reduces lever arm) reduces bulk? Or are they saying if I compared two people with different arm lengths then the person with the shorter lever arm would have less bulk (which makes sense)---wouldn't the person with the shorter arm then have reduced ROM?

Thanks

They are generally talking about how far away from the pivot point/fulcrum/joint the muscle is inserted. The farther the muscle inserts from the joint, the more powerfully that joint can be bent...however, the tradeoff is bulk (imagine if your biceps inserted even just 3cm farther down your forearm) and speed at the distal end of the limb.

Note that your muscles of mastication, which don't require a large ROM or speed of motion, but DO sometimes require strong forces, are inserted quite far forward, relatively speaking, on the mandible (aka the coronoid process). Your elbow flexors, on the other hand, are inserted quite close to your elbow joint, so a large force is required to bend your elbow against a load.

However, contracting that muscle by 1cm in 1s moves your hand by a large distance in one second...it allows for quick motions...whereas if it inserted more distally, a 1cm contraction would move your hand far less, and therefore more slowly for the same speed muscle contraction.

 

[eudovcic]

Thanks, that cleared it up quite nicely