Muscle "Isolation": Teres Minor Chest Flyes

Image courtesy of janellelora.blogspot.com

A lot of exercises are often considered "isolation" exercises by a good majority of the mainstream fitness community, meaning the exercise is believed to only work one muscle.  Typically, these exercises involve motion around only joint, which often leads to the false understanding of only one muscle being challenged.

In addition to this blatant misunderstanding that there are potentially certain neuromuscular demands placed on the system in order to simply hold all of the rest of the joints still (think about it like performing the almighty plank for part of/the entire body except for the one or two joints that is/are moving) as well as to maintain the integrity of the moving joint, I would like to demonstrate that muscles that oppose motions performed by the "isolated" muscle will need to contract as well in order for a specific position around one axis to be controlled while the majority of the motion occurs around another axis within the same joint.

Image courtesy of well-girl.com

For this post I want to discuss dumbbell chest flyes, which are often said to "isolate" the three divisions of pec major.  Mechanically, the three divisions of pec major have the ability to internally rotate the humerus (turn your arm in) in addition to horizontally adducting the humerus (bring the arm across the chest), which is the motion often associated with a chest fly.

Now, as I discussed in the post "Real" Weight, muscles control the distance between their two points of attachment.  They bring their insertion towards their origin or vice versa without any regard to which of the cardinal planes the trunk or limb is moving through.  Therefore, if the trunk or limb is moving through a plane that does not exhaust everything that one muscle is mechanically capable of, such as performing a chest fly motion without any associated internal rotation of the humerus, then there has to be another muscle or group of muscles involved in counteracting the mechanical ability of the targeted muscle to do so.

Going back to the chest fly example, because the targeted muscle group (pec major) has the mechanical ability to internally rotate the humerus, in order for pec major to horizontally adduct the humerus without internally rotating it as well, there have to be other muscles involved that are trying to externally rotate the humerus, such as teres minor.

What's the takeaway?  Next time you see someone performing chest flyes, compliment them on their sweet teres minor pump.

Your body.  Your training.

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