Carpal Extensor Biomechanics Question

[lehughes]

Laurie,

Hello!

I came across your blog/videos while trying to research an answer to a question I had about canine biomechanics on YouTube.

Specifically, I am trying to find out exactly why it is that when fully extending a dog’s elbow with my hands, their carpus also extends at the same time as well.

The elbow motion I am facilitating is also captured in your YouTube video titled “Suspected ECR Rupture 2” but with the carpus also extending instead of remaining neutral as in the video.

My best guess is that the mechanism must be an increase in tension on an extensor muscle. After finding out that the Extensor Carpi Radialis muscle’s origin was above the elbow joint on the Humerus, I assumed the length of the ECR muscle was being pulled taunt as I guided the elbow into full extension, and that this is what facilitates the extension of the carpus distally further down the limb on it’s own.

I am under the (perhaps mistaken) impression that as long as the dogs limbs/joints are being moved for them, their muscles should be relaxed and not contracting. This is also what led me to think about the increased tension as an explanation.

Do you have any thoughts on this? I would be deeply grateful for any insight you have.

Thank you so much!

Sincerely,
K

[lehughes]

Hi Kelly,

Okay, so, the mechanism is passive. It’s not a muscle contraction, which means we have to look at the biomechanics.

The main difference would have to be the fact that a dog’s leg is in a permanently pronated position as compared to a human.
Along with the fact that the dog tends to weight bear in a slightly flexed elbow position with an extended carpus - all to say that perhaps their extensor muscle groups are a bit more taught than our (or shortened in comparison if you will).
Thirdly would be to look at where the origin of the muscle(s) is (are) in relation to the axis of the joint. (i.e. when the elbow is flexed it increases the distance between the origin and insertion).

I did find a fascinating paper about the Deep Digital Flexor Muscle a number of years ago that goes into trying to understand the biomechanical / anatomical differences of that muscle. Essentially, why in a dog when you flex the elbow are you unable to get full wrist extension?… which is not affected in a human. I’ve attached portions of the paper here, as I think it lends a hand in understanding the arguments I present above (but in reverse… i.e. other side of the forearm!)

I hope this helps a bit!

Cheers,

Laurie