This is because of the Functional Line.
this line goes from the humerus, crosses over the back to the diagonal patella.
Thoracolumbar Fascia
The Thoracolumbar Fasia has a big roll to play in this line. From looking at the blue line above, you can see how this big sheet of fascia connects to the humerus via the Latissimus Doris (labeled as x) and then crosses over the back to the patella, via the Tensor Fasciae Latae (Labeled as o) and Vastis Lateralis.
The Trot
You can see this line doing its best work in trot. When the limbs are moving in a two beat, with the diagonal pair.
Lameness
If you go on all fours and lift one leg or arm up, you will feel your weight shift into the diagonal limb. So you can see the strain the can go into this line when a horse starts to favor a leg.
So it is very important when your horse recovers from a lameness to get in contact with you physio so make sure there isn't any compensatory issues because if they are not dealt with, it can be come a bigger problem and you will be left with wondering why is my horse still not right.
Pics from:
rodnikkel.com
escapestudiosanimation.blogspot.com
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