This is the multifidus, the tiny but mighty stabilising muscle that runs along the spine in both you AND your dog.
In humans, a weak multifidus can lead to back pain, instability, compensatory movement, and overloading joints.
In dogs it's exactly the same.
Except your dog won't tell you their back hurts, they'll just start
Taking the stairs slower
Choosing grass over pavement
Shifting weight when standing
Refusing jumps they used to nail
Tiring more quickly than you'd expect
It's not age or being lazy, your dogs multifidus (Like 95% of the dog population) might be crying for help.
The good news? It's completely trainable.
You can strengthen this muscle before it becomes a problem with core conditioning