This is why there is a need to provide the dog an opportunity to relieve the stress that is building up inside him. I know that certain forms of stress seem to impact on Zac's health - I can get him to behave nicely on leash near cats (most of the time) but he is holding himself in all the time and the stress is really building up. That's why I'm looking at building his tug drive so that after a "stressful" encounter Zac can relieve that stress through tug. He can also relieve stress through chasing but that is what I'm trying to avoid . That brings us full circle back to that natural training method doesn't it?I know people say that with reactive dogs, exposure to environmental triggers will lessen their reaction over time. I am not convinced of this with Brody. I believe he may be faking calmness because it earns him food, but the cortisol is still there. Maybe I haven't persisted long enough, but his behavior once we are back in the house is evidence that not much has changed. My interest in this research stems from the need to find ways to reduce cortisol and increase testosterone, exactly what the power poses are able to achieve.
From what you have said however I am not yet sure that Brody's homecoming scary zoomies are due to cortisol. What tells Brody that he is home and to start behaving like a loony? Where does he switch from being calmer on his walk to being the arrived home loony? It could simply be a habit that is triggered by your arrival home and removal of the lead or something like that. A lot of people, just for fun, deliberately trigger their dog to do zoomies by simple word association it is quite possible that Brody is being triggered by your return from the walk in some way.
So presumably if he is burning off cortisol if he had a really short relatively unstressful walk then he would have a bit less cortisol and he would be less reactive when he came home. If he habitually behaving this way it is possible he'd be just as loopy if you just leashed him stepped outside your gate and came back in, or if you just walked him to the end of the street and back on a quiet non raining day - it all depends on the trigger.