On Mon, 22 Jun 2020 17:43:59 -0400, Frank Krygowski wrote:

I have one friend who asked stability or handling questions about my
handlebar bags, but I've never noticed the slightest problem.

It could be my personal history. I spent years as a teen delivering a
large paper route by bike. I rode miles daily up driveways, hurrying
around parked cars and other obstacles with at least four cubic feet of
newspapers in a huge front basket, so I'm comfortable with a front load.
In fact, on some solo tours, I've used no rear panniers, just a big
handlebar bag and two front panniers.

It is from muscle development of both muscles on each 'side' of your arms
as you hold the steering steady whilst flinging/lobbing the paper.

Most people think of pull ups in arm muscles but not push up.