Biking riding to turn off the back muscles

I find riding a pre 90s style racer bicycle helps my back, core and neck.

Bending forward to use the low bars of a drop handle bar rounds my back which forces only the legs to flex. My lower back cannot add to the movement and even when it flexes it can only flex in a stretched position. My core turns on involuntarily to hold the compacted, hunched position, so no twisting of the hips. Yet I need to look forward so my thoracic extends while my chest pulls and shoulder blades pull into each other so I can reach the handle bars. It is essentially a yoga cat stretch for the thoracic. And lastly, as my legs are the only drivers, the bad coordination between back, hips and leg is successfully interfered with and I walk a bit less spasticated afterwards.

Pre 90s, the frame was European. European frames were simple in measurement and increased in height but not in length as they grew in size. All you had to do was to choose a bike was find one where you could stand on the ground, over the frame bar, and the bar gave clearance for your testicles. The post 80s Japanese frames turned up with mountain bikes, are over complicated, and do increase in length, which leaves me stretching to lean forward to hold the handles bars. A lean instead of a hunch  forward activates the back and my lower back becomes part of the driver and so my hips start twisting with each push. I found the Tokyo Bike line is an older style design, very close to my old 1984 Standish Adami racer, that lets me ride and retrain my strange movement patterns as well.