Mr. Hardy seems to indicate that the One Plane Swing(by his definition) is easier on the back and shoulders.
It seems to me, however, that if one is bent over more, and has more aggressive rotation of the hips and shoulders... this would be more strenuous for the back.
You're right...in both cases!
The key is to think "easier" or "strenuous" on the back in two parts. One is the mechanical structure of the spine* itself and the other is the muscles of the back.**
The neutral spine requires your back and other core muscles to maintain this position. Bending over into golf address increases the difficulty of maintaining this posture. Then the forces generating by swinging the club will make it even harder to keep neutral. This is strenuous for the back muscles, but as with any strength training, becomes easier as the muscles are developed.
However, if you can maintain the posture, the spine structure is protected from going beyond its normal range-of-motion or load limits. This is where we really hurt our backs (herniate disks, muscle spasms, etc.). Thus it is easier on the back from this perspective.
On the other hand, a rounded back posture at address*** will feel easier on the back muscles, which are asked to do a lot less, but the load will be passed along to other places such as the lower back. Over time, the repetitive nature of golf becomes very strenuous for the spine structure.
Lou
*the bones, ligaments, joints, discs, etc.
**even better to think of all the muscles of the core (back, abs, etc.)
***this is spine flexion...spine extension (think reverse-c follow-through) is bad for the same reasons
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