I’m sure Lag will chime in, but in my opinion, steepness or flatness has to do with the angle of the club at impact in relation to the ball and the body.
Ideally we would want the line of the shaft to intersect a point just below the navel at impact. Backswing really has nothing to do with how flat or upright the swing is.
Whenever I think of flat or upright, I’m thinking of the height of the hands at the top of the backswing and the position of the left arm (more horizontal or vertical).
Generally speaking… yes.
But one could fly the right elbow out and up also… so in that case… no.
I look at the location of the left arm against the shoulder line (down the line view)
Above the shoulder line would be upright… on the shoulder line would be ideal…
below the the shoulder line would be very flat (Doug Sanders)
So if the left arm is above the shoulder line… and you plan on a proper level rotation through the strike… then you HAVE TO reroute the arms and club down to get back on plane. It complicates the swing unnecessarily. Also, upright arm swings tend to compensate for poor shoulder rotation… so that is usually where to address the issue.
A flatter backswing really simplifies the golf swing because you don’t have to re route or reshift things around. It’s just not necessary to do that.
You can also swing with a faster tempo with a flat backswing… which keeps the whole swing more together and feeling like one complete motion.
A long slow upright golf swing will have to re route down and it’s just going to be harder to repeat and much more problematic under pressure for most people (except Nicklaus)
I’m not saying it can’t be done… it certainly can (Nicklaus) but it’s not necessary and the easy way.
It is possible to get too flat at the top of the backswing, if the right forearm is pointing counterclockwise i.e. 11 o’clock at the top the arms will get “stuck” almost every time.
Rickie Fowler, Zach Johnson, and Matt Kuchar had that problem and they weren’t statistically the best ball-strikers or perceived by their peers as such.
There’s a spectrum for right forearm orientation, Hogan’s was vertical with driver and Nicklaus’ was parallel to the angle of his spine at the top, anything outside of that spectrum won’t work out well.
Like many things in the golf swing… “it depends”. For a swinger with a lazy pivot action… I would agree. Too flat would be detrimental.
However, if you have adequate spine tilt at the top, then the arms will always have room to access the 4:30 line, so there will never be any “stuck” situation.
Stuck became a phrase when “stack and tilt” became the trendy method. It’s a deficient method for the longer clubs because the downswing is not able to be initiated with the legs properly and correct weight transfer which allows the proper delay and opening of the torso - shoulder rotation. With S and T the shaft comes down very steep and the torso - shoulders open too early which leads to over acceleration issues, loss of shaft flex and the compensations made to try to accommodate failed miserably.
That being said, flat can work well in a pivot driven hitter’s methodology if the weight transfer is executed through a proper lateral move with good preservation of spine tilt and and opening of the torso - shoulders is delayed so that it’s happening aggressively through the strike.
Doug Sanders won 20 times on the PGA Tour during the era of Nicklaus, Palmer, Player etc… and used a very flat swing to do so. Spine tilt with a great pivot action… lower body lateral move and aggressive Mod 1 application and use of ground pressures makes it work.