Quick answer: Down-the-line: a rounded back with the shoulders slumped forward, or over-flexed knees with the weight in the heels; the spine angle can't be sustained.
What it does to the ball
It limits the shoulder turn and forces compensations: inconsistent contact and lost distance with every club.
Root causes (by frequency)
Bending from the waist instead of hinging from the hips
Limited thoracic mobility from desk hours (TPI screen)
Standing the wrong distance from the ball
How to fix it
Hip hinge with a stick along your back: three contact points (head, mid-back, sacrum) and bend only from the hips
Drills that fix it
Stick along the spine · 10 min · 12 reps
Build a hip hinge with a neutral spine Hold an alignment stick against your back touching head, mid-back and sacrum. Hinge from the hips, keeping all three contact points, until you reach your golf posture. Drop the stick, take your grip, and hit a ball keeping that back.
Frequently asked questions
What is "Poor address posture (C-posture)" in the golf swing?
Down-the-line: a rounded back with the shoulders slumped forward, or over-flexed knees with the weight in the heels; the spine angle can't be sustained.
What does poor address posture (c-posture) do to the ball?
It limits the shoulder turn and forces compensations: inconsistent contact and lost distance with every club.
How do you fix poor address posture (c-posture)?
Hip hinge with a stick along your back: three contact points (head, mid-back, sacrum) and bend only from the hips
Sources
Titleist Performance Institute — Body-Swing Connection screening