The problem is due to tense muscles. When you take your practice swing you know the ball is not going anywhere so you swing nice and easy with relaxed muscles, the moment you put a ball down you have the need to muscle it a mile so everything gets taught and inefficient.

The way to improve is to convince yourself that a loose free flowing swing that lets your arms extend down rather than them cramp upwards will hit the ball farther. Work on the imagery, see yourself swinging freely through the shot, get that picture in your mind as you swing.

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Best Regards
Brian

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Funny o'l game!