GMC Denali 2004 Owner's Manual page 12

But don't have a seatback reclined if your vehicle is
moving.
1-6
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CAUTION:
Sitting in a reclined position when your vehicle
is in motion can be dangerous. Even if you
buckle up, your safety belts can't do their job
when you're reclined like this.
The shoulder belt can't do its job. In a crash,
you could go into it, receiving neck or other
injuries.
The lap belt can't do its job either. In a crash
the belt could go up over your abdomen. The
belt forces would be there, not at your pelvic
bones. This could cause serious internal
injuries.
For proper protection when the vehicle is in
motion, have the seatback upright. Then sit
well back in the seat and wear your safety belt
properly.