GMC 2007 Yukon XL Owner's Manual page 17

CAUTION:
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Sitting in a reclined position when your
vehicle is in motion can be dangerous.
Even if you buckle up, your safety belts
cannot do their job when you are reclined
like this.
The shoulder belt cannot do its job. In a
crash, you could go into it, receiving neck
or other injuries.
The lap belt cannot 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.
Do not have a seatback reclined if your vehicle is
moving.
17