This could get fun.
Dungeoneers Survival Guide page 20:
The Tight Squeeze
Tunnel ceilings that are lower than the height of the characters walking through them can penalize a party’s movement and combat capabilities. However, since the heights of individuals within a party may vary widely, the effects of the following conditions will also vary from one party member to the next.
If the ceiling is lower than a character’s height, but at least 2/3 of his height, he can walk upright at 2/3 of his normal movement rate. The character suffers no adverse combat effects.
Characters must travel on hands and knees through any tunnel that is less than 2/3 their height. Of course, this means that occasionally humans will be grunting painfully along on all fours while their halfling, and perhaps dwarven, companions trot along with no more precautions than an occasional ducking of their heads. In a tunnel whose diameter is 1/4 or less of the character’s height, the character must actually slither along in a prone position. See the limits below for the minimum heights and widths of tunnel that can accommodate characters of different races.
A character travelling on hands and knees moves at 1/2 his normal movement rate. If he should be so unfortunate as to engage in combat while in this awkward position, he suffers a -5 penalty to all of his attack rolls. He also loses all Dexterity and shield bonuses to AC. Additionally, weapons such as longbows, broadswords, and battleaxes cannot be employed in these close quarters. Note the “Space Required” listing on the Weapons Tables for AD&D rules; this gives a clear indication of what weapons can and cannot be employed in a narrow tunnel.
A character who is prone because of a low ceiling suffers even more serious penalties to his chance in combat. Of course, the space required limits his choice of weapons even further. In addition, the character suffers a -8 modifier to all of his attack rolls, and can gain no Armour Class benefits from a shield or a Dexterity bonus. When moving, the character is limited to 1/4 of his normal rate of movement.
