Mobility is given as the percentage of how fast the character can move compared to his/her maximum speed.
The characters base mobility is Speed ×5, and any physical penalties are deducted from this. Running (press [R] to toggle on/off) will double the mobility, at the cost of rapidly increasing fatigue. The character will be unable to move at all if mobility reaches zero.
Physical penalties are encumbrance, fatigue, injury and starvation.