Final answer:
The abdominal muscles, including the external oblique, internal oblique, and transversus abdominis, are indeed tonically active in an upright position to maintain posture and balance.
Step-by-step explanation:
The question of whether abdominal muscles are tonically active in an upright position can be answered with a definitive 'true.' When the body is upright, the muscles of the vertebral column, thorax, and abdominal wall, including the external oblique, internal oblique, and transversus abdominis, are tonically active to maintain posture and balance. These muscles continuously extend, flex, and stabilize parts of the body's trunk to ensure that the body remains upright, and to prevent any single muscle group from becoming fatigued too quickly. This is achieved by the brain sending out electrical impulses that control posture through the alternate contraction and relaxation of these muscle groups. The three layers of the abdominal wall also play a protective role for the internal organs in regions without bony enclosures.