Final answer:
The most limited trunk movement for older adults is rotation, as aging can restrict the movement in the joints between the vertebrae. This affects the ability to turn the head side to side or twist the body.
Step-by-step explanation:
The most limited trunk movement for older adults due to the aging process is rotation. This movement, which entails turning the head side to side or twisting of the body, becomes restricted as the joints between the articular processes of adjacent vertebrae experience age-related changes. Flexion and extension are movements that occur in the sagittal plane and involve anterior or posterior bending of the neck or trunk. Although these movements can also be affected by aging, it is the rotational ability that typically becomes most limited in the elderly, affecting their range of motion.