Over the past two weeks, a personal trainer has been working with a client to...

Narrow the base of support, raise the center of mass (arms overhead), and shift the line of gravity (lean or rotate trunk).

Narrow the base of support, add an unstable surface, and alter sensory input (slowly move head up and down).

Raise the center of mass, alter sensory input (slowly move head up and down), and remove sensory input (close eyes).

Narrow the base of support, reduce points of contact (stand on one foot), and add an unstable surface.