A power tower, also known as a knee raise machine or knee raise station, and sometimes referred to as a captain's chair, is a piece of exercise equipment that mostly works to build abdominal/core strength.
The equipment most commonly has a backrest and fore-arm rests that form the chair, with vertical handles at the ends of the arm rests. This main piece is used for knee raises and the captain's chair exercise. There are often parallel horizontal handles as well for performing dips. Often, a pull-up bar is attached to the top, and push up handles to the bottom.
When it is only the forearm pads alone for doing the abdominal exercises and it lacks dip handles it is usually referred to as simply the captain's chair. The "power" tower comes from the addition of other powerful arm exercises like push ups, dips and pull ups. The captain's chair requires minimal arm strength as it is stable and movement occurs in the hips and torso.
Manufacturers make the equipment to various specifications and weight limits. Usually the pull up bars are widely set apart for wide grip, so the equipment may not be ideal for people who are weak at this movement and could benefit from starting off with narrow grip supinated pull ups.
The equipment comes designed for different weight limits, so some may only allow lighter users, while the sturdiest would allow even heavy users to increase beyond bodyweight by wearing ankle weights, holding a dumbbell between the feet, kettlebells on the toes, a sand bag between the thighs, plates strapped to a dip belt around the hips or a weighted vest or chains over the shoulders.