What is a Pilates reformer?

A Pilates reformer is indeed a piece of equipment specifically designed for performing Pilates exercises, particularly resistance training. The reformer features a sliding carriage that is attached to springs, allowing practitioners to use varying levels of resistance. This unique setup helps enhance the effectiveness of workouts by enabling users to work against or with the resistance provided by the springs. The reformer's versatility allows for a wide range of movements that can target different muscle groups, improve strength, flexibility, and posture, making it a central tool in Pilates practice.

The other options, while related to the context of Pilates, do not accurately represent what a reformer is. A Pilates mat is a different piece of equipment used primarily for exercises performed on the floor. A group of Pilates practitioners refers to people who engage in the practice but does not pertain to the equipment itself. A style of Pilates practice could describe various methodologies within Pilates but does not define the equipment used. Thus, the answer highlights the essential role of the reformer within Pilates training.