Which of the following is NOT a variation of the "Hundred" exercise?

The Hundred exercise is a foundational move in Pilates, designed to engage the core, increase circulation, and promote breath control while being performed in various variations.

The classic version, along with the variation with bent knees, both adhere to traditional approaches found in the Pilates repertoire. They maintain the exercise's intention of core engagement and physical alignment. In contrast, the exercise performed on a stability ball does not align with the standard variations recognized in typical Pilates practice. While it may engage the core in a different way, it does not reflect a formal adaptation of the traditional Hundred, which is meant to be performed on a mat or reformer for specific benefits tied to Pilates principles.

Additionally, using the reformer with springs also remains consistent with the original exercise's intent, offering different levels of resistance while still staying true to the mechanics of the Hundred. Therefore, the Hundred on a stability ball, while it may serve as an alternative core exercise, detaches from the established variations of the Hundred.