Which of the following is NOT a benefit of Pilates?

The option stating "Increased mental stress" is indeed not a benefit of Pilates. In fact, one of the fundamental principles of Pilates practice is to promote relaxation and reduce stress levels. Pilates focuses on mindful movements, improving breath control, and enhancing body awareness, which contribute to a calmer mind and a more relaxed state of being.

In contrast, the other options reflect recognized benefits of Pilates. Improved muscle strength comes from the resistance training elements within the exercises, which target various muscle groups. Enhanced flexibility is gained through the dynamic stretching and stabilization aspects inherent in Pilates routines. Better posture alignment results from the core strengthening focus, which helps stabilize the spine and improve overall body mechanics. Thus, while Pilates promotes numerous positive physical and mental health benefits, increased mental stress is definitively not one of them.