Expand-A-Lung: Low-cost version that has a similar inhalation-exhalation function as the PowerLung, but with a simpler design. Likes: One-piece unit with single resistance-adjuster dial. Lightweight.

Box breathing: Emphasizes equal duration of inhalation, breath retention, and exhalation. (1:1:1 ratio) Cyclic hyperventilation: Emphasizes robust inhalation, short retention, and rapid exhalation.