CrossFit is a branded fitness regimen that involves constantly varied functional movements performed at high intensity. The method was developed by Greg Glassman, who founded CrossFit with Lauren Jenai in 2000, with CrossFit its registered trademark. The company forms what has been described as the biggest fitness chain in the world, with around 12,000 affiliated gyms in over 150 countries as of 2022, under half of which are located in the United States.

High-intensity interval training (HIIT) is a training protocol alternating short periods of intense or explosive anaerobic exercise with brief recovery periods until the point of exhaustion. HIIT involves exercises performed in repeated quick bursts at maximum or near maximal effort with periods of rest or low activity between bouts. The very high level of intensity, the interval duration, and number of bouts distinguish it from aerobic (cardiovascular) activity, because the body significantly recruits anaerobic energy systems. The method thereby relies on "the anaerobic energy releasing system almost maximally".