It's a timer app suitable for HIIT training.
All features are completely free to use.

Features:
- Quick start-up and comfortable operability. Create unlimited timers.
- Freely set warm-up, ready, workout, interval, cool-down minutes / seconds, and set counts.
- Customize colors, texts, background notifications, sounds, speeches, vibrations, screen flashes and icon badges.
- Dark mode
- All free

URL scheme:
hiittimer://

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* High-intensity interval training

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, which thereby relies on "the anaerobic energy releasing system almost maximally."

* Procedure

HIIT exercise sessions generally consist of a warm up period followed by repetitions of high-intensity exercises separated by medium intensity exercises for recovery, then a cool-down period. The high-intensity exercise should be done at near maximum intensity. The medium exercise should be about 50% intensity. The number of repetitions and length of each depends on the exercise, but maybe as little as three repetitions with just 20 seconds of intense exercise. The specific exercises performed during the high-intensity portions vary. Most of the research on HIIT has been done using a cycling ergometer, but other exercises like a rowing ergometer, running, stair climbing and uphill walking can also be effective.
There is no specific formula for HIIT. Depending on one's level of cardiovascular development, the moderate-level intensity can be as slow as walking. A common formula involves a 2:1 ratio of work to recovery periods, for example, 30–40 seconds of hard sprinting alternated with 15–20 seconds of jogging or walking, repeated to failure.

The entire HIIT session may last between four and thirty minutes, meaning that it is considered to be an excellent way to maximize a workout limited by time constraints. Use of a clock or timer is recommended to keep accurate times, the number of rounds, and intensity.

Quoted from:
https://en.wikipedia.org/wiki/High-intensity_interval_training