The "30 Day Sit Up Challenge" app is designed for iPhone users looking to enhance their fitness routine with a focus on sit-ups. The app's main function is to accurately count sit-ups completed each day, aiding users in tracking their progress over the 30-day challenge. Additionally, the app offers features for managing achievement days, receiving notifications, and recording active energy and workouts to Apple Health.

To use the app, simply place your iPhone on your abdominal area, tap the screen to begin, and hold the device with both hands while performing sit-ups. The app calculates active energy expenditure based on factors like METs, weight, and exercise duration, providing users with insightful data on their workouts.

The app supports multiple languages including Japanese and English, and offers optional permission requests for accessing Apple Health data, enabling tracking for personalized ads, and sending notifications to users.