Sit-ups should be done every three to four days. As sit-ups are not the most effective way to build muscle or burn fat, they should be done as part of a more involved exercise routine.
Sit-ups target a small portion of the core muscles and should be done in sets of 12 to 20 repetitions, and weights should be used to add resistance and improve results when needed. These should be done once every three to four days to allow the body to rest and build muscle while also allowing time and energy for other exercises.
Solely focusing on sit-ups as exercise can lead to muscle imbalances. This can also lead to middle and lower back problems. A balanced exercise routine should include sit-ups along with core strength training and cardio to achieve the best results possible.