Another good exercise to do in strengthening your core and lower back is the plank. Form is extremely important in order to strengthen the core muscles rather than hurting. For instance, you must plank in a way that keeps everything in a straight line. This is especially true for your back. If your back is not straight, it will be unnecessary stress on your lower back and could lead to an injury. Since a plank is a common exercise for people to hold up to five minutes or so, form comes into play more so than other exercises. By doing this exercise right, your back will greatly thank you by being able to support you throughout your years.