It's a mistake to think that front planks are the best exercise for people who have low back pain. Front planks have a heavy engagement of the psoas (hip flexor) muscles and can put more stress on the spine, especially low back.
Side planks can be a better alternative for a lot of people. They will target the side stomach muscles (transverse abs and obliques) which is more important for overall core stability. These transverse abdominals and obliques cover more surface area along the pelvis and are overall more important for the core.
