when it comes to the core, it is a must to include exercises that challenge your stability as well. most of us focus mainly on using tougher resistance rather than challenging the abs with balancing movements. TRX band, stability ball, bosu ball, and medicine ball are just a few tools that can help us in this manner. like always make sure your breathing is in control(breathe out while contracting) and make sure you are flexing and extending your spine to do the job. you do not need to bend the hip.