>>279743068You don't need to lift that high. Just raising your head and upper body off the floor will activate your upper/outer stomach muscles. For the lower and deeper muscles, raise and lower your legs while keeping them and your back straight.
Finally, if you're stupid like me then you can do static exercises where you lay on your back, hands together above your head with straight arms (make like an "I") and then push your stomach/lower back towards the floor so that your feet and hands just barely lift off the ground. Hold for 30s. Really shitty painful but great for your deep stomach muscles