You can get a yoga mat for very little money, but the benefits are tremendous, especially if you workout in a room with a hard floor.
A yoga mat lets you comfortably perform regular crunches, bicycle crunches, planks and a variety of other ab movements no matter what type of floor your gym or workout room has.
Using a stability ball turns just about any exercise into a core exercise, making every second of your workout a bit more beneficial.
Performing ab movements, like crunches, on your stability ball allows you to safely increase the range of motion and get a better ab workout in a shorter amount of time.
An ab wheel is a bit more of an advanced piece of ab equipment, but you certainly can't discount it's effectiveness.
Just a few seconds of using an ab wheel and your entire core will be begging for mercy. If you are a complete beginner, you may not want to dive right into full repetitions on the ab wheel, but intermediate trainees could reap great benefits from this piece of equipment.
In order to stave off injury, you need to train your entire core, including your lower back. A hyper-extension/Roman chair bench will allow you to do flat, decline and twisting sit ups as well as hyper-extensions for your lower back to give you complete core workout.
A simple door mounted pull up bar can let you perform leg raises with a full range of motion. By simply hanging beneath the bar and pulling your knees up toward your chest you can effectively blast your lower abs and obliques for a very intense ab workout.
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