"People say to me 'oh you're so lucky that you're so fit' or 'you're so lucky that you're so strong'," Cass wrote in the caption. "The thing does not come out of bed at 4:00 in the morning. Luck does not put my trainers on. Luck really does not have anything to do with it. To be brutally honest it comes down to the combination of two things - hard work and consistency."
She says it's certainly not easy and feeling strong didn't happen overnight.
The 27-year-old was a competitive runner before she first stepped foot into a CrossFit studio about five years ago.
"I was completely out of my comfort zone. I remember thinking 'what have I gotten myself into?'. I had little to no upper body strength. I could barley do a push up on my knees let alone my toes, and the idea of doing a pull up was just insane. But the very combination of those two things is what has earned me the strength I have today."
And whether you want a six pack or you just want to set some new fitness goals, Cass has some wise words to keep in mind.
"The hard truth is that it will take time, it will take energy, sweat, tears and maybe even a little blood, but the feeling of being able to say "wow, I couldn't do that last week" will be worth every bit of it."
Cass says her abs are the result of consistently good nutrition and a core activating workout routine.
"To be honest I never actually do any abdominal specific or focused exercises," she wrote in a previous post.
"I never do 'leg days' or 'back & bi's" days or target one area over another. My workouts are nearly always full body functional exercises and just that's what works for me. What gets you results in anything you're looking to achieve is consistency. Set a goal and then be consistent in doing what needs to be done to achieve it."