First, I would not fake a back injury nor would I advise anyone to. Second, I am not a doctor so I cannot give medical advice. Third, I would also strongly advise not faking an injury as part of an injury claim. At least in Nebraska, where I practice law, you could be subject to civil and criminal penalties for insurance fraud. FInally, from representing people injured in car accidents, the most common injury complaint is "whiplash", which is usually described as a soft tissue injury to the neck or back. "Whiplash" injuries are often difficult to prove in court as there are very few tests that definitively show a "whiplash injury". However, trained medical providers can usually detect something to show that a person sustained an injury.
As an insurance professional, I am truly impressed at the other answers. It's good to see some folks think faking an injury and committing insurance fraud is a bad thing. I can tell you that insurance companies carefully scrutinize "soft tissue" claims, claims for injuries that have no objectively verifiable symptoms, such as "whiplash". If you believe in Karma (and I do not) faking an injury might just bring about a real injury you would rather not have.