Jason Derulo -- RISKING HIS NECK ... To Stay Jacked

Question: If you were thiiiiis close to being paralyzed after fracturing your neck in a freak accident in 2011 ... and you had to rehab for months to get back to normal ... would you risk it all for a killer physique??

Answer: ABSOLUTELY ... if you're Jason Derulo.

The "Whatcha Say" singer hit up a park in Sydney, Australia this weekend ... and bent over backwards for some kind of a handstand, pushup thing during a topless workout.

As we previously reported, J.D. screwed himself up bad just 2 years ago ... breaking his neck while doing tumbling and acrobatics during rehearsals for his tour.

Clearly, the guy feels confident his body is fully healed.