Michigan Compiled Law (MCL) - 750.226a. Pocket knife opened by mechanical device states the following:
Any person who shall sell or offer to sell, or any person who shall have in his possession any knife having the appearance of a pocket knife, the blade or blades of which can be opened by a flick of a button, pressure on the handle, or other mechanical contrivance shall be guilty of a misdemeanor. The provisions of this section shall not apply to any one-armed person carrying a knife on his person in connection with his living requirements. (Emphasis added.)
Thus, it depends how the courts construe "mechanical contrivance." Is a steel spring or bar a mechanical contrivance? Possibly. However, according to the statute, butterfly knives are legal because they are opened with inertia and/or gravity, and no mechanical contrivance!
I have not seen any cases on this yet here in MI, but if I do, I'll post the results.