All consumer retail (mass market) treadmills have a deck surface laminate composed of melamine formaldehyde. It is essentially the same material found in Formica countertops. Its hardness is relatively high for a polymer, which provides good wear resistance. It also gives fairly low friction, especially when polished for smoothness. And there is another similarity between motorized retail treadmill decks and cheap countertops: MDF base material. Conversely, many of those extremely inexpensive self-powered treadmills (which are suitable only for walking) utilize plywood instead of MDF, but paradoxically, plywood is the superior of the two materials!
Plywood is not only far more dimensionally stable, but is also far stronger for its weight than MDF. Moreover, MDF is subject to fatigue cracking, which is not a problem with plywood. The one and only reason why MDF is found in retail motorized treadmills is COST. It is simply cheaper than plywood. So why then is plywood used for those cheap nonmotorized treadmills? Weight savings. Those machines are marketed mainly for petite single women having limited living space (such as an apartment), who may need to fold and stow the machine in a closet. For space and weight savings, their plywood decks typically measure only about 41" long x 15" wide, contributing to a total machine weight under 60 lbs.
If the deck of a motorized treadmill has a lengthwise crack propagating along its centerline, its MDF material is mostly to blame, but part of the cause is the poor design of the machine's cushioning feature. Essentially that cushioning is achieved via simple deflection of the front end of the deck, like a diving board. In order for that to happen, the frame cannot have crossmembers supporting any part of the deck where deflection occurs. So without crossmember support, the centerline of the MDF deck gets concentrated shear-loading along its centerline.
True commercial treadmills never have MDF decks. Moreover, they generally don't employ "diving board" type cushioning, because their frames are robustly built with crossmembers supporting the front, middle, and rear portions of the deck. So true commercial treadmill decks never undergo centerline cracking.