It does not make the belts of all treadmills up of a single body but has a heat seal. , precisely this type of welding causes the belt at the junction point to be thicker than its normal thickness.
This greater thickness means that the belt is slightly noisier during the first hours of use. In fact, with passaging time, the excess welding becomes thinner, causing less noise as it passes over the treadmill rollers.
Sometimes a zig zag is also visible as the two ends of the tapes are cut in a zig-zag pattern before welding and then fit together and heat welded.