A neat little article via Low Tech Magazine
In the characteristic Chinese design a much larger wheel was (and is) placed in the middle of the wheelbarrow, so that it takes the full weight of the burden with the human operator only guiding the vehicle. In fact, in this design the wheel substitutes for a pack animal. In other words, when the load is 100 kg, the operator of a European wheelbarrow carries a load of 50 kg while the operator of a Chinese wheelbarrow carries nothing. He (or she) only has to push or pull, and steer.
The result was an extremely powerful and agile vehicle. In 1176 AD, the Chinese writer Tsêng Min-Hsing noted enthusiastically: "The device is so efficient that it can take the place of three men; moreover, it is safe and steady when passing along dangerous places (cliff paths, etcetera). Ways which are as winding as the bowels of a sheep will not defeat it."