I'm a runner (cross country) and I'm always amazed at how fast Olympic sprinters are. There's a lot of hype about those in the 100-meter dash being the fastest in the world, and we're constantly seeing the 100-meter time dropping. But is there a limit? How fast can human being, propelled only by its own energy (i.e. without any external assistance) run?
An older answer that referred to the physiological feats of ancient humans, and why modern humans are wimps made me look up a couple of papers on running speeds a while ago. One of the examples used to support the statement that modern humans are "wimps" is a study of ancient fossilized footprints from Australia, which claim that the individuals making the footprints were running close to Olympic record times. The footprint studies (Webb, 2007) basically attempts to describe the humans that have created the footprint paths, and also tries to calculate the speeds they were running at. For one of the individuals (T8), the estimated running speed is 37 km/h (i.e. 10.3 m/s). The interpretation by Peter McAllister in his book Manthropology seems to be (I haven't read the book) that these barefoot individuals running on mud could easily have outrun current day sprinters, especially with training and on a rubber track (also see this huffintonpost article on the book). If the estimates in Webb (2007) are close to accurate, it would seem that modern top-trained athletes might not have reached the edge of what is humanly possible when it comes to top speeds.
There is also a biomechanical study on potential human running speeds (Weyand et al., 2010) that you will probably find interesting. This paper looks at factors such as gait, stance, force application etc, and includes empirically estimated values from 10 subjects to discuss the limits to human speed. They conclude that (natural) human speeds are likely to be limited to ~50 km/h (as reference: Bolt's top speed during his 9.58 WR was ~44 km/h).
Consequently, human running speeds in excess of 50 km/h are likely to be limited to the realms of science fiction and, not inconceivably, gene doping.
They also look at different scenarios and e.g. point out how important limb length is for top speed:
Second, limbs lengthened through evolution or perhaps prosthetically could substantially increase the top running speed attained at the minimum period of foot-ground force application. For example, a relatively small increase in leg length of 10 cm would increase contact lengths by 9 cm and the top speeds of the subjects tested here from 9.1 to 9.8 m/s.
In general, important factors for maximum speed in animals are ground reaction force and ground contact time. An earlier study by Weyand et al. (2002) also show how faster human runners in general have shorter ground contact times compared to slower runners. As a sidenote, this seems to be in contrast to cheetahs, which have longer ground contact times than greyhounds, while still being much faster (Hudson et al. 2012 and popular summary).
View these as a couple of interesting examples and a starting point. This is not my area, and I do not know the literature well.