From Oztrack/futuresprinting:
Michael Johnsons turnover was higher & would have resulted in higher efficiency due to less loss of energy in the early stages of each foot contact. But it cant be forced to be faster. Faster turnover is a result mostly of quicker contact times with the ground and to a lesser extent better leg recovery dynamics. The way to improve turnover is to get stronger and more powerful - be up higher on the support leg, run with less of a quad pushing action and with more with hip extension. I have also found that a small volume of hurdle bounces on two legs eg 6 x 2 hurdles set at a high height will help in this area.
Contact times are also to do with how “active” the contact is and whether the athlete is too high up on the front of their feet and then need to roll onto the ground more (creating a delay before toe off). Foot contacts can also be improved by athletes doing smart low volume [plyometrics and making sure the athletes ankles are strong with heel/toe walking, sandpit mini bounces done barefoot.
Good core stability will also help decrease contact times as it will decrease energy lost on ground contact through absorption in the centre of the body.
I believe also that faster turnover can only be optimized by doing relaxed tempo training on the actual race surface - excessive use of grass tracks will result in increased contact times - as the surface returns less energy and because the athletes hold themselves slightly lower to improve stability because the surface is not smooth.
Charlie and others. What do you think of the last bit in bold?