I had a look at wikipedia, and it seems that there are conflicting studies about this:

This view was largely unchallenged until the late 1980s. Since that time, several studies have shown that transitioning from walking to running actually resulted in an increase in energy expenditure, while other studies have supported an energetic benefit from the transition. In the time since the energetics optimization view was first challenged, a number of mechanical, kinetic, and kinematic factors have been explored to explain the transition. Weak to moderately strong correlations have been found between several variables and the PTS, but work from a variety of researchers in the 1990s and 2000s agrees that ultimately it is fatigue and discomfort (or imminent fatigue/discomfort) in the tibialis anterior and other dorsiflexor muscles of the ankle that is the primary stimulus for the transition from walking to running in humans