Visualize his bouncing up and down.

Study: 97 athletes. Measured vertical movement vs running economy (e.g., how much energy is spent in running).

Results:

Greater vertical oscillation was the biggest factor impacting energy cost as it requires more work performed against gravity.

Source:

Medicine and Science in Sports and Exercise. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5473370/