What is it?
Vertical Oscillation (Vert. Osc.) is the vertical movement of a runner's center of mass between steps when running. A human's center of mass is typically at the pelvis or hip, so an easy way to think about vertical oscillation is the vertical rises and fall of a runner's pelvis over one gait cycle (expected minimum vertical position at mid-stance of ground contact, expected maximum vertical position during float period). Typical Vertical Oscillation values will range from 3-15cm.
Why does it matter?
Ideal Vertical Oscillation will vary based on the individual. It is best used as a metric to monitor trends, rather than to aim for specific values (i.e. striving to attain a Vert. Osc. of 4.0cm). Trends will also differ from one runner to the next as they get fitter or faster over a training cycle, but it can be useful in considering how a runner's body may be responding to their training.
Vertical Oscillation can also be impacted by running uphill or downhill versus running on level ground. Again, the way in which one runner's Vertical Oscillation responds to uphills and downhills may differ from another runner.