I own a HRM Pro plus and I can get several running dynamics (ground contact, cadence, balance); I wonder why same HRM is not able to measure step speed loss and associated running economy. I suppose that hardware of HRM Pro Plus is similar if not identical to HRM600, so it may be able to calculate Running Economy.
i suggest Garmin to include Running Economy for those owning HRM Pro plus and not only limited to HRM 600.