CO is a product of blood volume
per heart stroke
rate. The higher/better the stroke volume
(weak, strong, stronger contractions) the fewer heart beats (more for weak, less for strong, least for stronger) are required to meet demand (held constant for illustration) for blood oxygen. A well-condition individual requires fewer heartbeats due to strong contractions. It is the stroke that determines EF...not the CO, as CO can vary by an increase or decrease of HR.