Tina, good evening I hope you are well and having a great Tuesday night.  With regard to your question, consider the following.   The oxygen demand of working muscles is directly related to mass, metabolic efficiency and level of work. In general in order to calculate maximal oxygen uptake the Fick equation is used: VO2 = Q * a-vO2 difference

In general if you do not have access to VO2 max testing you can determine training heart rate via the Karvonen formula as 60% to 80% of VO2 max is equivalent to 60% to 80% of training intensity as calculated via the Karvonen formula.

We hope this helps.  Take care and have a great night.