Resting heart rate and even heart rate variability despite some claims are not direct indicator of VO2 max since VO2 max is dependent on the ability to absorb and utilise oxygen - not just transport it - pump it round the system.  A better way to assess VO2 max quickly is to do a sub-max test and use two different exercise levels to assess the increment in heart rate for a given change in work rate - the YMCA sub-max exercise protocol is a classic example.  This can be done in literally 9 minutes with only moderate exertion by a subject and is very safe for people form a wide range of backgrounds.

Mark, do not trust any method that can assess VO2max without the person work out. The evaluation of resting HR does not allow that the VO2max can be calculated indirectly because many factors affect the values of HR at rest, therefore invalidating your data or your value of VO2max. You can use submaximal tests that allow, by inference, that VO2max can be established. They are more reliable, but even so, the real value of VO2max can only get in maximum dynamic effort.

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