Garmin FR620 – max curiosity

In mid December 2013 I bought a Garmin Forerunner 620 (FR620); my first ever GPS watch. I bought it primarily because my trusty Garmin FR60 was becoming less trusty, partly because Garmin were offering a 30% pre Christmas discount and perhaps partly because I had sprained my ankle earlier in the month and was feeling a little sorry for myself. However this post isn’t a review of the 620, though if you need one you’ll be hard pressed to find one more thorough than that at DC Rainmaker’s blog, nor is it an investigation or explanation of the 620’s VO2 max feature provided by Firstbeat Technologies; since I have little or no insight to provide. I am just curious to know where the 620’s estimate of my VO2 max is going …

historic VO2 max graph to 8 June 2014

My FR620’s various revisions of its estimate of my VO2 max. Taken from the ‘classic’ view at Garmin Connect.

Whilst not every graph tells a story this one does: Point A is New Years Eve 2013 and my first run with the 620. Previously it had remained shiny and new in its box, or at least on my wrist in purely decorative fashion, and had never been used. An initial value of 50 seems to have been the 620’s default starting point. Fitness wise I was about three weeks complete rest off full fitness having sprained my ankle earlier in December. By point B in mid February 2014 I’d returned to full training and was about to PB at the Brighton Half Marathon though my time, 88:16, was well short of the 74:50 predicted by the 620! At the end of April point C was my first run for 43 days, having sprained my ankle for a second time on 18 March, hence the straight line immediately preceding it. The 3 point drop in VO2 max estimate would seem a reasonable reflection of 6 weeks rest. A few days into May I reset my 620 to address an apparent issue and it appears to have returned to its default estimate of 50. Point D is a couple of runs after this reset. Point E is the 620’s latest estimate after five weeks or so of regular running. The table below summarises the five points and the 620’s associated race time predictions alongside my current PBs.

point date VO2 max 5k 10k HM marathon
A 31 Dec 2013 51 20:24 42:19 1:33:41 3:15:07
B 13 Feb 2014 63 16:19 33:53 1:14:50 2:36:33
C 30 Apr 2014 60 17:01 35:20 1:18:04 2:43:12
D 7 May 2014 50 20:50 43.12 1:35:40 3:19:08
E 15 June 2014 57 18:04 37:29 1:22:53 2:53:05
current PBs 18:55 39:33 1:28:16 none

Reviewing the rise from A through B it appears the 620 was initially acquiring data on my fitness, revising its estimate of my VO2 max accordingly and that it had settled on a figure somewhere between 60 and 63 since it had ceased to rise even before the tail off in fitness to C due to inactivity. Whilst the initial shape of the graph is understandable the actual VO2 max estimate at B seems excessive. Or at least the related race predictions seem highly optimistic given my current targets.

In this initial period I am not entirely sure that I had entered good estimates of my maximum or resting heart rates into the 620 and in fact it may have been an adjustment to one of these entries which prompted the end of the relentless rise in VO2 max estimate just after point B. I have assumed for now that these inaccurate entries were the reason for the apparent over estimation of my VO2 max / race performance.

At the point of reset I ensured accurate estimates were entered; 172 for my maximum HR and 45 for resting HR. The maximum HR may even be a little conservative; it’s the maximum value I’ve recorded during the closing seconds of a 5k PB. The resting HR I am more confident of as it is the average of a series of careful measurements which themselves varied very little. Either way these values should not encourage the 620 to over estimate again. Perhaps coincidentally the 620’s latest race predictions at point E appear just about realistic given my current PBs, or at least the 5k does; as the race distance increases the predictions again disappear into what I would consider unattainable optimism. The difference from event to event though I expect is more to do with my own ability/preference for shorter distances rather than any error in the 620’s VO2 max estimation.

So, as I said at the outset, at this point I am curious to know just where the VO2 max estimate is going: If it continues to revise its estimate upward along a similar trajectory to that originally recorded from A to B, I will probably have to conclude that the feature is interesting, but not useful except perhaps as a measure of relative fitness. On the other hand it may be that the more accurate values for maximum and resting HR enable the 620 to come to a more believable conclusion. Unfortunately as my ankle continues to falter in its recovery I’m again inactive as a runner so there’ll be no new data for the 620 to consider in the short term. Given that points A and B were just over 6 weeks apart and the 620 was still revising its VO2 max estimate it would seem that more than 6 weeks running, and all of it at full fitness and running health, are required to enable the 620 to settle on its best estimate.

I’ll post again when points F and G come to pass …

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