I pretty much do the same exercises for the same duration/repetitions/resistance and have been doing so for years. At 72 my goal is to just maintain fitness rather than improve. I alternate days at the gym with days of a brisk walk.
I use a Versa 2 with an iPhone 13 mini both at the most current patch level.
I have noticed over the last couple of weeks that I'm getting almost double the zone minutes for a given workout compared to what I used to get for the same workout before 2 weeks ago. When I compare the workouts, heart rate (avg & peak) are almost exactly the same, duration is almost exactly the same, personal health metrics (HRm HRV, SP02, sleep) are almost exactly the same and yet my zone minutes are double what they were 2+ weeks ago. I have not had a birthday since July - age hasn't changed.
Anyone else experiencing the same? Any suggestions as to why this is happening.
