i know this sounds a little strange but hear me out...
when you are training properly, your body has enough "energy" to go about fixing all the damage we do during training in time for the next work bout. unfortunately, we can not physically see these repairs, so it is hard to tell if we are working out at the proper rhythm.
here's an idea. can external wounds act as a data point for recovery rate? i knicked the palm of my hand about 10 days ago. nothing major, just snagged a corner of rough metal or something and didnt even notice it. however, the little scab is still there 10 days later. normally, a little thing like this would have healed. however, i have also recently bumped up my mileage as well.
i know from interactions with runners with eating disorders that bruises and cuts take a very long time to heal without proper caloric intake, especially while running high volume.
can this be used as a diagnostic? imagine that - a standardized "finger-prick" that, when monitored, can tell an athlete or coach truly how recovered an individual is from a workout, hard race, hard week, etc.
any ex phys guys want to add some science to this absurd claim?