Here’s a recent case study featuring HRV changes in a pro soccer player throughout preseason training. Thanks to my colleague Alirezza Rabbani for inviting my collaboration on this one.
The main findings were that post-waking HRV and the HR-Running Speed Index (an indicator of aerobic fitness) during the warm-up jog progressively improved and were highly correlated. Additionally, we found that 1-min HRV measures were no different than 5-min HRV measures. The paper is free open-access at the link below.
HRVtraining 