This may already be planned as part of the journaling feature, but since it hasn't been mentioned specifically, I'm creating this as a separate feature.
The idea is to prompt the user through a guided questionnaire every day, and generate a “subjective score” that may be used as part of the Recovery calculation – but, as @Markus Eisen points out below it should probably be a separate Subjective score and displayed as its own trends chart.
The questions presented to the user can be enabled/disabled so the user only needs to answer questions that are relevant to their training goals.
This is similar to how HRV4Training uses both HRV and subjective annotations. I'm currently using this, and log my subjective values when I do a Breath session every morning. The questions I configured are:
  • How was your sleep quality last night?
  • How motivated are you to train today?
  • How is your current physical condition?
  • How is your mental energy this morning?
  • How is your muscle soreness this morning?
  • How fatigued are you this morning?
  • Are you currently injured?
  • How would you describe your lifestyle? (Unstable/travel/stress ↔︎ Routine/can focus on training)
  • Were you traveling yesterday?
  • Are you sick today?
Our bodies already accurately report a “recovery score” by producing feelings of soreness, fatigue, etc., which is generally accurate – we should use it! Generating a Recovery score based only on biometric data is difficult, but by including subjective experience into its calculation we can get more useful clues as to whether and how to adjust future training. I'm not sure if it should be included as part of the Recovery score (is that how HRV4Training does it?) or to display it only as a separate Subjective score. What do you think?