Cool to read... π Have you tried the Norwegian Protocol? 3 min slow, then 4 min 85-95% max HR, then repeat 4 times. Gold standard training method for increasing VOβ max. It's been shown to improve VOβ max in as little as one training per week.
A study involved 50-year-old untrained individuals who used the Norwegian 4x4 method. After 2 years of training, researchers found their hearts was β20 years youngerβ after including the protocol once a week in their training regime.
Another study compared low intensity, lactate threshold, 15/15 intervals (15 seconds on & 15 off) & the Norwegian protocol. The types of cardio that increased V02 max the most were the 15/15 intervals (+5.5%) & The Norwegian Protocol (+7.2%)
https://pubmed.ncbi.nlm.nih.gov/17414804/