Jakobs trouble with his sacrum flared up during XC-champs in Lisboa 2019 (he did a quick turn at the last downhill stretch). According to his father (former coach), Jakob was very tired, both mentally and physically, after a long track season (World Champs in Doha was arranged as late as October due to the heat).
During the winter, he did a lot of elliptical and aquajogging. When he gradually build up his mileage, Jakob mention he had som difficulty with his adductor muscle when he ran his threshold session on the treadmill due the "slippery surface" from all sweat, but also because his aerobic engine was so strong that he had to reduce his pace to "only 2.45-50" pace (imagine having that problem...)
That was also someting his father Gjert saw when Jakob came back from high altitude camps. He was so strong aerobically that he could not* (*should not) getting over 2mmol/l during his threshold sessions on the treadmill due to too much muscular forces at the paces he was undercoving.
But again. Jakob has have a lot of "niggles" and sickness during the last years. What separates Jacob from his brothers is that his father never did the same mistake with Jakob as he did in the past with Filip and Henrik (Henrik in particular).
The thing that´s worrying me now is not that Jakob has the motivation and dedication to come back as strong as he was before he got more serious issuses with his achilles. What's worrying me more is the he will compensate his running stride in a way that will cause him trouble in other areas. Then he is stuck in a vicious spiral that Henrik has had to suffer through for many years.
We all know how smooth and relaxed Jakob looks when he is in his best shape. Everyone around him is suffering and he is just slips away.