In an article entitled “Three Norwegian brothers all European 1500 m champions: What is the secret?” in the International Journal of Sports Science and Coaching, Tjelta concludes that “an active childhood, a gradual progression in training volume, strong family support, mental toughness, a high volume of training at and above anaerobic threshold, and mindful monitoring and regulation of training intensity have brought these brothers to a top international level in distance running.” The brothers train hard and smart, their weekly mileage averaging 140K to 160K with 23 to 25 per cent at or above anaerobic threshold pace, and they seem mostly to avoid injury.
https://runningmagazine.ca/the-scene/team-ingebrigtsens-formula-for-success/