18+ Megami no Sprinter

Takase is the opposite of his brother. He's weaker and struggles in school. He recently moved into his married brother's house in order to receive coaching to improve his physical health. The coach, also known as the "Venus of Track," is none other than his sister-in-law. The Venus of Track is researching and developing a new technique to improve male performance by testosterone management. In other words, manipulation of sexual desire. She has dubbed it "Absolute Obedience" and Takase must undergo it to improve himself. Takase's life is in for a dramatic change...

(Source: MangaDex)