Futei no Olympia

A dull, unreliable middle-aged salaryman, Tamaki Kuroudo, is treated as a hopeless old man by everyone. Teased about his pot belly, treated coldly by his daughter at home, and when he happens to glance at his wife’s phone, he sees suspicious call logs with one of her coworkers. As he spends his pathetic days, his capable subordinate Ayase Mizuki invites him to the equipment room. While helping her move some equipment, he accidentally touches her breast and she records that very moment on video. After work, he’s blackmailed with the video and taken to, of all places, a sports gym? The price of sexual harassment is body transformation? A secret muscle-training comedy he hides from his wife!

(Source: MangaDex)