He played soccer and, in 2353 at around thirteen years of age, played against Worf in a school league championship game. While trying to gain possession of the ball during a corner kick in the second half, Mikel clashed heads with Worf and suffered a broken neck. Unaware of Mikel's injuries, Worf went on to score the goal and win the match for his team.
Mikel died in the hospital the next day and Worf felt extreme guilt ever since, restraining himself at all times lest he accidentally hurt someone with his Klingon strength. (DS9: "Let He Who Is Without Sin...")