He formerly played for Worcester Warriors and Harlequins and has made over 175 Premiership appearances to date.
Horstmann plays as number eight or flanker, and represented the England Saxons for the Churchill Cup tour of North America in July 2006, receiving Man of the Match twice.
[2] He was also a member of the IRB World Sevens Series squad in the 2003–04 season, winning in Hong Kong, Dubai and Twickenham.
[3] In 2014 he was part of the team that won the LV Cup and was rewarded man of the match in the final against Northampton Saints.
In the autumn of 2017 Horstmann launched a new training company, Team Kai, teaching leadership and teamwork skills and strategies based on his experiences operating in high performing teamwork environments during his rugby career.