Having won the Vuelta a Espana at the age of 41, Chris Horner has found himself in a cloud of suspicion, with several people doubting his performances. In a lengthy interview with the ProCycling Magazine, the American has responded to the claims, giving a detailed description of the gradual improvement of his result as the testing has become better.

Chris Horner enjoyed the best moment of his sporting career when he stepped onto the podium in Madrid to be celebrated as the winner of the 2013 Vuelta a Espana. In a time when the doping debate continues to make headlines, the win has not been a cause for pure joy as the 42-year-old American has found himself at the centre of several doping accusations.

What has triggered the debate is the fact that Horner's win came at the age of 41 and at the end of the season where he had barely raced due to a knee injury. Between the Volta a Catalunya in March and the start of Vuelta in August, he only rode the Tour of Utah but showed fantastic form right from the beginning when he won the third stage of the race in Galicia.

To quell the speculation, he released all of his biological passport data but it has not been enough to silence his critics. In a lengthy interview with the Pro Cycling Magazine, Horner has tried to end the accusations by explaining how his results have become better at the same time as testing has become more advanced.

Horner got his first taste of European racing with the FDJ team in 1997 but failed to make much of an impact in cycling's heartland. He returned to his native USA to ride on the domestic scene in 2000 and rode for American teams for several years. He returned to Europe in late 2004 with the Saunier Duval team.

His first big win came at the 2005 Tour de Suisse when he won a stage and went on to become a trusted lieutenant for Cadel Evans at the Tour de France. He had a similar role for riders like Lance Armstrong, Alberto Contador, Levi Leipheimer and Andreas Klöden on Astana and Radioshack but gradually improved his performances.

The culmination came at the 2010 Vuelta al Pais Vasco when he won the World Tour race overall and he went on to finish in the top 10 in the Tour de France. One year later, he finished 2nd in Pais Vasco, 3rd in the Volta a Catalunya and won the Tour of California but a bad crash in the Tour de France put his career temporarily on hold.

He came in early 2012 and led the Tirreno-Adriatico for several days before ending 2nd overall. He failed to live up to those results in the final part of the year but was back at his best when he returned to the Tirreno in March. He climbed alongside the likes of Chris Froome, Vincenzo Nibali and Alberto Contador and finished in the top 10. A few weeks later he was, however, sidelined with his knee injury.

According to Horner, that gradual progression goes a long way to prove that the doping accusations are unfounded.

"I had a difficult time in Europe," he said. 2It’s difficult to say whether the drug problem was there or it wasn’t there, and that was the reason. I was 10 pounds overweight; that was one of the reasons. Maybe 2002, when the EPO test started coming out, then they started perfecting it, then they started doing at-home out-of-competition testing, then the biological passport. All that has helped my career. As the testing has gotten better, my results have gotten better.

"When all the information is out there, you can go look. You can go see, in 1997 I didn’t finish any World Cup races except for the one in England. There’s a reason why I finished that one; nobody wanted to be there. But I couldn’t finish Flanders and Liège-Bastogne-Liège. And I couldn’t get results at those races. In 2001, I got 50th at Fleche Wallonne.

"In '97, I could finish third at Plouay. Nobody over here [in the States] knows what Plouay is. I couldn’t finish the big races. The first time I started getting results was at the end of 2004, I think I was eighth at the world championships after getting dropped on the last climb and then coming back with [Erik] Zabel. I stayed with Zabel and he’s a sprinter, and he had two or three German teammates and I knew they’d pull the whole way to get us back on. His boys did all the work and I sat on the whole time. I did the sprint, put myself in a good position and good eighth. That was the first real result I had.

"In 2005, I went over there (with Saunier Duval) and I did okay at Catalonia, I think I was 16th or something. I then won a stage of the Tour de Suisse."

Contacted by Cyclingnews earlier this year, Horner refused that he was the one known as Rider 15 in Levi Leipheimer's affidavit in connection with the Lance Armstrong affair. The United States Anti-Doping Agency (USADA) has redacted the names due to possible links to ongoing investigations.

In the ProCycling interview, Horner is clear in his response to the allegations.

"Nope," he said when asked whether he was Rider 15. "Well, I don’t know who rider number 15 is but I never had that conversation. So, it’s not me. USADA has never contacted me. Clearly I’ve never been in touch with the Puerto investigation, the Italian investigation, yada, yada, yada."

You can read the entire interview with Horner in the ProCycling Magazine.