Stuart Holden hasn't played since 2014. George Frey/Getty Images

The next step in Stuart Holden's long, tortuous comeback from injury has seen him invited to United States national team camp in Carson, California.

Holden, who has suffered a horrific run of injuries over the past five years, confirmed he is participating in the camp that started on Jan. 11 and is scheduled to continue into the first week of February.

U.S. manager Jurgen Klinsmann invited Holden as a guest, a U.S. Soccer spokesman said. The former Bolton Wanderers midfielder is not on the squad that will play in friendlies against Iceland on Jan. 31 and Canada on Feb. 5.

While playing for Bolton in March 2011, a tackle from Manchester United's Jonny Evans left Holden with a gash in his left knee that required 26 stitches to close, as well as a fractured femur near the knee joint.

Holden later required microfracture surgery to fix a problem with the articular cartilage in the same knee.

A subsequent loan to Sheffield Wednesday in May 2013 suggested that Holden had recovered, but he tore the ACL in his right knee while playing for the U.S. in the 2013 Gold Cup. He then reinjured that knee during a reserve match for Bolton in March 2014.

Holden has been sidelined ever since and done television work for a variety of outlets including ESPN FC, leading to speculation that he was heading towards retirement.

But he has continued to train in a bid to return to fitness, and had been working out in California this winter with a group of MLS players that included Benny Feilhaber and Dom Dwyer of Sporting Kansas City, the New York Red Bulls' Sacha Kljestan, Servando Carrasco of Orlando City, the LA Galaxy's A.J. DeLaGarza, Hector Jimenez of the Columbus Crew, and D.C. United's Steve Birnbaum.

Now Holden is taking the next step in his recovery.