|Active years||1979 - 1997|
|Championships||500cc - 1993|
Martens' first motocross race was in 1979, and by the mid 1980s he made the breakthrough into Grand Prix motocross, originally on a privately entered Honda, before a switch to the KTM factory racing team, culminating in a best result of second in the 500cc world championship in 1991.
For 1992 Martens switched to the Husqvarna factory team, riding the fearsome Husqvarna 610 four-stroke machine. Many felt this was a backward step for Martens, as unlike today, four-stroke machines were seen as slow, bulky and uncompetitive compared to the two-stroke machines. At times Martens had a difficult 1992, but some encouraging results later in the year saw him finish 11th in the series.
However, 1993 was a different start. After a steady start to the series at Hawkstone Park, Martens quickly proved himself and his machine, rising to the top of the 500cc World standings, with Swede Jorgen Nilsson. The championship was contested until the very final round, where Martens clinched the title, the first four-stroke rider to do so in over 20 years. In addition, Martens performances on the fearsome four-stroke machine earned him something of a cult following, particularly amongst British fans, where he was every bit as popular as the home riders.
1994 saw Martens once again challenge for the title, his main rival being another Swede, Marcus Hansson. Once again the title was decided on the very final round, but a fall in the final round cost Martens the title, as he narrowly took second overall.
1995 and 1996 were blighted by injury, and Martens final season as a rider was in 1997. By this time, Martens was seen as something of an also-ran, but he managed to upset the form book and produce some impressive results, including a moto win at the British Grand Prix at Hawkstone Park, which was popular with Martens' large contingent of British fans.
After retiring, Martens stayed in the sport by assisting to run the factory Husqvarna team. However in recent years, Martens has been a successful team principal, running the successful factory JM Racing KTM team. Under Martens' leadership, the team has won the World MX3 title with Belgian Sven Breugelmans, and also support young Belgian Joel Roelants in the World MX2 series.