sentences:  His speed coupled with rugged power enabled him to be ranked as one of the best middleweights in the world and ultimately challenge Marvin Hagler for his world crown.A Middleweight, Sibson fought during the 1970s and 1980s, winning 55 of his 63 bouts, including 31 by knockout.He fought Marvin Hagler for the WBA and WBC middleweight titles in February 1983, losing by a technical knockout in the sixth round.