Capes was a shot putter and represented his country over a span of 11 years, winning two Commonwealth Games and two Indoor European Championship titles. His first major games were the 1970 Commonwealth Games in Edinburgh, where he finished fourth. In the next two games in 1974 and 1978 he took the gold medal. In this period he also became the European Indoor Champion in both 1974 and 1976. His first Olympic experience was in 1972 when he competed in Munich. The 21-year-old Capes did not make it past the qualifying round, but improved on this considerably four years later. Having thrown his personal best on 28 May 1976 at Gateshead of 21.55 m. Capes went into the 1976 Montreal Olympics as one of the favourites for the gold medal. He came second in his qualifying group but went on to come sixth overall in the final, the winner being Udo Beyer of East Germany. 1980 was the year that saw Capes put the best distance of his career and increased the British record to 21.68 m (71 ft. 3.5 in.) in Cwmbran on 18 May 1980 being a new Commonwealth and British record. He went into the Olympics as the athlete with the best distance of the year so far and was once again a favourite for the title. However, he eventually placed fifth, the winner being Vladimir Kiselyov who although putting an Olympic Record of 21.35 m was well short of Capes' distance prior to the Olympics. Capes said of his performance at the 1980 Moscow Olympics that the result that left him "numbed with disappointment".