You don't have to look far in sumo to find links with Japan's imperial family.

Every two months the top rikishi compete for a whole host of prizes, the most prestigious of which is the Emperor's Cup.

The sterling silver trophy stands 108 cm high and weighs almost 30 kg.

It is so named because Emperor Hirohito (posthumously called Emperor Showa) a keen fan of the sport, gave it to the Japan Sumo Association in 1926 before he ascended to the throne; it was originally known as the Prince Regent Cup.

Sumo itself initially was only performed in front of the emperor with the legendary origins of the sport dating back to a one-on-one battle that took place in Nara in 23 B.C. where Nominosukune defeated and killed Taimanokehaya on the instructions of Emperor Suinin.

In modern times members of the imperial family still attend sumo on a semi-regular basis.

When the emperor, crown prince or any of the imperial family are at a tournament they sit in the large convex box on the front side of the Kokugikan, accompanied by the chairman of the Japan Sumo Association.

The emperor's presence is never announced in advance for security reasons, but if metal detectors and a heavy police presence is in place from early morning, it's a sure sign that he is coming.

Princess Aiko had a noted love of sumo when younger and would follow the action when in Ryogoku Kokugikan with her parents — the current emperor and empress.

The ring entering ceremony is changed whenever the emperor is present. Instead of walking around the ring in a circle, rikishi crouch in rows facing the Imperial box then stand up and bow when their name is announced.