This is a reference to the 'Physical Energy' statue, a bronze sculpture by the sculptor and painter George Frederic Watts (1817-1904). It is located in Kensington Gardens, a Royal Park situated to the west of Hyde Park, and immediately adjacent to it.
Watts's statue depicts a naked man on horseback. It is based on an equestrian memorial that Watts made in 1870 of Hugh Lupus, an ancestor of the Duke of Westminster. Two castings of the 'Physical Energy' statue were made in 1902 and 1905. The first became part of the Rhodes Memorial in Cape Town, South Africa, and the second was placed in Kensington Gardens. A third, slightly modified, casting was made in 1959 and is currently in the grounds of the National Archives in Harare, Zimbabwe.