Aikido has both external and internal aspects to its dynamic movements. We study '6 Harmonies' movement principles to develop internal strength as taught by John Burn Sensei (Chishin Dojo, Coventry Aikido Club). John Sensei is the UK's most senior student of both Hiroshi Ikeda Shihan and the renowned internal arts teacher, Mike Sigman. Learning internal strength develops a connected body and consequently improves the martial efficacy of the aikido waza.
Aikido without 6 Harmonies Movement, or, internal movement as Ikeda Sensei would say, is "empty practice and therefore not aikido".