“Aiki requires an enormous amount of solo training. Only amateurs think that techniques are enough.”

– Yukiyoshi Sagawa

(more from Yukiyoshi Sagawa linked in the comments)

Here’s an interesting passage from Kisshomaru Ueshiba on solo training, from 1963. Unfortunately, he was unclear on exactly what these movements were, or how they would be performed:

“Solo Practice (Tandoku Dōsa)

​These movements are designed for practicing Aikido techniques alone, without a partner, in order to cultivate “breath power” (“kokyū-ryoku”).

The primary element of what is called “breath power” in Aikido is “ki-ryoku” (“willpower”). Uniting the different types of physical strength, this manifests in a trinity of spirit (ki), soul (tamashii), and body (tai) that is said to be the ultimate strength a human possesses.

​Through solo practice, one aims to fully perfect this power.”