Joel von Lerber plays the concerto by R. Glière in Cracow, Poland under the baton of Szymon Nasciszewski Audio: Borys Kunkiewicz Allegro Moderato Tema con variazioni, Andante Allegro giocoso
His legs could appear to be shaking because there are 7 pedals at the bottom of the harp, each with 3 different positions, that have to keep being adjusted while his fingers playing. (I hadn't particularly noticed any shaking myself.)
