(color online) Variations of dimensionless mean radius of gyration of the soft boundary with (a) propelling force and (b) the initial particle area fraction, where R₀ is the natural radius of the boundary. The inset in panel (a) shows the magnified parts of curves to show the data at small forces. (c) Linear–log plot of the ratio of the mean radius of gyration of the boundary to the persistence length of a free active particle, versus propelling force which quantifies the degree of confinement.