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The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials [1-5]. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be determined [5-10]. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques enables to clearly demonstrate how the light excitation can lead to a modulation of the symmetry in ferroic materials [11]. By studying two asymmetric Bragg reflections (i.e. h0l and -h0l for instance) of a (001)c single BiFeO3 crystal, we show how it is possible to disentangle at the picosecond time scale the light-induced longitudinal and shear strain in the unit cell [11]. Due to a difference between the value of the longitudinal and shear velocities, the strain develops within a two-step mechanism. This temporal evolution of the strain within the unit cell highlights the transient symmetry breaking of the pseudocubic unit cell of BiFeO3. Moreover, recent experiments with thin films of multiferroic nanostructures having different ferroelectric domain organizations and different elastic and electrostatic boundary conditions will be presented. In these systems, we discuss the different physical mechanisms at play in the light-induced strain and in the ferroelectric polarization modulation. These results provide insights for the understanding of the physics of photo-induced strain including the modulatuon of the ferroelectricity with the light.