I will use the ground-breaking spectroscopic LEGA-C survey to understand the star-forming phase of disk galaxies at a look back time of 7 billion years by studying the dynamics of ionised gas. The new data is of unprecedented high quality for its sample size of thousands of galaxies and contains for the first time detailed information about the dynamics, ages and chemical composition of the stellar content. I will constrain, for the first time, the radial profiles of gas velocity dispersion, which is of paramount importance to accurately measure their total masses and to test different hypotheses on the origin of disordered motions in distant galaxies. The radial profiles will set a new standard for future observations with the upcoming generation of telescopes and will provide improved constraints on scaling relations such as the Tully-Fisher relation of stellar mass and rotational velocity. The ultra-deep data can also be used to trace the faint outskirts of galaxies and to derive constraints on the amount and distribution of dark matter. My work will significantly improve our knowledge of the evolutionary history of galaxies in the universe today, including the Milky Way, and will serve as an explicit test of the current generation of high-resolution cosmological simulations.