The relaxation of energetic quasiparticles in superconducting nanostructures involves many cascaded interactions between electrons, phonons and Cooper pairs. These dynamics are central to the performance of devices such as qubits or photon detectors. However, they are still not well understood, as they require experiments in which quasiparticles are injected in a controlled fashion. Until now, such experiments have typically employed solid-state tunnel junctions with a fixed tunnel barrier. Here we use instead the scanning critical current microscopy technique that we developed by taking advantage of a cryogenic scanning tunnelling microscope to tune independently the energy and the rate of quasiparticle injection through, respectively, the bias voltage and the tunnelling current. For high-energy quasiparticles, we observe a reduction in the critical current of a nanowire and show it is mainly controlled by the injected power and, marginally, by the injection rate. Our results prove a thermal mechanism for the reduction of the critical current and give insight into the rapid dynamics of the generated hot spot.