Lightweight pre-cast flooring systems using post-tensioning to increase strength but not stiffness are increasingly popular, and vibration serviceability problems tend to govern design of such floors where human occupants are increasingly concerned with vibrations. At the same time as inducing response, stationary human observers can also participate in the response as mitigating influence and it is clear that a human behaves as a highly effective damper, even when seated.Experiments were done to study energy flow and storage in a 1.2 tonne vibrating concrete plank with a human occupant. Results showed that damping could increase to as much as 10% frequency shifts (usually decreases) in the slab apparent resonant frequency, depending on occupant posture. Simple lumped mass mathematical models were also used to study the vibrating human-structure system through dynamic simulations, corroborating the findings.Further corroboration was provided from measurements on a prototype full-scale floor slab occupied by several hundred people who were either jumping or sitting. Modal analysis of vibration response signals showed that normal floor resonance associated with jumping at a sub-harmonic of the floor natural frequency was almost completely damped out by the passive (seated) people.