System identification of human leg stiffness during rhythmic jumping on a perceptibly oscillating surface

In recent years, various man-made structures, such as grandstands and footbridges, have shown significant vibration problems during human jumping causing concern for maintenance and serviceability. To understand the interactions observed between a human and a structure, a 5.5m timber beam was constructed and instrumented. This was designed to simulate a cantilever tier of a grandstand, with similar natural frequency and damping ratio to the full-scale structure and with a similar mass ratio of a single human to the beam as for a crowd to the full-scale structure. Measurements of accelerations and displacements of both the jumper and beam, and of the contact force between them, were acquired. Tests for a range of target jumping frequencies, from below to above the structure’s natural frequency, were performed to identify the jumper’s dynamics and the induced vibrations of the structure and to observe interactions. System identification techniques were performed to evaluate and model the human leg mechanics of a jumper on a vertically oscillating surface. Force-displacement curves, for both relative (jumper to beam) and absolute displacements, have been obtained. The corresponding leg spring stiffness has been evaluated using a linear fit. Variations in the stiffness for different jumping frequencies is discussed.