I recently exercised on the treadmill at Rachel’s Body Shop Gym with an inclination of 3% and a speed of 3.6 mph. I measured the resulting vibration with my Android Smartphone which was mounted against the console screen. My footfall or step frequency was about 2 Hz counting the total impact rate from both my left and right feet. My stride frequency was about 1 Hz.
Alternate footsteps apply forces in opposite lateral directions, the predominant lateral forcing frequency is one-half of the total footfall frequency. The spectral response in each of the three axes is shown in the following plots. The 1 Hz stride frequency has the highest corresponding amplitude in the lateral axis as expected. The various peaks represent the stride and footfall frequencies with integer variables thereof.
The treadmill is designed and built to withstand the impact forces and resulting vibration. But structures such as the London Millennium Bridge have required retrofits to mitigate resonance effects from pedestrian footfall.
Accelerometer time history data: treadmill.txt
– Tom Irvine