Transmission of vertical vibration to the human foot and ankle.

The load absorbing capability of the foot and ankle system (FAS) was characterized by measuring the transmissibility and the phase delay at the medial malleolus and the tibial tuberosity. The FAS of twenty subjects were exposed to sinusoidal vertical excitation (10-50 Hz with 5 Hz increments and peak to peak acceleration of 17.9 m/s(2)) while sitting as a function of the external mass (0, 2.3, and 4.5 kg) and the foot postures (midstance, plantarflexion, and dorsiflexion). The results showed that the FAS plays important role in vibration transmission of lower leg. Adding extra mass affected a resonant frequency at the medial malleolus: 15-25, 30-35, and 35 Hz for with no additional mass, 2.3, and 4.5 kg, respectively. However, the changed postures of the FAS did not show significant effect on the resonant frequency. The applied mass affected the stiffness increase of the FAS and consequently resulted in the increase of the resonant frequency. This result supports the assertion that the resonant frequency of overweight or obese persons is similar to the major frequency component (25-35 Hz) of the heel strike.