How to determine RC filter cutoff of an ADC reading?
Usually data acquisition with ADC uses a filter to avoid "aliasing" a phenomenon in which a spurious signal appears on the digital side while it does not exist in the real life analog signal, this is due to the ADC bandwidth limit. Another reason for the filter is to avoid noise.

In your specific application, the foot movement probably won't need a great bandwidth as we humans don't make extra fast movements. In theory you should put the filter frequency somewhere in the middle between the ADC bandwidth (half the sampling rate) and the signal bandwidth of interest.

In your case a 1 kHz or so can be good. Beware of how the RC filter will load the output of your pressure sensor.