Hi Ionel, there are numerous types of sensors for insole measurement. The two most common forms are capacitive and resistive/FSR. The most suitable type will depend on the specific application and in particular, demands of the activities you are interested in. I've attached a nice review article by Abdul Razak et al (2012), published in Sensors. This may be useful for your requirements.

Lonel, 

I cannot suggest the Piezo-Electric sensors 

because they respond only to varying pressure applied. 

However,

we did a study in the 1980's, using FSR pressure sensors.   This study in Wheel Chair seating characteristics was done to support the work of D.A.Hobson, Ph.D., in our Rehabilitation Engineering Program, at the University of Tennessee.

Among the sensors tested for use was a type of rubber resistive strip ("RS") , with variable resistance properties.   These RS  were supplied in 8 inch lengths, 1 inch wide, 1/16 inch thick.   Resistance varied as pressure was applied to a one inch square piece of this sensor.  We spaced them with about 1/16 gaps.  Attaching wires was a problem.  One of the options we investigated was the use of two simple silver contacts near the center of each "RS" pad.  

I have located a source for Interlink Electronics 0.5" Circular FSR on the internet.   These devices work well with constant force applied, and with varying force applied.    

You can google for "Wheel Chair Seating Hobson " .   I have just downloaded severral PDF files by D.A.Hobson, Ph.D., who was our director at the Univ .of TN in the 1980's.  Dr. Hobson is Prof. Emeritus at Univ. of Pittsburgh. 

My profile is listed on ResearchGate.Net. 

I wish you success. 

any kind of pressure sensor should work.

I would second the suggestion on 'piezoelectric sensors'

Lonel,

I cannot suggest the Piezo-Electric sensors

because they respond only to varying pressure applied. 

You would have to gather samples of this data and subtract the 'dead' sections, via computer processing in order to see the history of the Insole Pressures.  The data is non-continuous and requires computer processing. 

However,

we did a study in the 1980's, using FSR pressure sensors,

which are constantly sensitive to existing pressure.  

No conversion of multiple data reads would be required. 

These provide a continuous measurement of resistance during both active and non-active periods of Insole usage. 

The FSR allow both D.C. non-varying measurements 

and also A.C. varying measurements.  So, given Time(0) to Time(n), you can make continuous resistance measurements, and collect all the data, and analyze the data  in a very standard manner.  

I have located a source for Interlink Electronics 0.5" Circular FSR on the internet. These devices work well with constant force applied, and with varying force applied.

This is a sample table of readings and characteristics. 

Force (lb) Force (N) FSR Resistance (FSR + R) ohm

Current thru FSR+R Voltage across R 

0.04 lb 0.2 N   30 Kohm   40 Kohm  0.13 mA   1.3 V

0.22 lb 1 N      6 Kohm     16 Kohm  0.31 mA   3.1 V

2.2 lb   10 N    1 Kohm     11 Kohm  0.45 mA   4.5 V

See the link below for more scientific information.

Pads are about $8 (U.S.) each,

and have good electrical contacts. 

Good Luck, 

Glen Ellis

https://learn.adafruit.com/force-sensitive-resistor-fsr/using-an-fsr

http://www.robotshop.com/en/interlink-electronics-en.html

Lonel, 

The linked URL is a new type of sensor from M.I.T., 

our leading tech university in the U.S.    

You may find an idea for future developement. 

Good Luck. 

https://www.youtube.com/watch?v=lvtfD_rJ2hE

Thank you everyone for all your answers. The debate can continue as I am interested in as many opinions as possible.

Dear Ellis your suggestions are very promising but I'm looking for something that could fit in a shoe and your last idea isn't very successful for me, but still interesting, I have other ideas that arise from this video.

Thank you.

Please don't stop answering my question.

Lonel,

OK. 

I am back to considering   FSR pressure sensors . 

And I did find several primitive strips of this material, 

and "yes" if I press on the strip with my thumb, the Ohm meter moves.   Gee, this looks like fun !  

The FSR available ( as posted previous )

are far more usable.  

Good Luck.

We collected FSR data via an array of op-amps

( Instrumentation Amp circuits ) 

and scanning ADC circuit,

which data were fed into an old Apple-II

( almost the original Apple with the 6502 processor

and the wide flat box cabinet.) . 

The FSR approach, for me, was a one-man-project ! 

We did "seating studies" for persons confined to wheelchairs. 

* We used Air Bubble Sensors,

with many air tubes and transducers.   

* We used  FSR pressure sensors, 

with many small electrical leads.  

* Others do not come to mind at the moment, 

but we considered all the available tech at year 1980. 

http://www.geocities.ws/glene77is/

Thank you.