For the short answer, no, you cannot determine force production from EMG. At best, you might determine the relative force production within a muscle (or muscle group) but that assumes you had a trial in which the subject did a maximal voluntary contraction (MVC) and that the relationship between EMG amplitude and force production is linear, which may or may not be true. If the relationship is linear, you could normalize submaximal contraction EMG amplitude values to the MVC one and from that you would get relative force production values.

You also cannot compare different muscles (or muscle groups) on their absolute force production using EMG. At the same level of contraction force production, it is doubtful that two muscles would have the same EMG amplitude.

For the short answer, no, you cannot determine force production from EMG. At best, you might determine the relative force production within a muscle (or muscle group) but that assumes you had a trial in which the subject did a maximal voluntary contraction (MVC) and that the relationship between EMG amplitude and force production is linear, which may or may not be true. If the relationship is linear, you could normalize submaximal contraction EMG amplitude values to the MVC one and from that you would get relative force production values.

You also cannot compare different muscles (or muscle groups) on their absolute force production using EMG. At the same level of contraction force production, it is doubtful that two muscles would have the same EMG amplitude.

You can not. You can measure the MVC using the Isokinetic Dynamometer (n/m) and at the same time connect the EMG to the muscle or the muscle group to measure the muscle activation during the MVC.

Probably this model could help:

MS = nEMG .TFSA.F(l).F(v).σ

Where,

MF= muscle force

TFSA= : transverse physiological section área

nEMG= normalized EMG value

F(I)= force x length function

F(v)= force x velocity function

σ =specific muscle tension

Actually this is a good question we couldn't have the vital answer for this question. There were some studies finding a liner relationships between EMG and muscle force on the other hand some finding there was non liner relationships. Really is complicated because the variety of EMG measurement situations 

I think there were a lot of studies on the internet explain that 

Although surface EMG (sEMG)  amplitude has been used in the past as an indirect estimate of muscle force this is not a reliable measure. In fact, contrary to the opinion of many the relationship between muscle force and sEMG is not linear for many muscle groups but likely  curvilinear.

HUMAN MUSCLE IN VIVO'? YOU CANNOT RELIABLY DO SO ON A FINER SCALE.

You couldn't get a data regard force production by the EMG. I support the comment of Mr. Giuseppe, and the only one method that's will be gold, is the Expierment with EMG in targeted muscles groups during some tests by the Isokinetic dynamometer. In this direction you can find a lot of articles in researchgate 

Thanks for your answers.

In my opinon it´s not posible to measure the muscle force just with the EMG ... It would be necesary to use another devices that are actually made to measure the muscle force like the Isokinetic Dynamometer (i´m not sure, but i think it is the Gold Standard to measure muscle strength). Since the EMG may asses the ability of the muscle to generate tension by measuring the electrical activity of the muscle and its motor units, but doesn't measure the amount of force generated (like newtons, or kilograms) it´s necesary to use other devices wich actually asses muscle strength

I would like to add one more thing about measuring the force with the Isokinetic Dynamometer.  While using the Isokinetic Dynamometer you have to notice that you are measuring the Torque and you will need to use another equation to convert it to the Force.  This equation could be easily found in the related previous publications.

check this article it may help

Predicting maximum eccentric strength from surface EMG measurements

https://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/6536/1/pain2009[1].pdf

The second question is:

how can i find what kind of relationship there is between EMG and Force? (if is linear o not linear)