Barring secret technology I don't know about, it takes too much power for such a chip to transmit continuously. Most RFID chips have no power supply at all; they get their power from the antenna that is scanning them. This includes the implantable RFID tags (the cliche is "size of a grain of rice") that you can buy for your pets
Active RFID chips have power supplies; they can transmit longer distances, can autonomously initiate transmissions, and also perform other actions such as collecting and storing data. However, the battery makes them larger; a device implantable in the arm can't be very large (though much larger devices, such as pacemakers, can be implanted in other ways). Most likely, if it's been designed for the stated purpose of collecting biomedical data, it has just enough battery power to collect and store the data over the time period required. There's no need to waste power on autonomous transmission because it's cheap and easy to scan the study subjects as needed. (As military personnel, they can be ordered around, though more likely the researchers will try to instead put the scanners into places where the soldiers' normal routine will bring them into contact, to avoid the chance of special procedures altering the results.)
I have mixed feelings about the whole thing. Misuse of technology is an issue, no doubt. But there's certainly nothing new about soldiers being subjected to things that most civilians would object to. The rest of us don't live in barracks, march in step, or even salute each other, despite the military doing those things for millennia. There's not much precedent for testing things on soldiers as a sneaky way to introduce those same things into the general population.
I also see wanting to study the sources and effects of stress as positive progress. Stress does a lot of harm, mental and physical, some of it tragically long-lasting, to large numbers of soldiers. The commanders could just accept the status quo and say, "if they can't handle it, screw 'em," and hardly anyone would blame them for not trying to solve a problem that's existed for as long as there have been armies. Trying to do something about it is admirable.
U.C., if you're like most soldiers, you are willing to put yourself at risk primarily for the sake of your brothers in arms. Since you're not being given a choice anyhow, why not think of this as taking a little extra risk of a different kind, to help save them from something that lays almost as many of them low as bullets and bombs?
Respectfully,
Myriad