IIRC, one of the TNG eps (I think it was Chain of Command) had a portable single-direction forcefield, and there was a similar unit in Insurrection.
There were forcefield tripods against the firestorms in "Lessons", and there were transporter jammers (that did not resist weapons fire in any observable way) in ST:INS. The putative platoon shield would probably be more like the former than the latter - although one would expect Starfleet ground troops to carry those jammers, too, and for this to be the reason why Picard had them on stock.
Whenever a bad guy's got a magic personal forcefield, he isn't bouncing around the walls.
Indeed, the guys most famous for their forcefields, the Borg Drones, are notorious for their zombie gait. There might be a connection there.
The black-and-white people of Cheron had something looking like the TAS skintight envirofield, yet with the ability to resist phaser fire and apparently a function in Cheronian-to-Cheronian wrestling as well. But again, that's advanced aliens.
A shield wouldn't need to be bubble-shaped either.
Certainly not. Although we don't know why some shields are skintight (over a starship or a person) and others have a simpler shape and some standoff distance. We have seen both types resist kinetic impact - but when skintight shields again became the VFX norm in late DS9 and ST:NEM, ramming also resurfaced as a valid combat maneuver. So we might weakly speculate that skintight fields are going to be poor performers in stopping bullets, shrapnel or knives. (Worf McGyvered a non-conformal shield for the purpose, mind you.)
Wouldn't a personal shield suffocate the user?
We've seen TOS era breathing gear, and it's extremely compact. Something like that could easily fit within the belt that erects the TAS life support field, leaving only the problem of air circulation. And simple fans or then peristaltic movement of the skinfield would take care of not only air supply, but of thermal regulation as well.
If you know a person can take a few hits before going down, use another way.
Which would seem to be our best proof that personal forcefields aren't in use by the UFP-level cultures of Star Trek. That is, their undeniable effect on combat is never taken into account either by the heroes or the villains.
OTOH, the total lack of wide-beam or beam-sweeping tactics might suggest that our heroes and villains indeed believe that they need to punch their opponent with concentrated firepower. This, when the natural assumption would be that even the weakest glancing blow or "side lobe leakage" effect from these extremely powerful death rays should be enough to take a fragile humanoid out of the fight. So the conclusion ought to be that troops trained for combat believe their targets aren't vulnerable to glancing blows in the general case. (That a Federation starship landing party indeed is vulnerable is a statistically insignificant element in that training, and something the average Klingon or Cardassian doesn't have to take into account.)
The annoying thing is that this personal shielding issue is actually threefold: we have evidence that there are no phaser-stopping personal shields, we have evidence that forcefields aren't commonly used for stopping lesser threats like sticks and stones - and we still see physical armor worn a lot. Klingons have it, Cardassians have it. For all we know, the Federation pajamas are also armor; the TMP security gear certainly appears to be. These folks opt to wear obviously clumsy and movement-hindering gear. But the litmus test fails: all of these "armor" types have been shown to be vulnerable to knives, kicks, fists and karate chops.
Are we to think that Klingon and Cardassian and possible Federation armor is worn because, while it's useless against kinetic attack, it reduces phaser impact by X %? We did see one Federation trooper survive a disruptor blast to his chest apparently by virtue of his uniform in "Nor the Battle". And we can't really tell if all those Klingons hit squarely in their chest armor really die, or merely get stunned enough that they play no further role in the episode.
Timo Saloniemi
