I agree with what Fundibularius writes. Ancient shields can be two griped (a hoop through which the arm goes, a grip on the edge of the shield) or single gripped (a handle in the centre of the shield board). The Greek "aspis" (called "hoplon" only after the Classical period) is an example of the first type, the Roman scutum (and Celtic shield) of the second. Weight > Say 8 kgs. The sword might weigh 2 kg or less.

The shield stays "up" during manoeuver and battle; arm muscles locked into place (as any Dark Ages or earlier reenactor will tell you). Not clear what actually happens at the sharp end of edged weapon combat; e.g. Adrian Goldsworthy v. good on this. Probably front-fighters, in loose but not too lose formation, two deep, jostle and fight behind raised shields, with spear and sword, aiming for vulnerable bits and aiming for the knock out blow (throat; arteries in thigh. groin). Possibly men fall back to catch breath, and odd pauses happen. A lot of shoving with shields. As melee progresses, lines grow closer, spears break and sword work means fighters step in and can't fall back away from the killing zone anymore-- and eventually the two masses square off. But the actually mechanics by which one battle line physically drives away the other not clear (do people start panicking at the back ?).

Anyway, this is the model now proposed (Goldsworthy, Krentz, van Wees-- see e.g. the recently publishee Cambridge Companion to Ancient Warfare)-- rather than the old model of battle lines physically crashing into each other (Keegan, Hanson). But all the while, men are lugging contraptions the weight of small suitcases with their left arms.

Aussie: of course, I meant right arm.

Lycurgus: note that the idea that "aspis-fighters were useless outside of the phalanx" is now challenged. H. van Wees, Greek warfare, reality and myths, gives the strongest statement of this. Some references (I haven;'t got texts to hand): In Herodotos 5, there is an Argive, Eurybates, who likes to fight duels; the man who kills him, Sophanes of Dekeleia, is then famed for his prowess in battle (Hdt 9.73, i think); Sokrates, in the retreat after Delion, kept his shield and his cool, and scared off pursuing Boiotian cavalry (Plato Symposion); early C5th vases regularly show one on one fighting between aspis fighters; C7th Crete shows weapons captured by individual fighters, presumably in duels (Hoffmann, Cretan armorers); Thucydides (e.g. Book 4, Brasidas' fighting), and Xenophon (Anabasis; also Hellenica) show you hoplites, with aspis, carrying out all kinds of loose formation fighting. Of course, this is not to say that most hoplites or panhoploi fought in serried ranks; just that they were more of all rounders than sometimes admitted. The pyrriche, dance in arms, with its violent shield movements and twists and turns and stamps (very recognizable to anyone who fences), is presumably related to this

Finally, as a past reenactor (I stopped when I got stabbed in the eye with a blunt sword), I can say that double grip shield is no problem for one on one duelling (it's the single grips that are much trickier). Mediaeval guys do double grip shield duelling all the time (say C13th century guys)

FWIW-- and away from our Balearic humeruses.

Mark: sorry for delay in replying. Will look to see what I have-- but the answer is not very much at the time being. I still recommend Hans van Wees, Greek warfare, myth and realities !