JaM,
See how the spears are held at shoulder level? That means they are above the heart. See the angle of their elbow? They are activating the muscle fibers of the medial deltoid, similar to the manner in which the exercise called "lateral dumbbell raise" is done. Which means while blood flow should increase (as the muscle fibers are strenuously activated), gravity makes it so less blood will pump, so there will be an increase in muscle fatigue for the shoulders.
Now you're going to counter this by saying that they are resting the dory's shaft on the rim of the aspis. And then I'm going to counter that with an overhand grip a hoplite can do the same thing, when not directly fighting someone in front of them, they can hold their spear at a 45 degree angle and rest the portion of the shaft forward of the hand, behind the tip, on the upper rim of the aspis, without interfering with the man behind them, while resting the muscles of their shoulder, which still provides a guard for the front and a reasonable response time to attack with the spear again.
See my well illustrated and highly detailed Paint drawing below.
Now let me discuss some highly obvious issues with the picture from Chris Matthew's book of his "ready position." *Please reference the well illustrated and highly detailed Paint drawing below.
Picture A. are two opposing hoplites facing off from one another in Matthew's Underhand ready position. As we can see, they are well armored, solid aspis shields, greaves, and Corinthian helmet. Though there is still some distance between them its obvious from the angles what can and cannot be attacked with the spear. So what exactly is being threatened? Feet? Torso are out, the shield completely covers it, no need even for torso armor with a straight thrust blocked easily by the shield held in normal ward. With a underhand thrust, the angle of attack will either be parallel to the ground, or it will come upwards, which leaves a tiny bit of face not covered by the shield as the only target capable of being attacked. Even the neck isn't a target, as the shield rim protects it. Now compare the angles to any picture or video of an overhand attack, which can go parallel to the ground or over the enemy's shield on a downward angle, so even should an enemy hoplite raise their shield to cover their face, their neck, shoulders, and upper chest are still exposed, as are those of the men of the second rank.
Picture B. shows what happens when the two sides begin closing the distance. They are roughly at fencing distance from one another but neither side has any edge, even with the longer reach advantage, because both sides are too well protected for the spear to be of any obvious use using an underhanded grip. Merely by raising the aspis a measly few inches and covering the mouth/nasal area of the helmet, while leaving the eyes open, the hoplites are nearly invulnerable to spear attacks possible with an underhand grip. Again, with an overhand thrust, being at spear fencing distance from one another, the hoplites could thrust over each others shields to attack more vulnerable spots, or they could parry those attacks more easily (as evident by this video, the overhand grip allows for a more solid parrying ability).
Picture C. shows the two sides having closed the distance, about to start the famous Othismos portion of a battle, when both sides go shield to shield. Now at this point the underarm carry of the spear for the front ranks of both sides is useless, at this point the enemy front two ranks would have raised their shields to cover their faces, used the angle of their aspis front to deflect the spear points and passed under the spear points, tipping the shafts up in the air at angles that would mean the spear tips would hit nothing but air (similar to "push of pike" illustrations). So that means the only method the front ranker hoplites who started out with the underhand grip have to cause physical harm to their opposition is to drop their spear and use their sword. Interesting enough, when shield to shield, the only sword thrust or cut that would be effective would be one that goes over the enemy's shield, in the same manner that an overhand grip would allow for. Also worthy of noting is that when in othismos position, the overhand grip of the spear, and that its gripped more or less in the middle of the shaft shortening its reach, still allows the front two ranks of both sides to continue using their spears in an effective manner that is actually capable of producing casualties to the enemy.
When fighting in formation, attacking over an enemy's shield is a deadlier attack than attacking into the shield or under it.
One last thing. There are times that the underhand grip would be useful too, just not in a close fought battle. More of a dueling or usage during a route. But shield against shield, even when one isn't constrained by a rigid linear formation, there is little in the way of available targets for underhand attacks, unless the arm is completely extended upwards to do a downwards attack, similar to those shown in fight scenes in Troy and 300 (when both sides had spears and shields). In that case, its still simpler to use an overhand grip.
