Bear_Nenno had a great series of posts on what a giant PITA it is to change a track on a tank, and the physical strength it requires. This kludge of a search function won’t let me find it, but perhaps he can link back to them?
For the infantry, the increased weight that soldiers have to carry now, as opposed to the past, is precluding many men from being able to keep up with the demands, never mind women. The new generation of protective equipment is literally a life saver—being able to reliably stop rifle bullets is amazing, and unprecedented in infantry warfare—but this equipment weighs a lot. Further, all of this weight is accelerating the rate of knee and lower back injuries as compared to times past. I’d like to find a confirming link with hard data for that statement, but have only been able to track down anecdotal evidence, such as this guy claiming that 70% of returning have serious chronic injuries. (Anyone have good data from, say, the VA on the subject?)
Still, it takes a toll on your body, jumping in and out of a Humvee or Stryker while carrying 50+ pounds. See, for example, these posts (.pdf) on the increased weight that today’s soldier carries, and efforts (.pdf) to lighten their load. From the Afghanistan operations link, page 7 and 8 list the different types of infantry loadouts and their recommended weights. This link goes into planning soldiers’ individual loads, and those loads that need to be carried by transport. An example of an individual infantry officer’s need for strength in combat, and a proposed strength training program for infantry, can be found here. His breakdown of what he had to do when a suicide bomber detonated within 10 meters of him, and the physical lifts those tasks corresponded to, is in the beginning of the article, and I found it rather interesting.
Artillery is about as bad. Moving shells, quickly, is exhausting work. (Also a concern in armored units—120mm shells don’t materialize in the breech by themselves.) This lengthy pdf from the Rand Corporation goes into mind-numbing detail about the MOS system the military uses to define its jobs. Within it, at page 28, the author notes that 43% of all MOSs have a “Very Heavy” physical demand category. This is the highest of the categories, and is the Army’s way of stating that people in that MOS can expect to lift things over 100 pounds and constantly carry more than 50 pounds. The MOS that has all “1”'s in the physical requirements serial is 13F, Fire Support Specialist, a specialty in the Field Artillery. Interestingly, it’s not 13B, Cannon Crewman. For those more interested in the subject, the Army did a study on physical requirements for artillery crew in the mid 1980s, and the pdf of their findings may be found here. Doubtlessly it’s been updated.
Anyway, if the guns are self-propelled then you also have a lot of the maintenance headaches that exist in armored units. Short version, the combat arms specializations in the U.S. Army require large amounts of physical strength and endurance, requirements that preclude their being performed by the majority of women in the Army. (And a good number of men.)
One factor that I don’t believe has been mentioned yet is that a large driver behind allowing women to serve in front-line combat is that—help me out on this, Bear—the path to upper-level and flag-level promotion in the Army is to command combat units (and attend Ranger school, it seems). Female officers would rather not have their careers top out at O-5 and O-6, and there just aren’t enough non-combat unit O-6+ jobs to go around. Have we already linked to Marine Capt. Katie Petronio’s article on the physical demands and desirability of allowing women to serve in the infantry? (We have in other threads on this subject.) It may be of interest here. FWIW, she disagrees with my thesis in the preceding paragraph about female officers being behind the impetus to allow service in combat specialties.