Since getting back, I think I may have a handle on what is going on.
One of the things I did shortly after getting back from the Grand Canyon was play tennis. As I’ve noted before, I do that barefoot, and I’ve shown this little bit of footage showing me doing so:
During the fall, winter, and spring I play in an indoor league for about 2 hours a week. However, when it gets warm out, the group of us also play outdoors. Then my playing time can go up to 5½-6 hours a week. I think this is relevant to my being footsore.
But first let me digress just a bit about doing activities barefoot.
There is the old saying, “What doesn’t kill you makes you stronger.” That is obviously metaphorical, since losing an arm or a leg doesn’t literally make you stronger (except quite possibly mentally, which is the point of the adage). But it is also true that using and stressing the body (short of real injury) forces it to respond by making it stronger in those areas.
I’ve highlighted that before when I’ve stressed how weak feet get in shoes because they are “supported” and because the constriction of the shoe greatly reduces the flexing and strengthening response of the muscles, tendons and bones. The best example of this I can think of is that Rafael Nadal’s right arm bones are way larger than his left, because those are the bones that get the constant stress.
Doing hiking and tennis barefoot provide different kinds of strengthening.
When hiking, the varied surfaces provide stimulation to the sole. It helps build up callus, often with even a bit of thicking under the arch, as various twigs or uneven surfaces have a chance to stimulate it. Hiking strengthens the ankles as you walk over varied surfaces. Your whole bone structure gets to flex and stretch your ligaments as your foot conforms to the variations of the trail. You even get practice with your proprioception, working on balance based upon the feedback directly from your soles (and toes!).
Tennis is different. With tennis you are stopping and starting. The stresses on your sole are not vertical, but side to side. That provides a different sort of stimulation, and a different sort of response: different tissues have to strengthen to support that. And they do.
Additionally, while I still run with a more bent-knee stride while playing tennis, there are times I slap my foot down (anethema to regular barefoot running), and the stresses and strains on the muscles are completely different.
There is also one more kind of conditioning tennis does: getting used to hot surfaces. When I play tennis, in the late afternoon, the tennis court is usually right on the edge of being tolerated. You can feel the heat; if you stand in one spot too long, you might get burned. But simply by moving around, and occasionally standing on the lines while waiting for the next point, I’ve been able to remain barefoot and to get my soles used to somewhat warmer surfaces than they might otherwise encounter.
And now we get to why I suspect I am getting footsore too easily (too easily from my wishful point of view):
Tennis also wears down my soles rather quickly!
All that sideways motion on concrete is like sandpaper. No, I’m not sliding my feet, and I’m placing them straight up and down as much as possible, but with tennis, “as much as possible” is not the same as “not at all”. When I am done playing tennis, you can always see the areas in which the skin has been rubbed away. Not surprisingly, it’s pretty much on my ball and my heel.
As usual, in the month before I headed to the Grand Canyon, my tennis playing time had increased. So had my hiking time, as I tried to get in shape for the Grand Canyon.
But it is a constant battle between the forces of hiking and the forces of tennis as to whether my soles thicken or thin. Overall, that means that my soles just never get thick the way that soles get thick in native populations.
It also means (in addition to other possible causes, such as aging and thinning of fat off the soles) that I am more liable to get footsore.
I’m not sure how to fix this, or if it is possible. I enjoy both of those activities quite a bit. However, if I am planning another bit of long hiking, I may just need to lay off tennis for a month or so beforehand, just to give my soles a chance to thicken . . . and keep it.