I was thinking about footwear recently, and asked myself the question: "What exactly are the advantages and disadvantages of sole stiffness?" By "sole" I suppose I'm referring to the mid-sole or shank - not the softness of the part that makes contract with your foot (insole) or the ground (outsole), but the middle part. By a stiff sole, I mean one that doesn't flex much. Ignoring other factors like weight, here's what I came up with:
Considering the extremes seems to be useful. First, imagine an extreme case of strapping wooden blocks to the bottom of your feet. Then image the opposite, where you have very light sandals on - almost nothing there at all apart from a bit of cushion.
What are the advantages of the wooden blocks as opposed to sandal-weight shoes? An obvious answer is pressure dissipation on uneven ground. Imagine, for example, stepping on a fairly sharp rock. The light sandals will flex, which places a lot of your weight onto that small point of contact, causing lots of pressure in that area. Gripping your foot around the rock (if it's not *too* sharp) will mitigate this somewhat, but not entirely. In contrast, the thick wood will flex very little, so that the pressure is evenly distributed across the foot, regardless of the ground underneath (a soft insole on top of the wood would help to blunt the pressure further, but that's a separate matter). The relative difference would increase with more weight on the back: more weight, more pressure.
Okay, so what are the disadvantages? One seems to be that a stiff sole interferes with the natural flexing of the foot, so it makes the gait somewhat unnatural and, I assume, less efficient. It also prevents the foot from conforming around uneven terrain, which I think would make you more likely to slip (holding the grip of the outsole constant). Consider that rock example above. Stepping on a pointed rock with the sandals will put a lot of pressure there, but your foot will also conform and perhaps grip the surface better. The wooden shoe won't flex as much, so the surface area contact with the rock will be smaller, which means less grip. Similarly, I think you might also be more likely to rotate your ankle. You could imagine, for example, putting all of you weight on a focused point on the wood block: this would create a fulcrum point that will rotate if there is much more weight on one side. (That's where high ankle support is needed.)
It would seem from this analysis that a stiffer sole is most useful when you are going over rough, rocky/branchy terrain with more weight on your back. One would have a less natural gait, and perhaps not grip the surface as well which would lead to greater risk of slippage and ankle rolling, but a grippy outsole and good ankle support would help. In contrast, one would want a flexible sole on softer, flatter ground, and with lesser loads. This seems to conform with common knowledge - although I think my claim that stiffer soles -> more ankle rolling might be unusual.
What do you think? What am I missing or getting wrong?
Considering the extremes seems to be useful. First, imagine an extreme case of strapping wooden blocks to the bottom of your feet. Then image the opposite, where you have very light sandals on - almost nothing there at all apart from a bit of cushion.
What are the advantages of the wooden blocks as opposed to sandal-weight shoes? An obvious answer is pressure dissipation on uneven ground. Imagine, for example, stepping on a fairly sharp rock. The light sandals will flex, which places a lot of your weight onto that small point of contact, causing lots of pressure in that area. Gripping your foot around the rock (if it's not *too* sharp) will mitigate this somewhat, but not entirely. In contrast, the thick wood will flex very little, so that the pressure is evenly distributed across the foot, regardless of the ground underneath (a soft insole on top of the wood would help to blunt the pressure further, but that's a separate matter). The relative difference would increase with more weight on the back: more weight, more pressure.
Okay, so what are the disadvantages? One seems to be that a stiff sole interferes with the natural flexing of the foot, so it makes the gait somewhat unnatural and, I assume, less efficient. It also prevents the foot from conforming around uneven terrain, which I think would make you more likely to slip (holding the grip of the outsole constant). Consider that rock example above. Stepping on a pointed rock with the sandals will put a lot of pressure there, but your foot will also conform and perhaps grip the surface better. The wooden shoe won't flex as much, so the surface area contact with the rock will be smaller, which means less grip. Similarly, I think you might also be more likely to rotate your ankle. You could imagine, for example, putting all of you weight on a focused point on the wood block: this would create a fulcrum point that will rotate if there is much more weight on one side. (That's where high ankle support is needed.)
It would seem from this analysis that a stiffer sole is most useful when you are going over rough, rocky/branchy terrain with more weight on your back. One would have a less natural gait, and perhaps not grip the surface as well which would lead to greater risk of slippage and ankle rolling, but a grippy outsole and good ankle support would help. In contrast, one would want a flexible sole on softer, flatter ground, and with lesser loads. This seems to conform with common knowledge - although I think my claim that stiffer soles -> more ankle rolling might be unusual.
What do you think? What am I missing or getting wrong?
