Rigid, inflexible soles are a common inclusion in most conventional footwear. Most shoe manufacturers have taken the idea of foot sole protection and gone well beyond what is necessary or healthy for your foot. In most cases, only a thin layer of material between your foot and the ground is required for adequate foot protection. And people who have grown up in a barefoot culture may not require any sort of external foot protection.
Immobilization & Deformity
The biggest problem with rigid, inflexible soles is that they hold your foot in a compromised and deforming position, both during activity and at rest. Stiff soles, then, when combined with other problematic shoe design features—tapering toe boxes, toe spring, and heel elevation—effectively immobilize your foot in an abnormal configuration and prevent your foot structures from performing the way nature intended. Think of it this way: What other body parts do you immobilize for prolonged periods? Why would it be effective or advantageous to do this to your feet? When you wear shoes with rigid soles, it’s like putting your foot in a cast and expecting it to get stronger.
An Impediment to Mindful Movement
Stiff-soled shoes are also an impediment to mindful walking or running. With a stiff-soled shoe, there is no need to consider your foot placement, how your foot is striking the ground, and what effect this is having on your gait, posture, and musculoskeletal health (including joint health). Stiff, inflexible soles cause you to lose the tactile sensation of the ground—a true detriment to anyone interested in the full array of sensations possible when hiking or running.
Rigid vs. Flexible Soles
Rigid soles, though believed by many to be beneficial, may actually hinder your foot’s propulsive ability, contribute to immediate and long-term foot, ankle, and other musculoskeletal problems, and increase your likelihood of injury from poor foot placement. Search for shoes that can easily be folded in half (sole of the forefoot touching sole of the heel), preferably with just one hand. This folding ability is an indication of a flexible and foot-healthy sole. Also consider the degree to which you can twist the soles of your shoes. A shoe that's easily twistable is another excellent indicator of sole stiffness and appropriateness. You should be able to bend your shoe in multiple directions with minimal effort.