One feature often overlooked when selecting the proper running shoe is shoe flexibility. How flexible or stiff the shoe is, and where the shoe is flexible or stiff, can make a big difference in the appropriateness of the shoe for your particular needs. The amount of flexibility and the location(s) of flexibility are determined primarily by the portion of the shoe below the level of the foot: the midsole and outsole. Firmer, more rigid materials stiffen the shoe.
The sole of the foot flexes during running in two basic ways: 1) the foot flexes upwards, or “dorsiflexes”, in relationship to the toes, at the “ball” of the foot (at the base of the toes, where they join the foot), as the foot is leaving the ground, and 2) The forefoot (front half of the foot) twists a small amount relative to the rearfoot (back half of the foot) during the entire portion of time the foot is on the ground.
As a general rule, the shoe should flex at the same locations that the foot flexes. If you hold the shoe firmly by the sole at the heel and bend the front of the shoe upwards, it should only flex at the ball of the foot (at about 1/3 the distance from the front of the shoe - Fig. 1). If it is flexing at the middle of the shoe, or not flexing at all, the shoe will not match normal running mechanics (Fig. 2). I recommend avoiding such shoes for the majority of runners, with two exceptions: If you have stiffness or pain when attempting to flex your toes upward at the ball of the foot, you may actually benefit from wearing a running shoe that is stiff and does not flex at the ball of the foot, as this could eliminate the painful symptoms by protecting those joints from motion. If this is your situation, you may have arthritis or other joint damage that should be evaluated by a qualified professional such as a podiatrist.
Another exception to the above rule is trail running. Due to the hazards of hard and jagged objects on the running surface, a stiff foot plate can distribute high focal pressures more evenly across larger areas, thus, reducing the possibility of injury. Additional cushioning can also provide protection from jagged objects on the trails.
The amount of “twist” flexibility in the shoe between the forefoot and rearfoot is termed “torsional flexibility”. This can be evaluated as follows: grab the shoe by the heel with one hand and by the front of the shoe with the other hand and twist the heel clockwise and the forefoot counter clockwise (like wringing out a towel - Fig. 3). If the mid part of the shoe (arch area) remains relatively stiff and resists the twisting of the shoe, there is less torsional flexibility. If there is a lot of motion between the two portions of the shoe, there is more torsional flexibility. By performing this test on several different shoe models, you will get a feel for this particular property.
The shoes that have a lot of torsional flexibility may increase the risk of injury, or aggravate existing injuries for runners with problematic biomechanics, structural damage, or foot weakness. Also, shoes stiffer in torsion can be more protective when used on uneven or irregular surfaces, such as off-road or trails. The amount of torsional flexibility, which is generally less in “motion-controlling shoes”, is a different property than the degree of ”motion control” of the shoe, which I will discuss in a future column. Understand that there are plenty of “neutral” running shoes (no “motion control”) that have a lot of torsional stiffness, although almost all motion-control shoes and most stability shoes are stiff in torsion as well. Virtually all minimalist running shoes, some neutral shoes, and even a few stability shoes are relatively flexible in torsion.
The ideal amount of torsional flexibility may require a “trial and error” approach on your part, but recognizing and paying attention to this property will help you understand a feature that may be important to meet your needs for optimal comfort, function, and injury prevention.