Flip-flops are a ubiquitous symbol of warm weather and casual comfort. However, this convenience comes at a biomechanical cost, prompting questions about their long-term impact on orthopedic health, particularly the knees. The simple design—a flat sole held by a thong strap—does not interact with the foot and leg in the same way as structured footwear. Whether they are detrimental is nuanced, involving subtle changes in how the foot moves that end with altered forces traveling up the leg to the knee joint.
How Flip-Flops Change Foot and Ankle Motion
The primary mechanical challenge of a flip-flop is keeping it attached to the foot without the security of a backstrap or a closed upper. To prevent the shoe from flying off during the swing phase of walking, the wearer must constantly flex or “grip” the toes. This abnormal muscle engagement disrupts the smooth transition of weight during the push-off phase and increases the activity of stabilizing muscles in the shin and ankle, such as the tibialis anterior.
People walking in flip-flops instinctively adopt a shorter stride length and a slower walking speed compared to when they wear athletic shoes. This is a subconscious compensation mechanism to reduce the force needed to keep the footwear on the foot. Furthermore, the typical thin, unstructured sole of a flip-flop offers little to no arch support, which can exacerbate the foot’s natural tendency to roll inward, a motion known as excessive pronation.
The lack of lateral stability means the foot and ankle complex is less controlled with each step. When the foot over-pronates, it becomes a less rigid lever for propulsion, which further contributes to the shortened, less efficient walking pattern. These fundamental changes in foot and ankle kinematics set the stage for forces that travel upward through the body’s kinetic chain.
The Direct Impact on Knee Joint Loading
The altered biomechanics originating in the foot translate directly into abnormal forces at the knee joint. The excessive pronation of the foot causes the lower leg bone, the tibia, to rotate slightly inward. This inward rotation, even if small, changes the alignment and tracking of the knee joint, placing abnormal stress on the inside, or medial compartment, of the knee.
The medial compartment of the knee is often the first to wear down in conditions like osteoarthritis, and chronic, abnormal loading in this area can accelerate wear and tear. While some studies show that the peak external Knee Adduction Moment (KAM)—a measure of medial knee loading—in flip-flops is not significantly different from walking barefoot, this is attributed to the low heel height and flexibility of the shoe. However, this finding must be balanced against the altered gait mechanics, such as persistent inward rotation, which still introduce unusual stresses.
The lack of substantial cushioning in the thin sole minimizes the foot’s natural shock absorption. Although some research shows a lower peak vertical force at heel contact compared to sneakers, the overall force transmission and the rate of loading are modified. The knee joint must absorb these altered forces without the benefit of the stable foundation provided by supportive footwear.
Identifying Risk Factors and Supportive Alternatives
Individuals with pre-existing joint conditions, such as knee osteoarthritis, or those with underlying foot issues like flat feet or high arches, are at a higher risk from chronic flip-flop wear. The unstable nature of the footwear compounds existing biomechanical inefficiency, increasing the likelihood of pain or injury. People who spend extended periods on their feet, such as those with jobs requiring a lot of walking or standing, should also be cautious about relying on flip-flops for daily use.
For those who prefer open-toed summer footwear, there are supportive alternatives that mitigate the risks of traditional flip-flops. Look for sandals that feature contoured arch support to control excessive pronation and promote better alignment. A firm, slightly elevated heel cup can help stabilize the rearfoot, reducing the tendency for the lower leg to rotate inward.
The most significant improvement comes from a shoe that secures the foot with adjustable straps over the instep, rather than just a thong between the toes. This security eliminates the need for the constant, detrimental toe-gripping action, allowing the foot to move more naturally through the gait cycle. Limiting the use of unsupportive flip-flops to very short distances or non-walking activities, such as a quick trip to the pool, is the most practical way to minimize potential orthopedic strain.