The longitudinal arch, specifically the medial arch, is a complex mechanical structure of the foot formed by multiple bones, ligaments, and tendons. Its primary function is to act as a load-sharing system, distributing the body’s weight and absorbing the impact forces generated during movement. The arch provides the foot with the necessary compliance to adapt to uneven terrain while maintaining stiffness for efficient propulsion. Determining the height of this arch is a fundamental factor in diagnosing foot health and understanding individual biomechanics.
Understanding Arch Type Classifications
Arch height is categorized into three main types, which influence how the foot interacts with the ground. The low arch, medically termed Pes Planus (flatfoot), is characterized by a collapsed arch where nearly the entire sole contacts the ground when bearing weight. This structure often leads to excessive inward rolling of the foot, known as overpronation.
Conversely, a high arch, or Pes Cavus, is a pronounced, rigid arch that leaves a large space between the floor and the middle of the foot. This type results in supination, an excessive outward roll of the foot during gait. The neutral arch is the third category, exhibiting a balanced curve that allows for optimal weight distribution and shock absorption.
Simple At-Home Measurement Techniques
A straightforward method for preliminary arch assessment is the “Wet Test.” To perform this, dip a foot in water, ensuring the sole is wet, and then step onto dark construction paper or cardboard. The imprint left behind provides a visual clue to the arch type.
A neutral arch footprint shows the heel and ball of the foot connected by a strip roughly half the width of the foot in the mid-section. A low arch leaves an imprint that is wide or complete across the entire sole, showing little to no inward curve. If only the heel and the ball of the foot leave a print, with a thin connecting line or no connection, it suggests a high arch. A quick visual inspection can also offer insight, especially when checking for flexibility. While standing, a person with a flexible flatfoot may have a collapsed arch that reappears when they rise onto their toes. Observing the wear patterns on older shoes is also helpful, as excessive wear on the inner edge suggests low arches, while wear concentrated on the outer edge points toward high arches.
Objective Clinical Assessment Methods
For a definitive and quantifiable arch height measurement, professionals use specialized clinical techniques. One objective method is the Arch Height Index (AHI), calculated using a caliper-based measurement system. This ratio provides a precise, numerical classification, with an AHI value below 0.345 typically classifying the foot as pes planus and a value above 0.364 indicating pes cavus in certain populations.
Pressure plate analysis offers another quantitative assessment, capturing thousands of data points to create a digital map of the foot. This technology measures static (standing) and dynamic (walking) pressure distribution, highlighting areas of high force concentration and the trajectory of the center of mass during movement.
Diagnostic imaging, such as a weight-bearing lateral X-ray, provides a non-subjective view of the bony architecture. Radiologists frequently use the Meary’s Angle, which measures the angle between the longitudinal axis of the talus and the first metatarsal bone. In a neutral foot, this angle should be close to 0 degrees (a straight line). An angle greater than 4 degrees with the apex pointing downward suggests a low arch, while an angle greater than 4 degrees with the apex pointing upward indicates a high arch.
Biomechanical Implications of Arch Variations
Arch height directly affects the biomechanics of the entire lower limb, influencing how forces are transmitted up the kinetic chain. A low arch, due to overpronation, causes the foot to roll inward excessively, leading to internal rotation of the lower leg. This rotational force can contribute to soft-tissue injuries such as shin splints, plantar fasciitis, and tendinitis, and may also strain the knees and hips.
A high arch, often characterized by rigidity and supination, is less effective at absorbing ground reaction forces. Because the foot is less able to flatten and attenuate shock, the impact is transferred more directly to the bones and upper joints. This lack of natural cushioning is associated with bony injuries, including stress fractures, metatarsalgia (forefoot pain), and an increased risk of lateral ankle sprains. Both extremes alter the foot’s natural ability to spring and adapt, potentially leading to discomfort and movement inefficiencies.