Of all the fundamental human movements, gait—the pattern of walking—appears deceptively simple. It is an automated, rhythmic process most take for granted until injury or illness disrupts its fluidity. However, this apparent simplicity belies a breathtakingly complex orchestration of neurological, musculoskeletal, and sensory systems. Clinically, the analysis of gait is broken down into six core determinants, a conceptual framework pioneered by biomechanists Verne Inman and Howard Eberhart in the 1950s. These six determinants of gait are not merely observations of how we walk; they are the fundamental engineering principles the human body employs to transform the naturally inefficient, up-and-down, side-to-side motion of the legs into the smooth, energy-conserving forward progression we recognize as normal walking. They are: pelvic rotation, pelvic tilt, knee flexion in stance, foot and ankle mechanisms, knee mechanisms, and lateral pelvic displacement.
The first two determinants involve movements of the pelvis, the foundational platform for the gait cycle. The first determinant, pelvic rotation, occurs in the horizontal plane. As an individual steps forward with their right leg, the entire pelvis rotates slightly forward on the right side and backward on the left. This rotation, typically amounting to about 4 degrees on each side (for a total of 8 degrees), has a profound effect on the effective length of the leg. By rotating the pelvis forward, it effectively positions the hip joint further ahead at the point of heel strike, thereby functionally lengthening the limb and reducing the height of the apex of the arc that the body’s center of mass (COM) would otherwise have to travel. Without this rotation, the COM would be forced to rise and fall with a much greater amplitude, a wasteful and jarring expenditure of energy.
The second determinant, pelvic tilt, operates in the coronal (frontal) plane. During the mid-stance phase on one leg, the pelvis tilts downward on the non-weight-bearing side. This action, controlled primarily by the hip abductors on the stance limb to prevent an excessive drop, also serves to minimize the vertical displacement of the COM. By lowering the pelvis on the swinging side, the high point of the COM during single-leg support is reduced. This tilt, approximately 5 degrees, further flattens the arc of the COM’s trajectory. Together, pelvic rotation and tilt are the body’s first line of defense against the inherently inefficient bouncing gait that would result from rigid, pole-like legs.
The third and fifth determinants focus on the critical role of the knee joint. The third determinant, knee flexion during the stance phase, is perhaps one of the most crucial energy-saving mechanisms. Immediately after heel strike, the knee begins to flex, reaching about 15-20 degrees of flexion during the loading response and mid-stance. This flexion acts as a shock absorber, dampening the impact forces transmitted up the skeletal system. More importantly, it prevents a sharp rise in the COM just after heel strike. If the leg remained perfectly straight, the COM would be forced to pivot over a fixed, long lever arm, resulting in a significant upward displacement. By flexing the knee, the body effectively shortens the leg during this critical period, allowing the COM to continue its smooth, relatively level path forward. Later, the fifth determinant, knee mechanisms in swing phase, facilitates limb advancement. The flexion of the knee during the swing phase (to approximately 60 degrees) serves to functionally shorten the leg, much like a retractable arm on a machine. This shortening is essential to prevent the toe from scraping the ground, reducing the energy required to swing the limb through and allowing for a faster, more efficient step.
The fourth determinant encompasses the intricate interplay of the foot and ankle mechanisms. This is a multi-part process that manages the transition of weight from heel to toe. At heel strike, the ankle is in a neutral position. As the body moves forward over the foot, the ankle dorsiflexes in a controlled manner, which helps to smooth the forward progression of the tibia over the stationary foot. During the final phase of stance, push-off is initiated by powerful plantar flexion of the ankle. This action, primarily by the gastrocnemius and soleus muscles, provides a significant propulsive force for forward momentum. Furthermore, the foot itself is a master of adaptation and rocker mechanics. It functions sequentially as a heel rocker (at contact), an ankle rocker (during mid-stance), and a forefoot rocker (at push-off), each phase contributing to a smooth roll-over action that propels the body forward without jarring stops or starts.
Finally, the sixth determinant, lateral pelvic displacement, addresses the side-to-side balance of gait. Because the feet are typically placed with a narrow base of support, each located slightly to either side of the body’s midline, the COM must shift laterally during each step to remain balanced over the single, weight-bearing foot. This shift, controlled by the hip abductors, is minimal in normal gait—only about 2-5 centimeters. Without this small but critical displacement, the body would be unable to maintain balance during single-leg support, and walking would resemble an inefficient waddle with a wide base of support. This determinant ensures that the sinusoidal, lateral path of the COM is kept to a minimal, energy-efficient amplitude.
The six determinants of gait are not isolated phenomena but an integrated, synergistic system working in concert to achieve the primary goal of locomotion: efficient, stable, and smooth forward progression. They function to minimize the vertical and lateral displacements of the body’s center of mass, converting the potentially large, sinusoidal oscillations of a compass-gait model into the nearly level pathway characteristic of a healthy, efficient gait. Understanding these determinants is paramount in clinical practice. Deviations from these norms, such as a lack of knee flexion (leading to a vaulting gait) or insufficient pelvic control (leading to a Trendelenburg gait), are key diagnostic indicators of underlying neurological or musculoskeletal pathology. Therefore, the six determinants provide more than just a description of how we walk; they offer a fundamental biomechanical lexicon for assessing, diagnosing, and ultimately restoring one of humanity’s most essential and defining movements.