The human walk, or gait, is a deceptively complex feat of engineering. To the casual observer, it’s just putting one foot in front of the other. However, from a biomechanical perspective, walking is a continuous effort to prevent the body from falling while moving forward as efficiently as possible.
In 1953, researchers Saunders, Inman, and Eberhart introduced the Six Determinants of Gait. Their core theory was simple: walking requires energy, and energy is consumed primarily by moving the body’s center of mass (COM) up, down, and side-to-side. To walk efficiently, the body uses six specific kinematic adjustments to “smooth out” the path of the COM, turning a series of jerky movements into a fluid, energy-saving arc.
1. Pelvic Rotation
When we take a step, the pelvis doesn’t remain fixed in a forward-facing position. Instead, it rotates forward on the side of the swinging leg.
- The Mechanism: As the right leg swings forward, the right side of the pelvis rotates anteriorly (forward) about $5^\circ$ in the horizontal plane.
- The Benefit: This rotation effectively lengthens the limb. By extending the reach of the leg, it allows for a longer stride without requiring the center of mass to drop as low as it would if the pelvis remained rigid. It flattens the “valleys” of the vertical displacement curve.
2. Pelvic Tilt
In a static position, if you lift one leg, your hip would naturally want to drop significantly. During gait, however, the pelvis undergoes a controlled tilt.
- The Mechanism: As the swing leg leaves the ground, the pelvis tilts downward about $5^\circ$ toward the unsupported side.
- The Benefit: While this might seem counterintuitive (lowering the COM), it actually acts as a “ceiling” for the COM’s peak height during mid-stance. By lowering the pelvis slightly on the non-weight-bearing side, the body’s overall center of mass doesn’t rise as high when the stance leg is vertical, smoothing out the “peaks” of the arc.
3. Knee Flexion in the Stance Phase
If we walked with perfectly straight legs, our center of mass would follow a series of harsh, abrupt semicircles.
- The Mechanism: Immediately after initial contact (heel strike), the knee begins to flex. It reaches a maximum of about $15^\circ$ of flexion during the early part of the stance phase (loading response).
- The Benefit: This “shocks” the system in a good way. By flexing the knee while the foot is on the ground, the body effectively shortens the leg at the exact moment the COM is passing over it. Much like pelvic tilt, this prevents the COM from rising too high, further flattening the path of movement into a gentle wave rather than a jagged zig-zag.
4. Foot and Ankle Mechanisms
The foot and ankle work in tandem to ensure the center of mass follows a smooth path at the lowest point of the gait cycle.
- The Mechanism: At heel strike, the effective length of the leg is increased by the distance from the ankle joint to the heel. As the foot rolls forward into “toe-off,” the leg length is extended by the distance to the forefoot.
- The Benefit: This smooths out the transition between the end of one step and the beginning of the next. Instead of the COM dropping abruptly when the leading foot hits the ground, the “rolling” action of the foot ensures a rounded, continuous transition.
5. Knee Mechanisms (Coordination with Foot/Ankle)
This determinant is often grouped with the fourth, as it describes the synchronization of the knee and ankle to keep the COM’s path linear.
- The Mechanism: As the ankle undergoes plantarflexion (pointing the toes) during the end of the stance phase, the knee begins to flex.
- The Benefit: This coordination ensures that the “lengthening” of the limb provided by the ankle is offset by the “shortening” provided by the knee. This prevents the COM from bobbing upward during the transition from stance to swing.
6. Lateral Displacement of the Pelvis
Walking isn’t just an up-and-down affair; it’s a side-to-side one. Because our feet are not fused together, we have to shift our weight from one side to the other to maintain balance.
- The Mechanism: The pelvis shifts laterally toward the weight-bearing (stance) leg by about 4–5 cm.
- The Benefit: Humans have a natural “genu valgum” (the inward angle of the femur toward the knee). This allows our feet to stay closer to the midline of the body. By keeping the base of support narrow, the amount of lateral shifting required is minimized. This reduces the side-to-side “wobble” and saves the energy that would otherwise be spent hauling the body’s weight across a wide gap.
Summary: The “Cost” of Walking
Without these six determinants, the path of our center of mass would be highly inefficient. We would consume significantly more oxygen and tire much faster.
| Determinant | Primary Action | Effect on Center of Mass (COM) |
| Pelvic Rotation | Horizontal rotation ($5^\circ$) | Prevents COM from dropping too low |
| Pelvic Tilt | Lateral dip ($5^\circ$) | Prevents COM from rising too high |
| Knee Flexion (Stance) | Early $15^\circ$ bend | Prevents COM from rising too high |
| Ankle Mechanism | Heel-to-toe rolling | Smooths the low points of the arc |
| Knee Mechanism | Synced flexion with ankle | Maintains a level path during transition |
| Lateral Shift | Side-to-side movement | Minimizes energy lost to wide swaying |
Modern Perspective
While Saunders and Inman’s model remains the gold standard for teaching gait, modern biomechanists often point out that these “determinants” don’t just reduce COM displacement—they also facilitate momentum. Walking is often described as “controlled falling,” where we use gravity to our advantage. These six factors don’t just smooth the path; they ensure that the energy we spend is used for forward progression rather than fighting gravity or lateral instability.
Understanding these determinants is crucial for physical therapists and prosthetic designers. When a patient has a “trendelenburg gait” (excessive hip drop) or a “vaulting gait” (staying on the toes to clear a stiff leg), they are essentially losing one or more of these determinants, making every step a much more expensive endeavor for the body.