Are the Six Determinants of Gait Valid?

The six determinants of gait represent a foundational concept in biomechanics, first articulated in 1953 by J.B. deC.M. Saunders, Verne T. Inman, and Howard D. Eberhart. These determinants describe kinematic mechanisms that purportedly optimize human walking by controlling the trajectory of the body's center of mass (COM). The six include: (1) pelvic rotation in the horizontal plane, (2) pelvic tilt or obliquity in the frontal plane, (3) knee flexion during the stance phase, (4) foot and ankle motion (particularly heel rise), (5) knee motion throughout the gait cycle, and (6) lateral displacement of the pelvis. According to the original paper, these elements work synergistically to produce a smooth, sinusoidal path for the COM, minimizing abrupt changes in velocity and direction, which in turn reduces energy expenditure during locomotion. This framework has influenced gait analysis, rehabilitation, and prosthetics for decades, appearing in textbooks and clinical guidelines as a model for efficient walking. However, as gait research has advanced with technologies like 3D motion capture and metabolic measurements, questions have arisen about the validity of this theory. Critics argue that the determinants may not achieve the energy savings claimed, and some interpretations of the model are based on misconceptions. This essay examines the historical context, empirical evidence, criticisms, and clinical implications to assess whether the six determinants remain a valid explanation of gait mechanics.

The original proposal by Saunders et al. emerged from post-World War II research at the University of California, Berkeley, where the authors studied amputees and normal gait to improve prosthetic design. They contrasted human walking with a "compass gait"—a stiff-legged model where the COM follows a high-arched path, requiring significant energy to redirect momentum at each step. In contrast, the determinants were seen as evolutionary adaptations that flatten and smooth the COM trajectory into a low-amplitude sine wave, with vertical displacement limited to about 5 cm. For instance, pelvic rotation was said to extend the effective leg length during double support, while stance-phase knee flexion absorbs shock and lowers the COM peak. The authors emphasized that this undulating path avoids the energy costs of rapid accelerations, aligning with principles of mechanical efficiency. Early validations relied on observational data and simple models, and the theory gained traction because it provided a coherent kinematic description of gait phases: initial contact, loading response, mid-stance, terminal stance, pre-swing, initial swing, mid-swing, and terminal swing. By the 1990s, however, quantitative studies began challenging these assumptions, revealing that the determinants' impact on COM displacement and energy use was overstated or misinterpreted.

A key issue is the widespread misconception about the theory's core aim. Many textbooks and studies, including influential works by Perry (1992) and Kuo (2007), have portrayed the determinants as strategies to minimize COM vertical and horizontal excursions for energy conservation—as if the goal of gait is to keep the COM as flat as possible along the line of progression. Yet, a close reading of the 1953 paper shows no explicit claim that minimization of displacement is the primary objective; instead, it focuses on achieving a smooth, gradual deflection to reduce forces needed for velocity changes. This distortion likely arose from implicit references in the original text, such as how pelvic tilt "halves" vertical displacement, which later interpreters exaggerated into a minimization paradigm. Experimental tests of this misinterpreted view—such as training subjects to walk with a flattened COM path—have shown metabolic costs nearly doubling compared to normal gait, due to increased joint work and step frequency. This highlights that the determinants are better understood as mechanisms for smoothing the COM trajectory rather than reducing its amplitude to a minimum. The misconception has persisted in clinical education, leading to confusion about gait pathologies where exaggerated determinants (e.g., Trendelenburg gait with excessive pelvic tilt) are viewed as compensatory rather than inefficient.

Modern empirical evidence further questions the validity of the six determinants as originally framed. Studies using 3D gait analysis and mathematical modeling have quantified each determinant's contribution to COM vertical displacement. For example, research on 30 healthy adults found that heel rise (part of foot and ankle motion) accounts for up to two-thirds of the reduction in COM excursion, while pelvic rotation contributes about 10% beneficially. However, ipsilateral and contralateral knee flexion were actually detrimental, increasing rather than decreasing displacement, and pelvic obliquity and stance knee flexion had minimal effects at the actual COM peak (though more apparent in simplified compass models). Another review concluded that the determinants contribute only slightly to COM reduction, and forcing a lower COM trajectory requires higher metabolic energy than natural walking. This contradicts Saunders' assumption that minimizing displacement lowers costs; instead, normal gait optimizes a balance where moderate oscillation leverages passive dynamics, like pendulum-like energy transfer during stance.

Dynamic walking models, such as the inverted pendulum or spring-mass systems, provide alternative explanations that integrate better with observed kinetics. In the inverted pendulum model, the COM vaults over a stiff stance leg, conserving energy through gravitational potential-kinetic exchange, but this cannot fully replicate ground reaction forces without additions like compliant elements in the spring-mass model. Forum discussions among podiatrists and biomechanists note that the determinants are descriptive kinematics, not a predictive theory, and their linked proposal of COM minimization conflicts with these models. For instance, the spring-mass approach better explains running and walking energetics by focusing on leg stiffness and collision losses during step-to-step transitions, rather than kinematic smoothing alone. Critiques emphasize that the theory lacks falsifiable predictions and has been discredited as a scientific hypothesis, though individual determinants retain observational value for identifying deviations, such as in pronation pathologies. Attempts to enforce low COM displacement in experiments increase joint torques (e.g., tripled knee extensor demands) and overall work, suggesting the determinants do not primarily serve energy minimization.

Clinically, the theory's implications have been misleading for rehabilitation. In conditions like hemiparesis or amputation, therapies aiming to "normalize" determinants by reducing COM excursion can exacerbate fatigue without improving stability. Instead, modern approaches favor dynamic models that prioritize efficient energy transfer, informing devices like energy-storing prosthetics. While the six determinants offer a useful framework for describing gait kinematics and have historical significance, their validity as an explanatory theory for energy economy is limited. They remain partially valid as observational tools but require refinement to align with contemporary evidence. Ultimately, gait is a complex interplay of passive mechanics and active control, and dismissing the determinants entirely overlooks their role in sparking biomechanical inquiry. As research evolves, integrating them with newer models will better serve understanding and treating gait disorders.

Craig Payne is a University lecturer, runner, cynic, researcher, skeptic, forum admin, woo basher, clinician, rabble-rouser, blogger and a dad.