The six determinants of gait theory, proposed by Saunders, Inman, and Eberhart in 1953, represents a landmark conceptual framework in biomechanics that sought to explain how the human body minimizes energy expenditure during walking. This theory posits that six specific kinematic mechanisms work synergistically to reduce the vertical displacement of the body’s center of mass, thereby decreasing the energy cost of locomotion. While this model has profoundly influenced clinical gait analysis and orthopedic practice for decades, contemporary research has increasingly questioned its validity, revealing significant limitations in both its underlying assumptions and empirical support.
The six determinants consist of pelvic rotation, pelvic tilt, knee flexion during stance phase, foot and ankle mechanisms, knee mechanisms, and lateral displacement of the pelvis. According to the original theory, each determinant smooths the trajectory of the center of mass, converting what would be a series of arcs into a sinusoidal pathway with minimal vertical excursion. The model’s elegance and intuitive appeal made it widely accepted in medical education, rehabilitation, and prosthetic design, where it continues to inform clinical decision-making.
However, the scientific validity of this theory rests on several key assumptions that warrant careful examination. The primary assumption is that minimizing vertical displacement of the center of mass is the body’s principal strategy for reducing energy expenditure during gait. This premise, while logical, oversimplifies the complex metabolic processes involved in human locomotion. Energy consumption during walking involves not only the mechanical work of raising and lowering the body’s mass but also the metabolic costs of muscle contraction, the efficiency of energy transfer and storage in tendons, and the coordination of numerous muscle groups across multiple joints.
Contemporary biomechanical research has challenged the six determinants theory through sophisticated experimental designs and computational modeling. Studies using three-dimensional motion capture, force plates, and metabolic measurement systems have revealed that the relationships proposed by the original theory are more complex than initially conceived. For instance, research has demonstrated that selectively restricting individual determinants does not consistently produce the predicted increases in energy expenditure. In some cases, constraining certain movements results in only modest changes in metabolic cost, suggesting that these mechanisms may not be as crucial to energy economy as the theory proposes.
One particularly compelling study by Gard and Childress in the early 2000s systematically tested each determinant by using braces and orthoses to restrict specific movements in healthy subjects. Their findings were striking: while some restrictions did increase energy cost, the magnitude of these increases was often much smaller than predicted by the theory. Moreover, the researchers found that subjects could adapt to these constraints through compensatory mechanisms not accounted for in the original model, maintaining relatively efficient gait patterns despite the imposed limitations.
The determinant of pelvic rotation, for example, has been scrutinized extensively. While the original theory suggested that forward rotation of the pelvis on the swing side reduces vertical displacement of the center of mass, subsequent research has shown that the metabolic benefit of this rotation is minimal. Some studies have even suggested that pelvic rotation may serve other functions, such as facilitating leg swing or maintaining balance, rather than primarily reducing vertical displacement.
Similarly, the role of knee flexion during stance phase has been reconsidered. The original theory proposed that the stance knee flexes during mid-stance to lower the vault-like arc of the center of mass trajectory. However, more recent analyses indicate that this flexion pattern is influenced by multiple factors, including shock absorption, forward propulsion, and the coordination of muscle activity patterns. The energy-saving function attributed to this mechanism appears less significant than other biomechanical considerations.
Despite these criticisms, completely dismissing the six determinants theory would be premature. The model succeeded in identifying genuine kinematic patterns that characterize normal human gait, even if the functional explanations for these patterns require revision. The determinants describe real movements that occur during walking, and understanding these movements remains clinically relevant. Pathological gait patterns often involve disruptions to these kinematic features, and recognizing these deviations can aid in diagnosis and treatment planning.
Furthermore, the theory’s limitations should be understood within its historical context. The original researchers worked with the technology and methodological approaches available in the 1950s, before modern motion capture systems, sophisticated metabolic measurement techniques, and advanced computational modeling capabilities. Their work represented a significant intellectual achievement that stimulated decades of gait research and clinical application.
The ongoing debate about the six determinants highlights broader issues in biomechanical theory development. Models that appear elegant and parsimonious may oversimplify complex biological systems. Human gait represents an optimized solution to multiple competing demands—not only energy efficiency but also stability, adaptability to terrain, speed modulation, and injury prevention. A comprehensive theory of gait must account for this multifaceted optimization rather than focusing on a single objective function.
Modern alternatives to the six determinants theory incorporate more comprehensive frameworks. Dynamic walking models, spring-mass systems, and inverted pendulum models offer different perspectives on gait mechanics. These approaches often emphasize the role of passive dynamics, elastic energy storage and return, and the integration of neural control with mechanical properties. Rather than focusing solely on minimizing vertical displacement, contemporary theories recognize that energy-efficient gait emerges from the complex interaction of anatomical structure, neuromuscular control, and biomechanical constraints.
The six determinants of gait theory represents an important but limited framework for understanding human locomotion. While the model successfully identified key kinematic features of normal gait and provided a conceptual foundation for decades of clinical practice, empirical research has revealed significant gaps between the theory’s predictions and observed reality. The relationship between these kinematic patterns and energy expenditure is more nuanced than originally proposed, and the mechanisms underlying efficient gait are more complex and multifactorial. Nevertheless, the theory retains educational and clinical value as a descriptive framework, even as its explanatory power has been questioned. The evolution of thinking about the six determinants exemplifies how scientific understanding progresses through critical examination and refinement of established theories.