Human walking requires the body to move forward while maintaining balance, absorbing impact, and limiting unnecessary energy expenditure. In 1953, Saunders, Inman, and Eberhart proposed a highly influential model describing six biomechanical features that smooth the trajectory of the body’s centre of mass during normal gait. These became known as the classic six determinants of gait: pelvic rotation, pelvic tilt, knee flexion during stance, the ankle mechanism, the foot mechanism, and lateral displacement of the pelvis. The model argued that these coordinated movements reduce excessive vertical and lateral motion of the centre of mass, thereby producing a more efficient gait. Although later research has challenged aspects of this explanation, the six determinants remain valuable for understanding walking and interpreting gait abnormalities.

The first determinant is pelvic rotation in the transverse plane. During walking, the pelvis rotates forward on the side of the advancing limb and backward on the side of the trailing limb. The total excursion is commonly described as approximately 8 degrees, although the amount varies among individuals and with walking speed. Pelvic rotation effectively lengthens the step without requiring an equivalent increase in hip motion. At initial contact, the forward rotation of the pelvis allows the heel to contact the ground farther ahead of the body. According to the classic model, this flattens the arc through which the pelvis travels and reduces the fall of the centre of mass at double support. Pelvic rotation also contributes to coordinated counter-rotation between the pelvis and thorax, assisted by reciprocal arm swing, helping control angular momentum.

The second determinant is pelvic tilt, or pelvic obliquity, in the frontal plane. During single-limb support, the pelvis normally drops slightly on the side of the swinging limb. This controlled descent is restrained primarily by the hip abductors of the stance limb, particularly gluteus medius and gluteus minimus. The classic account suggested that a small pelvic drop lowers the peak height reached by the centre of mass during midstance, reducing its vertical excursion. Excessive pelvic drop, however, may indicate hip-abductor weakness, pain, poor motor control, or structural factors and is associated with a positive Trendelenburg sign. Conversely, a rigidly elevated swing-side pelvis may interfere with foot clearance and increase compensatory movements.

The third determinant is knee flexion during the stance phase. Following initial contact, the knee flexes to approximately 15 degrees during loading response before extending toward midstance. This flexion has several important functions. It helps absorb impact, allows controlled lowering of the body, and prevents the lower limb from behaving as a completely rigid strut. Within the classic model, knee flexion reduces the peak vertical elevation of the centre of mass as the body passes over the supporting foot. Eccentric quadriceps activity controls the motion and prevents the knee from collapsing. A stiff-knee pattern may increase vertical movement and loading, whereas excessive flexion, as seen in crouch gait, increases the muscular demands placed on the quadriceps and can substantially raise energy expenditure.

The fourth determinant, the ankle mechanism, operates mainly during the beginning of stance. At initial contact, the heel acts as a rocker while the ankle plantarflexes in a controlled manner, lowering the forefoot to the ground. Tibialis anterior works eccentrically to regulate this motion and prevent the foot from slapping. As the tibia progresses over the planted foot, controlled ankle movement smooths forward progression and moderates the lowering of the body’s centre of mass. The rounded heel and controlled plantarflexion therefore transform what could be an abrupt collision into a gradual transfer of weight. Impairments such as restricted ankle motion, dorsiflexor weakness, or an inappropriate prosthetic-foot alignment can disrupt this mechanism and provoke compensations elsewhere in the limb.

The fifth determinant is the foot mechanism, which becomes especially important during terminal stance. As the heel rises, the forefoot and metatarsal heads form a pivot, often termed the forefoot rocker. Ankle plantarflexion and movement over the forefoot effectively lengthen the supporting limb as the body moves ahead of it. In the classic theory, this delays and reduces the downward displacement of the centre of mass near the end of stance. The foot must also adapt from a relatively flexible structure for shock attenuation to a more rigid lever for propulsion. Subtalar and midfoot motion, plantar-fascial tension through the windlass mechanism, and activity of the plantarflexor muscles all contribute. Pain, deformity, restricted first metatarsophalangeal-joint dorsiflexion, or loss of plantarflexor strength may compromise this rocker and shorten the opposite step.

The sixth determinant is lateral displacement of the pelvis. Walking alternates the base of support from one foot to the other, so the body’s centre of mass must shift from side to side. Humans limit this movement through a relatively narrow step width, physiological knee valgus, and the orientation of the femur from the wider pelvis toward the knees. These structural relationships position each foot nearer the body’s line of progression and reduce the distance through which the pelvis must move to place the centre of mass over the stance limb. Excessive step width generally increases lateral displacement and may raise energy cost, although it can also improve stability in people with impaired balance.

The six determinants do not function independently. Pelvic, hip, knee, ankle, and foot movements overlap throughout the gait cycle, while muscles provide active control and sensory systems continually regulate balance. Consequently, an impairment in one region can produce compensations in another. Limited ankle dorsiflexion, for example, may encourage early heel rise, knee hyperextension, foot pronation, or altered step length. Clinical gait analysis should therefore examine the whole person rather than interpreting any determinant in isolation.

Modern biomechanics has also revised the original claim that minimizing centre-of-mass displacement necessarily minimizes energy consumption. Experimental studies suggest that some determinants have smaller effects on vertical movement than originally proposed. Pelvic obliquity may even increase vertical excursion under certain conditions, while stance-phase knee flexion is strongly associated with impact attenuation rather than simply flattening the centre-of-mass pathway. Contemporary approaches emphasize step-to-step transitions, collision reduction, pendular exchange of kinetic and potential energy, muscle work, stability, and individual variation. Thus, the classic model is best regarded as a foundational descriptive framework rather than a complete theory of walking economy.

In conclusion, the six determinants of gait describe coordinated pelvic rotation, pelvic tilt, stance-phase knee flexion, ankle and foot rocker mechanisms, and control of lateral pelvic displacement. Together, these features help produce smooth, stable progression and provide a practical vocabulary for assessing pathological gait. Although current evidence questions whether their principal purpose is solely to minimize centre-of-mass movement, their historical and clinical importance remains substantial. Used alongside modern kinetic, muscular, and neuromotor analysis, the classic determinants continue to offer a useful introduction to the complexity of human walking.

References

• Gard, S. A., & Childress, D. S. (1997). The effect of pelvic list on the vertical displacement of the trunk during normal walking. Gait & Posture, 5(3), 233–238.
• Gard, S. A., & Childress, D. S. (1999). The influence of stance-phase knee flexion on the vertical displacement of the trunk during normal walking. Archives of Physical Medicine and Rehabilitation, 80(1), 26–32.
• Perry, J., & Burnfield, J. M. (2010). Gait Analysis: Normal and Pathological Function (2nd ed.). SLACK Incorporated.
• Saunders, J. B. de C. M., Inman, V. T., & Eberhart, H. D. (1953). The major determinants in normal and pathological gait. The Journal of Bone & Joint Surgery, 35-A(3), 543–558.