Human walking may seem trivial, but in reality it’s extremely complex.
Each step involves a sophisticated choreography of muscle activation and joint rotations working together to create smooth forward progress.
The Six Determinants of Gait—the set of principles described by Saunders, Inman, and Eberhart in 1953—explain in a precise way why the body doesn’t waste energy during movement.
For clinicians, biomechanics researchers, and physical therapists, the Six Determinants of Gait are essential.
They guide our gait analysis, our rehab protocols, our orthopedic tactics.
Read on for a synthesis of the six determinants, the biomechanics behind each of them, and why we care about them today.
Understanding the Six Determinants of Gait: First Two Components
The body’s center of mass (CoM) moves in a wave pattern between the two extremes of each stride’s stance phase.
Exaggerated vertical movement would require a ridiculous amount of power, however, making walking an aerobic nightmare.
The first two of Six Determinants of Gait combat this trend by limiting vertical CoM travel through pelvic movements and hip mechanics.
Pelvic Rotation
Pelvic rotation refers to the pelvis rotating about the vertical axis during gait.
A transverse-plane swing of the pelvic socket occurs as the ipsilateral leg (either side) moves forward.
This rotation can be four degrees if normal, and efficiently increases leg length without taxing the hip flexors.
Pelvic rotation effectively causes the CoM to take a shorter arch, minimizing how high a person must vertically move to advance between steps.
Clinicians will notice pelvic rotation at the very beginning of a patient exam when the patient has lumbar stiffness or any gait waddle.
Pelvic rotation is also a key component of stance phase stability when a person becomes fatigued mid-walk.
Pelvic Tilt
The second sagittal-plane component of gait kinematics—the pelvic tilt—is also a key to relative stance phase stability.
This phenomenon occurs as the pelvis drops downward on the swing limb.
Typically the muscles that contract around the support limb lateral aspect—mainly the gluteus medius—unit into an eccentric control system to oppose the force of gravity acting to lower the unsupported pelvis.
This downward tilt, like the pelvic rotation, results in a smoother, more efficient swing.
Clinicians will see this action when a lumbar block is performed, or when a person develops gluteus medius weakness.
The drop in the pelvis causes a style of gait called the Trendelenburg gait that is characterized by one-sided drop-shifting of the torso.
Reinforcing the abductors dorsiflex the patients thigh to give a more stable limb.
The Second Half of the Body Operates to Control the Middle Three Determinants
The next 3 determinants (out of 6 for walking) deal with the action of the ankle and knee joints respectively.
Subtleties in ankle and knee movements greatly improve high- and low-foreword walking mechanics.
Knee Flexion During stance
The body purposely flexes the knees by about fifteen degrees.
Flexed knees serve to “absorb” some of the impact of the body’s weight, as well as lower the body’s center of mass during an important transition period.
Without flexed knees, each step would involve an abrupt, rigid vault over the support limb at an enormous expendature of energy.
Patients with weak quadriceps, knee pain, or knee replacements will advocate for a stiff-legged gait because it minimizes knee activity.
Elimination of this determinant can increase the person’s energy cost of walking 70 to 2.0.
Aspects of the Foot and Ankle; the 4th and 5th Determinants
The next determinant is the foot’s function as a rocker.
When the hind foot hits the ground at heel strike, the anterior tibialis works eccentrically to smoothly pronounce the foot and prevent the tibia from collapsing forward.
This active muscle control plus the powerful push off that occurs with ankle plantarflexion at terminal stance together round out the CoM.
Lack of strength or paralysis here will create a decrease in walking speed with an increase in McKernan’s cost of walking per meter.
The Last but not the Least of Six Determinants of Gait Provides the Final Piece of the Puzzle
The 6th and final determinant of gait is concerned with the mediolateral travel of the body’s center of mass.
This lateral motion is unaffected by the first 5 determinants of gait, but it still adds as much as 15 percent to a person’s energy expenditure.
Athletic performance experts are interested here—clinicians should be too.
Lateral Motion and the Lateral Trunk Tilt
The final determinant involves the inward angulation of the thighs.
If the thigh plus its femur glides inward, the foot lands closer to the body’s midline, lessening the mediolateral distance between the hip joints and the foot strap points.
However, the force is not solely an internal binarystional one, instead, the inward angulation takes advantage of a biomechanical aspect of the sit-to-stand transition to offset the net effect of lateral CoM travel.
This inward angulation of the femurs is known as the physiological valgus.
Good news for the women, who are naturally team “widespread” in this area: a comparative analysis of pelvis morphology in high versus low-efficient runners shows that women who run with a wider pelvis to start with tend to run worse overall.
Clinicians must be aware of this determinant when fitting lower limb prosthetics because it will have an effect on the mediolateral sway of the CoM.
Another subtle movement, the ipsilateral lean of the trunk—”swinging” the torso toward the stance limb—combats the mediolateral CoM excursion.
This line of action remains strictly controlled and is beneficial in healthy gait.
In this vein, however, if the person leans ipsilaterally more than expected, it can be indicative of a motor control problem.
Assessing this determinant is often useful when trying to analyze an abnormal gait.
Six determinants of gait can be summarized as a way of designing a “building diagram” for the most energy-efficient walking pattern.
Pelvic rotation, pelvic tilt, stance phase knee flexion, the anterior tibialis-anterior rocker ankle mechanism, the plantarflexion ankle pump, and physiological valgus create the ideal family of gaits and everyday walking patterns—smooth, functional, low-energy.
The clinician must be careful not to “throw out the baby with the bathwater,” however, because the above list of determinants is based on averages from healthy studies—and patients are thus individuals.
In each case, if just one of these determinants is compromised, the body adapts without global benefit, and there is an energy or mechanical expense (the voice in their head calling out to them to stop running) that slowly builds up in the system.