Biomechanics of propulsion: Implications for AFOs

For many patients, the ability of an ankle foot orthosis to enhance propulsion is key to improving gait efficiency and reducing fatigue. But experts are only beginning to understand the biomechanical complexities that influence propulsion, which start with push-off but don’t end there.

By Cary Groner, Lower Extremity Review January 2016

In assessing and treating conditions that affect patients’ ability to walk, it makes sense for researchers and clinicians to pay attention to the propulsive aspect of the gait cycle. Increasingly, however, experts have concluded that much of what we thought we understood about propulsion is oversimplified or just plain wrong.

Moreover, interventions to improve propulsion, such as energy-returning carbon fiber ankle foot orthoses (AFOs) or functional electrical stimulation (FES), are far more effective in some patients than others—a variability that may have more to do with the individuals themselves than with their diagnoses. It’s crucial that clinicians be able to sort out such factors if they’re to help their patients, but often they don’t even agree on terminology, and assessing outcomes is a subjective art.

Photo courtesy of Allard USA.

Disputed definitions
Propulsive force generation comprises two primary factors: ankle moment and the position of the center of pressure relative to the body’s center of mass. These, in turn, can encompass a variety of other complex biomechanical factors including ankle dorsiflexion and plantar flexion, knee extension and flexion moments, timing and magnitude of activation of the gastrocnemius and other plantar flexors, trailing limb angle, and energy consumption.

Each of these factors plays a greater or lesser role depending on the condition under consideration. Poststroke hemiplegia patients have different propulsion issues than kids with cerebral palsy (CP), or teenagers with Charcot-Marie-Tooth disease (CMT), or adults with multiple sclerosis—and therefore will require different therapeutic approaches. Disagreements over terminology and biomechanics aren’t just theoretical—clinicians will have a hard time optimizing propulsion if they don’t understand its complexities—and a few experts have recently begun an effort to clarify matters.

“People want the spring AFO to mimic the calf muscle, but then there needs to be clarity about what the calf muscle actually does and when it does it,” said Elaine Owen, MSc, MCSP, a pediatric physical therapist at the Child Development Center in Bangor, North Wales, UK.

Clarity, however, can be hard to come by in this area, for a number of reasons.

“Push-off and propulsion may mean something different to a podiatrist than to someone working in a three-D gait lab,” Owen continued.

Owen has recently traced part of the problem to a statement within an important text on gait analysis, David Winter’s 1991 Biomechanics and Motor Control of Human Gait.

“In that book, there’s a graph depicting the gait cycle that, at one phase, shows decreased dorsiflexion that’s described as plantar flexing. It seems that people ever since have passed down that semantic confusion,” Owen explained.

Photo courtesy of Surestep.

Owen added that further problems can result when researchers and clinicians fail to distinguish terminal stance from pre-swing. In terminal stance, she explained, one limb is contacting the ground and the ankle remains in dorsiflexion; in preswing, which follows terminal stance and technically constitutes push-off, both limbs are contacting the ground and the trailing ankle is moving from dorsiflexion to plantar flexion.

“It’s a big problem in terms of developing rehabilitation strategies and orthosis designs,” she said. “We can’t put ‘normal’ back into pathological gait if we aren’t clear exactly what normal is.”

Owen is a proponent of the gait cycle descriptions published by Jacqueline Perry, MD, of Rancho Los Amigos National Rehabilitation Center in Downey, CA. These include loading response, midstance, terminal stance, and preswing, as well as the point at which true ankle plantar flexion begins.

Despite this reliable source, misunderstandings about propulsion persist, and there is also disagreement about the way push-off occurs.

“There’s an argument that the lengthening muscle in terminal stance acts like a spring tensioner on the [associated] tendon,” Owen said. “When the muscle finally begins to shorten, some of the push-off may actually come from the release of the tendon.”

Evidence exists for this view: Japanese researchers have elucidated a mechanism by which the gastrocnemius medialis tendon may act as a spring from the beginning of single-limb support to toe-off, increasing walking efficiency.

Power & prejudice
Increasingly, researchers and clinicians are looking beyond push-off itself and exploring how other phases of the gait cycle contribute to propulsion. Owen said that experts see power generation in gait as important for two primary reasons.

“One is that something has to propel the lower limb into swing phase,” she said. “The other is that it’s important for general energy and momentum within the gait cycle, and for moving the trunk.”

One question that’s plagued researchers, she noted, is whether ankle plantar flexion and knee flexion in preswing merely propel the swing limb, or whether they’re also involved in maintaining trunk momentum. In 2014, findings presented during a Thranhardt Lecture at the annual meeting of the American Academy of Orthotists & Prosthetists supported the first view. The Taiwanese authors offered evidence that the pre-swing movement of the thigh required power from the hip in addition to power from the ankle, which was consumed by the shank, knee, and thigh.

“They redid the calculations and said that a lot more of the force for propelling the trunk through the gait cycle came from loading response early in the cycle rather than later,” Owen explained. “When you make initial contact, the heel lever forces the foot to the floor and the shank is pulled forward from reclined to vertical; I think it’s massively propulsive, almost like a catapult. If we apply that theory to orthoses, we have to get first rocker and entry into midstance correct.”

Owen has arrived at her own definition of push-off. “I’d say it’s from the time the calf muscles start shortening—forty to forty-five percent through the cycle—through toe-off,” she said. “But that’s with the understanding that in terminal stance, the calf muscles are shortening and the ankle is reducing dorsiflexion. Then, in preswing, the ankle moves from dorsiflexion to plantar flexion.”

Continue reading in Lower Extremity Review

The importance of being earnest about shank and thigh kinematics especially when using ankle-foot orthoses, Owen E. Prosthet Orthot Int. 2010 Sep;34(3):254-69. doi: 10.3109/03093646.2010.485597.

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