Introduction of a Fourth Rocker Could Affect Future Prosthesis Designs

RENO, Nev. — A fourth rocker, also known as the toe rocker, was introduced in the second edition of Gait
Analysis: Normal and Pathological Function
, by Jacquelin Perry, MD, ScD
and Judith Burnfield, PhD, PT, director, Institute for Rehabilitation Science
and Engineering at Madonna Rehabilitation in Lincoln, NE.

  Judith Burnfield
  Judith Burnfield

Traditionally, there are three functional rockers supporting the foot
during an individual’s gait — the heel, ankle and forefoot rockers.
The toe rocker expands understanding of the how the limb efficiently prepares
for swing during the final phase of stance according to Burnfield. She and
Perry discussed this topic in February at the Hanger Education Fair in Reno,

During the forefoot rocker, body weight progresses across the forefoot
and the heel rises from the ground. Vigorous but modulated activity of the
plantarflexors enables the tibia to advance in a controlled manner that
balances the need for limb stability with maintenance of forward progression.
The vigorous plantarflexor activity also stretches the Achilles tendon, leading
to storage of elastic energy during terminal stance, Burnfield explained.

What does this newly discovered toe rocker actually do?

“The toe rocker occurs during pre-swing as body weight advances
from the metatarsal head(s) towards the great toe,” Burnfield explained to
O&P Business News. “As body weight transfers from the
trailing to lead limb, calf muscle activity rapidly ceases. Elastic recoil
energy stored in the stretched plantarflexor tendon causes the posterior
portion of the foot to pivot over the toe rocker. The result is rapid ankle
plantarflexion that leads to a notable power burst at the ankle. This helps
thrust the tibia forward contributing to the 40° of knee flexion that
occurs during this period. The relatively large push-off force recorded during
the toe rocker results in part from the passive recoil.”

The question Perry and Burnfield asked was why is there rapid motion for
power burst at the ankle even though the muscle is not dynamically contributing
to it? According to Burnfield, part of this discovery emerged from other
researcher’s use of technology including the work of Tetsuo Fukunaga, PhD.
Previously, gait researchers did not have a means for interpreting the fourth
rocker because there was no capacity to study biomechanics at the tendon level
as the person moved.

“What we were able to learn from researchers and others using
ultrasound for different applications, is that during terminal stance’s
ankle rocker, the plantarflexors’ vigorous contraction actually stretches
the Achilles tendon,” Burnfield said. “Then as body weight starts to
unload off the limb during the toe rocker, the stretched tendon is able to
rebound. That rebound is what contributes to the rapid plantarflexion that you

The investigation and discovery of a fourth rocker may quell a small
debate involving the ankle.

“This helps resolve a historical controversy. While some
researchers interpreted where from an engineering perspective researchers were
calculating these high ankle power demands to indicate the need for active
muscle action,” Burnfield explained. “Others documented rapidly
diminishing ground reaction forces and EMG, suggesting that not much was
happening at the ankle.”

By introducing a fourth rocker, engineers will have additional answers
and information for their prosthesis designs, which could provide the
lower-limb amputee with a smoother transition from stance to swing limb

“This added understanding of the mechanics occurring during the
final period before toe-off should help guide material and design
criteria,” Burnfield explained. “What are the material properties or
designs that not only progress the person over the ankle rocker, but also allow
rapid plantarflexion during pre-swing’s toe rocker? Capturing these
mechanics during the pre-swing toe rocker should facilitate improved
positioning for limb advancement and foot clearance during swing.”

According to Burnfield, effectively capturing that power burst through
prosthetic componentry may assist the patient during swing limb advancement.
The componentry should facilitate a smoother transition from stance to swing.
by Anthony Calabro

For more information:

  • Perry J, Burnfield J. Gait Analysis: Normal and Pathological
    . 2nd ed. Slack Inc.; Thorofare, NJ; 2010.


The plantar surfaces of the great toe and the head of the first
metatarsal were identified about 50 years ago as the final weight-bearing area
of the foot during a step. The lack of further study was attributed to the
absence of adequate instruments. Recent study, with more modern
instrumentation, has identified the dynamic function occurring during this
final phase of stance, i.e. the pre-swing toe rocker.

During pre-swing the normal person is seeking maximum step length. The
forward momentum generated by the limb, advances the pelvis and the alignment
of the trailing stance limb. The effect is visualized by the magnitude of
dorsiflexion at the metatarsal-phalangeal joint. In normal barefoot walking,
the toes dorsiflex 55·, accompanied by heel elevation. Both actions
contribute to body advancement by increasing the trailing limb angulation
during pre-swing.

The application of the pre-swing events to prosthetics requires finding
new materials with the essential qualities of mobility and stability. What is
needed is a flexible fourth rocker tip on the prosthetic foot.

— Jacquelin Perry, MD, ScD
emeritus, department of orthopaedics, University of Southern

Leave a Reply

Your email address will not be published.