Sand path
Dirt path
Home concrete
Sidewalk
Warehouse
Flat shoe
The Science — Triplanar Motion Control
Your feet aren't
the problem.
The ground is.
Flat ground doesn't just misalign the foot's primary control mechanism — it takes it offline. The passive geometry that's supposed to convert each step's force into rotation has no angle to work with on a flat surface, so it doesn't run. Muscles take over instead, contracting actively to force the job the geometry was built to do for free. Protalus Landing Gear is the science of putting the passive mechanism back to work.
The Problem
The body was designed
for variable terrain.
The ground changed. The foot didn't.
Natural ground is varied, oblique, dynamic. Every surface shifts, tilts, and yields — giving the foot the mechanical input it needs to move correctly. Industrial civilization replaced that with a single, universal surface: flat. The consequences are not cosmetic.
The subtalar joint (STJ) — the oblique hinge connecting your heel bone to the rest of the foot — governs the entire kinematic chain above it. It is not designed to operate on flat ground. Its axis runs at a precise angle, expecting terrain that loads it along that angle at every step.
The architecture of the heel bone itself tells part of the story. The lateral aspect of the calcaneus sits more plantar (lower) in humans than in apes — a documented anatomical difference, not incidental. It's part of why the subtalar joint axis runs at roughly 42° from horizontal and 16° from the sagittal plane. On natural terrain, the outer heel contacts first, and the rotational arc unfolds gradually, paced by the give of the ground.
Put that foot inside a modern shoe on a flat floor and the passive mechanism has nothing to work with. There is no angle for the axis to convert into rotation on its own. It doesn't run inefficiently — it doesn't run. Muscles contract actively instead, forcing the rotation the geometry was supposed to produce for free. Every step from that point is active compensation standing in for a passive system with nothing to work with — thousands of times per day.
"The foot is not a pressure problem. It is a control system — one that expects a specific motion environment. When that environment is replaced by flat industrial floors, the system is forced to run faster than it's built for, in predictable, measurable ways."
— Dr. Martyn R. Shorten, Ph.D., Former Director, Nike Sport Research Laboratory · BioMechanica LLC100 Years of Debate — Now Facing a Formal Challenge. The biomechanics paradigm that built much of the insole industry — the mobile adaptor–rigid lever model — has been questioned for decades. A 2023 peer-reviewed review in Biological Reviews argued the scientific community should move past it.
Behling, Rainbow, Welte & Kelly · Biological Reviews 98:2136–2151 (2023) ↗Natural rocky terrain
Varied, oblique surface loads the lateral calcaneus at the correct angle, gradually. STJ axis paced correctly.
42/16 pacedDesert scrubland
Irregular, yielding surface. Micro-variation keeps the proprioceptive loop engaged and the rotation gradual.
42/16 pacedCommercial concrete walkway
Zero-degree surface. No angle for the axis to work with. The passive mechanism fails outright; muscles compensate instead.
42/16 offlineWarehouse industrial flooring
10,000+ steps per shift on the same flat geometry. Muscles compensate for the offline mechanism on every one.
42/16 offlineEngineered footwear on flat surface
The shoe compounds the problem. Heel cushioning softens the load but doesn't restore the pacing.
42/16 rushedThe Mechanism
A published axis.
An engineered solution.
The subtalar joint axis was mapped and published in peer-reviewed biomechanics literature decades ago (Manter, 1941). It is not theoretical — it is a documented mechanical specification of normal human gait.
This asymmetry is significant. The calcaneus does not move straight up and down — it rotates around an oblique hinge. That rotation is the drive shaft of the entire kinematic chain above. The tibial rotation it produces governs knee tracking, hip alignment, and lumbar positioning with every step.
No flat surface recreates this geometry. No symmetrical heel cup can guide an asymmetric rotational path. The engineering gap was not filled by any product on the market — until the axis was used as the actual specification.
"If gravity does the deciding at heel strike, every structure above it absorbs the cost. Thousands of times a day."
— Dr. Martyn R. Shorten, Ph.D., Former Director, Nike Sport Research Laboratory · BioMechanica LLC
The drive shaft of your entire body
The STJ is the universal joint coupling foot tilt to tibial rotation. When the axis is loaded at the pace it's built for, it converts ground reaction force into clean rotational torque that travels up the kinematic chain — governing knee tracking, hip alignment, and lumbar positioning with every step.
Why It Works for Everyone
No measurement.
No custom fitting.
One geometry.
The subtalar joint axis is the same in every human foot. What varies is how far each person's gait has drifted from it. The Landing Gear platform doesn't need to be customized — it needs to be correct. And correct is universal.
Custom orthotics are prescribed because they assume the problem is anatomical — unique to your foot's shape. But the root cause isn't your anatomy. It's the environment: flat industrial surfaces that removed the terrain geometry every human STJ was built to operate on. That problem is the same for everyone. So is the solution.
The 42/16 axis was published as the specification for normal human gait — not for a specific foot type, arch height, or pronation pattern. When the Landing Gear platform recreates that geometry, every foot responds to it. Not because it was fitted to you, but because it was built to the axis your body already knows.
"It doesn't alter your gait. It restores the conditions your gait was designed for."
Self-regulating response
High drift
A foot significantly off-axis encounters stronger geometric guidance — the calcaneus is intercepted earlier and redirected along a longer corrective arc.
Low drift
A foot already close to neutral receives gentle confirmation — a light directional cue that maintains the axis without imposing unnecessary correction.
Result
Every foot arrives at the same destination — the correct axis — by a path proportional to how far it started from it.
What Makes It Different
Every insole is a shape.
This is geometry.
The difference is not cosmetic. Shape describes how something looks at rest. Geometry describes how forces and motion move through space. These are not the same discipline.
The symmetry problem. Every floor you walk on is symmetric — the same plane on both sides of every heel contact. The subtalar joint axis deviates 16° toward the body's midline. A symmetric surface cannot pace an asymmetric rotation the way it was built to unfold. Every other insole is also symmetric. Landing Gear is the only product engineered to match the asymmetry the STJ actually requires.
| Dimension | Every other insole — Shape | Protalus — Geometry |
|---|---|---|
| Design basis | — Static contour for foot at rest | → Dynamic guidance across full gait cycle |
| Dimensionality | — 2D: width, length, height | → 3D + time: angle, rotation, sequence |
| Timing | — Passive: reacts after motion occurs | → Preemptive: acts at heel strike, before the foot loads |
| Symmetry | — Symmetric: cannot pace an asymmetric axis | → Asymmetric: mirrors the oblique 16° STJ deviation |
| Arch support | — Pushes upward into a spring that needs to move freely | → Gives the spring its correct operating track |
| Motion philosophy | — Restricts subtalar movement — dampens the elastic energy cycle | → Paces motion along the correct axis — preserves the energy cycle |
| Scientific basis | — Built on mobile adaptor–rigid lever paradigm (challenged since 2023) | → Built on confirmed STJ axis geometry — tested with 3D motion capture |
"Flat ground gives your feet and your brain less to talk about. Triplanar geometry gives that conversation back — subtle angles the body can feel and respond to, restoring real stability and control."
The Mechanism in Motion
What happens at
every step.
Four phases. Four subsystems. One geometry restores all of them.
PHASE 01
Initial Contact — Heel Strike
The decisive moment.
If the heel makes initial contact without geometric guidance, the full force of bodyweight loads onto a joint already off-axis. Protalus intercepts this moment — guiding the heel into its designed 4–6° rotational arc at the pace the axis is built to run it, before any other phase begins.
PHASE 02
Loading Response — Weight Acceptance
Controlled rotation. Energy storage begins.
The STJ continues its small, controlled rotation. The tibia follows in phase — the critical coupling that determines whether forces are transmitted cleanly up the kinematic chain. The tibialis posterior tendon and plantar fascia begin loading elastically: the rubber band is stretching, storing energy for return at push-off.
PHASE 03
Mid-Stance — Body Over Foot
Re-centering. The spring fully loaded.
Motion peaks, then the drive shaft begins to re-center. Tibial rotation reverses smoothly — the direction of force transmission flips without shear. The rubber band is fully loaded. The servo controller receives clean proprioceptive input: the foot is reporting an accurate, directional signal.
PHASE 04
Terminal Stance / Push-Off
Energy return. The system completes its cycle.
The heel rolls back into inversion, the drive shaft locks the foot into a rigid lever at the right moment, and the rubber band releases its stored energy as propulsion. Energy stored in the tendon and fascia during loading returns at push-off — instead of being spent again by compensating muscles.
The Mechanical Case
The subtalar joint isn't a hinge.
It's a torque converter.
Every step, it takes the vertical force of your foot hitting the ground and turns it into the rotational torque your leg needs to move forward. That conversion works best within a fairly narrow window: a joint axis tilted roughly 42° up from the ground and 16° in from the heel line. Manter's original measurement — still the reference point today.
(sagittal tilt)
(transverse tilt)
Why the degrees matter for rotation to happen at all. Rotation needs two things: force, and distance from the axis that force is turning around. That distance is the moment arm. When the heel lands in a slight natural supination — 2° to 3° — the ground force lands just outside the STJ axis. That small offset creates a small, predictable pronation moment arm.
Torque is force times moment arm. Inside the right range, the incoming force has roughly the leverage it needs to start the joint rotating, cap the rotation at a controlled 4°–7°, and let the leg internally rotate the way it's built to.
controlled rotation — inside the correct window
The joint doesn't stop at its pronated peak. It reverses through neutral and past it, using the same torque-conversion mechanism to re-lock the foot into a rigid lever. Flat ground doesn't just rush entry into this sequence — it rushes the exit from it too.
Outside that range, the passive conversion doesn't happen — muscle takes over, and impact turns into damage instead.
Too much pronation. When the axis drifts inward, the ground force lands far outside the axis line — an oversized, less controlled pronation torque. The joint runs low on room to rotate cleanly. The lateral process of the talus can approach the calcaneus more closely than the design intends. With less room left to rotate, more of the impact energy is absorbed as joint compression and tissue shear rather than smooth movement.
Too much supination. When the axis shifts too far the other way, the ground force lands close to the axis line. Moment arm shrinks toward zero — and with it, the torque available to rotate the joint. More of the impact is passed straight up into the ankle, tibia, and knee instead of being converted into controlled motion.
Reduced efficiency
>7°
Too much pronation
Oversized moment arm. Joint runs low on rotational room. More energy converts to compression and shear instead of clean rotation.
Correct window
4°–7°
Controlled rotation
Moment arm sized correctly. Force has the leverage to rotate the joint, then cap the rotation on schedule.
Reduced efficiency
~0°
Too much supination
Force lands near the axis line. Moment arm shrinks toward zero, and so does available torque. More impact drives straight up into ankle, tibia, knee.
Why Flat Ground Breaks This System
The foot didn't evolve
for flat ground.
It evolved for sand, dirt, uneven terrain — surfaces that give and mold to the foot's angles.
Flat ground doesn't give. Two things go wrong the instant the heel lands.
It forces an almost-instant flat match. The STJ axis sits at roughly 42°, which means the heel bone needs to tilt as it rotates. Flat ground doesn't tilt with it. Instead it brings the entire heel into contact almost simultaneously — compressing the gradual rotation the joint is built to perform into a fraction of the time.
It compresses the timing. On a surface that gives, ground force builds gradually, and the joint has time to rotate under control. On flat ground, force ramps up much faster. That faster ramp loads the lateral side of the foot more abruptly, pushes the pronation moment arm toward the edge of its comfortable range faster, and gives stabilizing muscles like the tibialis posterior less time to respond before the rotation is already underway.
Mechanism 01
Near-instant flat match
The axis needs the heel to tilt through its rotation gradually. Flat ground forces near-simultaneous heel contact instead — the gradual rotation gets compressed into an instant.
Mechanism 02
Compressed timing
Force that should build gradually ramps up almost immediately. The moment arm moves toward the edge of its safe range faster than stabilizing muscles can respond.
Landing Gear rebuilds the terrain — not the foot.
It restores the 42°/16° geometry the joint uses, so the torque conversion the STJ is built to do can happen at the pace it's designed for — closer to how it would on the ground the foot evolved for.
The Geometric Proof
The STJ is not a hinge.
It is a helical axis in three dimensions.
A hinge joint rotates in one plane. The subtalar joint is not a hinge. It is a helical axis — an oblique vector fixed in three-dimensional space, around which the calcaneus rotates simultaneously in all three anatomical planes: dorsiflexion/plantarflexion, inversion/eversion, and abduction/adduction all happen as one coupled motion. This is called triplanar motion.
This architecture gives the foot its essential transition: from mobile adaptor to rigid lever. At heel strike, the STJ pronates — absorbing impact, adjusting to surface variation. At toe-off, it supinates and locks — the plantar fascia tensions via the windlass mechanism, the midfoot stiffens, and stored elastic energy projects forward as propulsion. That transition is the purpose of the STJ axis.
A flat surface gives the passive mechanism no angle to convert. It doesn't fire. Muscles force the rotation instead — actively, every step — and the midfoot locks late, if at all, because it's being pushed through the motion rather than driven by geometry. Every push-off becomes a push-off assembled by muscle effort instead of delivered by mechanism. More force leaks into give rather than projecting forward.
A flat surface is a two-dimensional plane: Z = 0. The STJ axis is a three-dimensional oblique vector. A flat surface, on its own, gives no gradual cue for a vector that isn't vertical. That mismatch is why flat surfaces work against the geometry of the human STJ, by definition — not by opinion — even if the joint still functions on them.
- The STJ axis is a 3D oblique vector: roughly 42° sagittal, 16° transverse
- 3D oblique vectors are best served by surfaces that yield gradually along their tilt
- A flat surface is a 2D plane — Z = 0 — with no gradual give
- A flat surface gives no timing cue matched to a non-vertical axis
- The STJ axis is not vertical
- Flat surfaces cannot passively drive the human STJ — by definition. The joint only rotates on them because muscle forces it to.
(side view)
(top view)
The gyroscopic principle
The heel lands in slight supination — lateral edge first, system cocked, pre-loaded. A gyroscope does not absorb force — it converts it. The direction of force entry relative to the axis determines what happens to that force. The STJ works identically. Vertical ground reaction force meeting the calcaneus at 42°/16°, already loaded in slight supination, creates torque around the oblique axis. The pronation arc unfolds not because the foot collapsed — but because the geometry channeled the force where it's designed to go.
On flat ground, that supinated landing angle still exists — swing-phase mechanics set it, not the ground. But the ground gives the gyroscope no angle to convert into spin on its own, so the passive mechanism does not fire. The rotation that follows is driven by active muscle contraction, not by the geometry. The elastic energy that would have stored in the tibialis posterior tendon and plantar fascia stores less, because the tissue is being forced through the motion rather than loading passively. The bow does not draw on its own. Muscle draws it by hand, every step.
The supination at landing is the pre-load. The oblique axis is the conversion mechanism. The pronation arc is the output. All three work best with ground geometry that paces them — not just any ground.
The asymmetry matters
A symmetric axis would be 45°/0°. The STJ axis is roughly 42°/16° — inherently asymmetric. This is why no symmetric surface, no symmetric heel cup, and no symmetric insole can pace correct STJ motion the way an asymmetric one can. The rotational path is asymmetric by design. The surface that paces it well should be too.
Protalus Landing Gear is engineered to match this asymmetry — medial geometry deeper, lateral shallower, mirroring the actual 16° medial deviation of the STJ axis.
What's Actually Been Measured
A number from real data.
Not a formula. A measurement.
Direct comparison studies have measured what changes when rearfoot-strike runners move from soft to hard ground. Zhang et al. (2023, PLOS ONE) ran 30 habitual rearfoot strikers across artificial grass, synthetic rubber, and concrete, capturing full 3D kinematics and kinetics on each surface.
The result: ankle joint moments differed significantly across all three planes between surfaces, and post-hoc comparisons showed ankle plantarflexion, eversion, and abduction moments were consistently lower on grass or rubber than on concrete. Separately, rearfoot strikers have been shown to exhibit greater foot pronation on harder surfaces than softer ones.
That difference doesn't disappear. It's absorbed by active muscle work — the tibialis posterior, peroneals, and hip stabilizers doing more active stabilizing work than they would need to on a surface that paces the load more gradually. This isn't a comfort problem. It's a measurable joint-loading difference, backed by force-plate and motion-capture data, not an invented formula.
Tibialis posterior
Primary STJ stabilizer
On a surface that rushes the load, this muscle-tendon unit may be asked to do more active stabilizing work than it would on ground that paces the load gradually.
Peroneals
Lateral stabilizers
Co-contract more to manage a faster rotational onset — a real, measurable difference in joint moments across surfaces.
Gluteus medius
Hip stabilizer
Activation patterns can shift as compensation runs up the kinetic chain — an area some research links to changes in oxygen consumption at matched pace.
Cushioning and material stiffness change how a surface feels. They don't restore the joint's 42°/16° geometry. The measured differences above track surface hardness and joint moments — not softness alone.
A Visible Pattern Worth Understanding
Why some people walk
with their feet turned out.
One plausible compensation pathway — not the only explanation.
Out-toeing gait has several possible contributors, and researchers don't treat it as having one single cause. One pathway some research points to starts at the subtalar joint, and it follows the geometry problem described above.
When the passive STJ mechanism cannot run — as on flat ground — muscle forces the joint through its rotation instead, and the talus may internally rotate more than it would with the passive mechanism engaged. That internal rotation can travel up the kinetic chain:
The idea: the body needs to walk forward, but part of the limb is rotating inward. One way to solve that is to rotate the leg outward at the hip — swinging the foot outward, recovering a more functional push-off axis. This is a mechanically coherent hypothesis, consistent with kinetic-chain logic, though it's one proposed pathway among several possible contributors to out-toeing, and shouldn't be read as a proven, exclusive cause.
Out-toeing may sometimes reflect a problem one joint lower, not just at the hip. When Landing Gear paces the heel correctly at entry, the aim is that the talus doesn't need to internally rotate as much, so the chain has less to compensate for.
One proposed pathway
Passive STJ mechanism offline
Flat ground gives the calcaneus no angle to convert passively. Muscle forces the rotation instead, and the talus may pronate beyond its typical arc.
Possible chain compensation
Internal rotation may propagate upward: talus to tibia to knee. The hip may end up facing a less efficient direction for forward travel.
Possible hip external rotation
One way the body could solve the direction problem is at the hip — rotating outward to swing the foot back toward the line of travel.
With Landing Gear
The 42°/16° geometry paces heel entry. The aim is less excessive internal rotation of the talus, and less for the chain above to compensate for.
The Performance Consequence
The cut that didn't happen.
The pivot that arrived too late.
Every cutting movement — a tennis split-step, a football plant-and-drive, a basketball crossover — requires the foot to rapidly supinate: STJ inversion, locking the foot into a rigid lever for explosive lateral push-off. That supination needs to happen in milliseconds.
If the STJ is already sitting in a more pronated position than usual, the range available for rapid supination is reduced. The foot has to travel further to reach the rigid-lever configuration before it can generate force. That extra travel is delay. In sport, delay can be the difference between beating the cut and getting beaten.
This applies at every level. Amateur or elite, the mechanism is the same. It also applies beyond sport: the stumble you catch, the step off a kerb, the pivot under a heavy load — all draw on the same reflexive STJ response.
There is also a loading consequence: excessive STJ pronation during cutting is correlated with increased knee valgus — inward knee collapse — which is a well-documented ACL injury risk factor. The foot's reduced ability to supinate on demand doesn't just slow the athlete. It's associated with load redistributing upward through the kinetic chain.
Your body has backup systems. But backup systems aren't designed to function as primary systems indefinitely — and they weren't designed to absorb thousands of repetitions per day. They compensate. They adapt. Any degradation from that tends to show up on a timeline that can take years to become visible.
| Movement | What the STJ must do | Off-axis consequence |
|---|---|---|
| Cut / direction change | Rapid supination → rigid lever in milliseconds | Lever less ready. Possible delay. More force leaks into give. |
| Plant and drive | STJ locks to transmit torque through plant foot | Torque transfer less clean. Knee valgus risk associated with this. |
| Catching a stumble | Reflexive supination — no conscious input | Reflex has to force a rotation the passive mechanism isn't providing. Potentially slower, less reliable. |
| Toe-off / propulsion | Midfoot locks via windlass for energy return | Midfoot locks later and less completely. Some energy leaks. Stride may shorten. |
| Rotational sports | Clean torque transfer through stationary foot | STJ instability can dissipate rotational energy. Some power lost. |
Why Custom Insoles Cannot Solve This
A scan captures shape.
Shape is not axis.
The custom insole industry is built on a seductive premise: measure the exact geometry of an individual foot, build a surface perfectly matched to it. The premise runs into a problem — not because the measurement is inaccurate, but because it measures a different thing than the one that matters here.
What a foot scan captures is plantar surface topography — the two-dimensional shape of the bottom of the foot at one static moment. Useful information about a surface. Not the same information as a joint axis.
The subtalar joint axis is internal. It doesn't show up in a scan. Its location and orientation relative to the calcaneus are what would actually need personalising — if personalising were what the foot needed most.
Here's the useful part: Manter's data, and work since 1941, shows STJ axis orientation falls within a fairly narrow, consistent range across the population. The foot that's been "customised" may be the one that needed personalising the least — because the geometry it requires is, within tolerances, similar in most human feet: roughly 42°/16°.
The more useful question isn't "what shape is your foot?" It's "what geometry does your subtalar joint axis need to work well?" The answer to the second question is broadly similar across human feet.
"Custom insoles are a photograph of your foot. Your foot doesn't primarily need a photograph of itself. It needs a surface that works with the geometry it was born with — geometry that's broadly similar across humans, and that flat surfaces don't pace well."
What the scan sees vs. what matters
Even a custom insole that perfectly matches the plantar surface of your foot has optimised for shape at one static moment — not for the axis that governs every dynamic moment. Shape is not axis.
The Heel Entry Problem
Much of what matters happens
before the foot is fully loaded.
No static scan can get there.
The calcaneus needs to land within a workable inversion/eversion range to load the STJ axis well — at heel strike, before much weight has transferred. This early window matters a great deal.
If the heel lands outside that comfortable range — even by a few degrees — the axis can be under strain from the earliest part of the step. The rest of the gait cycle involves the body managing the consequences of that early strain.
A scan captures the foot at rest. At rest is not heel strike. A snapshot of where you ended up says little about the geometry that mattered at the beginning.
Landing Gear's geometry is aimed at the entry condition. The 42°/16° surface is designed to help the calcaneus arrive within a better inversion/eversion range before much weight is loaded — addressing an early moment that has outsized influence on the rest of the step.
The core limitation of scan-based insoles: they have no mechanism for shaping heel entry angle. They optimise for the moment well after landing — not the earliest phase of the step, which is what this section is about.
A Well-Paced Landing
Two patterns.
Every step leans toward one or the other.
The STJ's passive mechanism either gets an angle to convert force on its own — or it doesn't run at all, and muscle does the job by force instead. Over thousands of steps per day, that difference accumulates in energy, performance, and the long-term load on a body substituting active effort for a passive system with nothing to work with.
- Foot completes mobile adaptor to rigid lever at the pace the tendon and fascia are built to load.
- Stabilizer muscles do less active work. More energy stays available for the whole shift, the whole run, the whole day.
- Rapid supination more available on demand — the reflex that catches a stumble, completes a cut, finishes the pivot.
- Proprioceptive signal reflects a passively-driven, geometry-correct load. Balance calibrated to a stable reference point.
- Kinetic chain from heel to hip runs cleaner. Less duck-walking tendency. Less added knee and hip load.
- Elastic energy stored in tendon and fascia returns at push-off — more of it as free propulsion, less as muscle expenditure.
- Midfoot locks later and less completely. More propulsive force leaks into give on each step.
- Tibialis posterior, peroneals, gluteus medius doing more active compensation work — burning more energy for the same movement.
- STJ has further to travel before it can supinate. That extra travel is what a slow cut or a slower pivot can feel like.
- Balance calibrated to a compensated position. The proprioceptive signal is less accurate.
- More knee load. More hip compensation. Feet may point out. The body adapts — quietly, over years.
- Elastic energy stores less efficiently. More of each step becomes muscular expenditure instead of return.
This is what the geometry predicts.
Here is what motion capture found.
BioMechanica LLC — an independent biomechanics laboratory in Portland, Oregon — measured what the STJ axis geometry predicts. 31 subjects. 20 NaturalPoint Optitrack cameras at 100fps. The subtalar joint axis tracked across four conditions through every phase of the gait cycle. This is a real, independently-run study, but it hasn't been published in a peer-reviewed journal — worth knowing when weighing it against the peer-reviewed citations elsewhere on this page.
On flat ground: 3 of 31 subjects moved into the defined correct STJ range. The M-100, built around the 42°/16° axis geometry, moved 28 of 31 into that range — and kept them there. The geometry pointed toward a joint under more pressure on flat ground. The motion-capture data is consistent with that.
The Anatomy Was Already Telling This Story
The axis is the answer
to the question the heel bone was asking.
Related structures. Mutually consistent. Rushed by flat ground.
The lateral aspect of the calcaneus sits more plantar (lower) in humans than in apes. This morphology is well documented (Boyle et al., 2018). It's part of why the heel bone tends to contact the ground with its lateral edge first — producing a slight supination at landing. Not a learned behavior. Not a style choice. The shape of the bone contributes to it.
The STJ axis at roughly 42°/16° is the mechanism that converts that lateral-first contact into triplanar motion. Research frames the LPP's plantar position as adaptive — likely helping distribute load during heel-strike (Boyle et al., 2018) — though the exact internal mechanics are still an active area of study (Koneru et al., 2024). The calcaneal morphology and the axis geometry are related parts of the same broader picture.
Natural terrain reinforced the system by pacing it. Varied surfaces loaded the lateral calcaneus first and gave the joint gradual, oblique geometry to work with. The whole system worked well together — the bone shape producing the entry, the axis converting it, the terrain pacing both.
Flat ground doesn't stop any of these three — it rushes all of them. Heel contact happens closer to simultaneous. The axis still receives an input, just compressed into less time than it evolved handling.
Calcaneal morphology
Lateral process sits more plantar in humans than apes. Heel tends to contact ground lateral-first. Slight supination at landing is structural — built into the bone.
STJ axis ~42°/16°
Converts that lateral-first, slightly supinated entry into triplanar rotation. The axis and the bone shape work together, not independently.
Natural terrain
Varied oblique surfaces reinforced lateral-first loading and paced it gradually — continuously, with every step.
Landing Gear aims to restore the pacing this system evolved with — not an arbitrary angle, but the specific entry condition the calcaneal morphology tends to produce, run at something closer to its intended speed.
Why Insoles Don't Just Underperform
They give the flight computer
an inaccurate altitude.
The instrument is reading. The reading is off.
A plane's autopilot doesn't make random decisions. It makes logical decisions based on the data its instruments feed it. If the altimeter reads the ground as 500 feet lower than it actually is, the autopilot can execute a flare at the wrong moment. Every system fires correctly — on inaccurate data.
Conventional insoles can do something similar to the subtalar joint system. A symmetric arch support gives the foot's proprioceptive system a static, symmetric reference — different from the asymmetric, dynamic signal the STJ evolved reading. The system initiates its rotation, but on a surface that doesn't correspond to the actual axis geometry. The pronation arc can begin from a less favorable position. The midfoot may try to lock at a less ideal moment.
This reframes the insole category. The question isn't whether insoles provide some surface — they do. It's what that surface tells the STJ system, and how well that matches what the joint actually needs. For a symmetric insole shaped to a static foot profile, that match is limited from the first moment of every step.
On custom insoles specifically: a custom insole gives a more precise version of the same limited reading. It measures the exact shape of your foot at rest and builds a surface that corresponds precisely to that shape. The measurement is accurate. It's still calibrated to a different variable than the one that matters most here. Shape at rest is not axis in motion.
Conventional insole
Symmetric surface geometry. Arch push. Static shape. The STJ system reads a signal that doesn't correspond well to the oblique rotation it's built around.
Landing Gear
42°/16° asymmetric geometry. Lateral calcaneus loaded first, paced gradually. The STJ system reads a signal that corresponds more closely to the rotation it's built around.
The name
Aircraft landing gear is not just what keeps the plane off the ground. It's the system that makes a controlled landing possible — the geometry, the sequence, the instruments all working together at the moment of contact.
A Debated Question in the Field
Does the body adapt to
a consistent, imperfect signal?
A plausible hypothesis, not a settled one.
Here's a genuine question in podiatric and PT literature, worth stating honestly rather than as settled fact: does the body's proprioceptive system adapt to whatever geometric signal it consistently receives — even an imperfect one?
The general principle — that repeated, consistent sensory input can shift what a system treats as "normal" — is a reasonable one drawn from broader motor-learning research. Applied here, the hypothesis is that a conventional insole's consistent, symmetric signal over months and years could lead muscles and proprioceptive calibration to reorganize around that signal as a baseline, rather than around the joint's ideal loading.
This is a plausible idea, and it's part of why some clinicians are cautious about long-term reliance on rigid, corrective orthotics. But it is not a settled mechanism with strong direct evidence behind it in this specific context, and this page hasn't found a citation that establishes it conclusively. We're presenting it as a hypothesis worth being aware of — not as proven fact.
If real, the implication would be: what matters isn't just whether a product changes how you feel in the short term, but what signal it's giving your system to adapt toward, over the long term.
To be direct: this section describes a hypothesis under discussion in the field, not a proven mechanism. We think it's worth including because it's a reasonable consideration — not because we can currently back it with a specific study on orthotic-specific proprioceptive dependence.
The Mechanism Is The How
Here's the so-what.
Three groups. Three different reasons this geometry is worth paying attention to.
On your feet all day
Why does my body feel more beat up at the end of a shift than it used to?
On flat ground, the passive mechanism doesn't fire — there's no angle for it to convert. Muscle does the job instead: tibialis posterior, peroneals, hip stabilizers, contracting actively on every single step to force a rotation geometry was supposed to produce for free. Multiply that by 10,000+ steps and end-of-shift fatigue isn't random. It's the accumulated cost of running on the backup generator all day.
Athletes / performance
Why does the cut or pivot feel a beat slow sometimes?
Rapid direction changes need the foot to reach a locked, rigid-lever position in milliseconds. If the passive mechanism isn't engaging and muscle is forcing the joint through a more pronated position instead, the foot has further to travel to reach that lock — and that extra travel is exactly the delay between starting a cut and finishing it.
Everyone else
Why should I care about a joint I've never thought about?
Because it's not occasional — it's every single step, all day, for your entire life. Small inefficiencies that would be irrelevant once become significant at 8,000–10,000 repetitions a day. This isn't about one injury event. It's about what happens to a body substituting active muscle effort for an offline passive mechanism, thousands of times a day, every day, for years.
What we're not claiming: that this causes any specific diagnosis, or that everyone with foot pain has this exact issue. What the evidence on this page supports is narrower and still worth acting on: the mechanism is real and measurable, it happens every step, and geometry that gives the passive mechanism an angle to actually run on is a reasonable thing to try.
The signal has been a little off
for most of the steps you've ever taken.
Your foot's system is intact. It's been running on a compressed timeline instead of the one it evolved for. Give it better pacing — and your own body does the rest.
Get Landing Gear — M-100 Start with T-100What This Page Is Built On
Every claim, traced to a source —
and labeled by what kind.
A peer-reviewed measurement, an independent unpublished study, and a clinical textbook all carry different evidentiary weight. We've labeled each source below rather than letting them read as equivalent.
Manter, J.T. (1941). Movements of the subtalar and transverse tarsal joints. Anatomical Record, 80(4), 397–410.
The original measurement of the ~42°/16° STJ axis — the foundational citation for the geometric argument throughout this page.
Maharaj, J.N., Cresswell, A.G., & Lichtwark, G.A. (2017). Subtalar joint pronation and energy absorption requirements during walking are related to tibialis posterior tendinous tissue strain. Scientific Reports, 7, 17958.
Direct measurement showing the STJ absorbs energy in early stance and returns it in late stance, and that this is explained largely by tibialis posterior tendon strain (R²=0.53) rather than muscle fascicle stretch.
doi.org/10.1038/s41598-017-17771-7 ↗Ker, R.F., Bennett, M.B., Bibby, S.R., Kester, R.C., & Alexander, R.M. (1987). The spring in the arch of the human foot. Nature, 325, 147–149.
Foundational paper establishing that the plantar fascia and arch store and return elastic energy during gait.
doi.org/10.1038/325147a0 ↗Boyle, E.K., McNutt, E.J., Sasaki, T., Suwa, G., Zipfel, B., & DeSilva, J.M. (2018). A quantification of calcaneal lateral plantar process position with implications for bipedal locomotion in Australopithecus. Journal of Human Evolution, 123, 24–34.
Confirms humans have a significantly more plantar lateral heel process than apes. This paper describes the structure as "hypothesized to" aid load dissipation, not proven — this page matches that hedge.
doi.org/10.1016/j.jhevol.2018.05.008 ↗Koneru, A. et al. (2024). Comparing lateral plantar process trabecular structure to other regions of the human calcaneus. The Anatomical Record.
Notes the LPP "likely plays a minor, supportive weight-bearing role" — included here to represent the field's actual uncertainty rather than overstating settled consensus.
Zhang, X. et al. (2023). Surface effects on kinematics, kinetics and stiffness of habitual rearfoot strikers during running. PLOS ONE, 18(3).
Direct measurement: rearfoot strikers show greater pronation and higher ankle/hip joint moments on concrete than on grass or synthetic rubber. The real citation behind this page's "hard ground changes loading" claims.
doi.org/10.1371/journal.pone.0283323 ↗Behling, A.H., Rainbow, M.J., Welte, L., & Kelly, L. (2023). Biological Reviews, 98, 2136–2151.
Review arguing the field should move past the mobile adaptor–rigid lever paradigm.
Root, M.L., Orien, W.P., & Weed, J.H. (1977). Normal and Abnormal Function of the Foot. Clinical Biomechanics Corporation.
Source of the standard clinical gait-phase degree ranges used on this page. A foundational podiatric textbook, not a fresh measurement — presented as the standard clinical reference range.
Shorten, M.R. (2019). 3D motion capture gait analysis, n=31. BioMechanica LLC, Portland, OR.
A real, independently-run measurement — no Protalus involvement in methodology or reporting — and the outcome measured (subjects in/out of a defined STJ range) is objective, not opinion. It has not been published in a peer-reviewed journal, so it hasn't been through external methodological review the way the citations above have. We label it this way to be precise about its status, not to cast doubt on the measurement itself.