Energetic gains at one joint were offset by compensations upstream
In the quiet mechanics of a single human step lies a complexity that resists simple solutions. Researchers studying carbon fiber foot orthoses have found that increasing their stiffness improves energy efficiency at the metatarsophalangeal joint, yet the midtarsal joint compensates by absorbing rather than generating power — a reminder that the body is a system of interdependencies, not isolated parts. The foot, it seems, redistributes its burdens rather than surrendering them.
- Stiffer carbon fiber orthoses do exactly what designers intended at the ball of the foot — the MTP joint becomes more spring-like, storing and returning energy with each stride.
- But the midtarsal joint, deeper in the arch, quietly absorbs the consequences, shifting from a power-generating motor into a force-dampening buffer.
- The ankle remains largely unchanged, leaving researchers with a picture of compensation rather than systemic improvement.
- For healthy walkers, the energetic gains at one joint are effectively cancelled out by increased demand at another — the insert solves one problem by creating another.
- Clinicians may need to abandon the assumption that more stiffness means better gait, and instead target orthosis design toward specific joint dysfunction rather than whole-foot correction.
A team of researchers has uncovered a counterintuitive finding about carbon fiber foot orthoses: making them stiffer helps one joint while quietly burdening another, leaving the net benefit for healthy gait uncertain.
The logic behind stiffer inserts had seemed straightforward. A more rigid orthosis should encourage the metatarsophalangeal joint — the hinge at the ball of the foot — to behave more like a spring, storing and releasing energy with each step. Prior research had supported this, and the new study confirmed it: the MTP joint did become more spring-like and more efficient as stiffness increased.
But when the researchers looked beyond that single joint, a more complicated picture emerged. The midtarsal joint, nestled deeper in the arch, began absorbing more energy rather than generating it. It shifted from acting like a motor to acting like a damper — soaking up force rather than contributing power. The ankle, meanwhile, showed only minor changes.
What the researchers observed was compensation. The foot redistributed its mechanical demands rather than reducing them. Energetic gains at the ball of the foot were offset by increased work at the midtarsal joint, leaving the overall ankle-foot system largely unchanged in healthy walkers.
The implication is significant for clinical practice. Increasing orthosis stiffness is not a universal solution — its benefits appear localized to the MTP joint. For patients with specific dysfunction there, a stiffer insert may still offer real value. But for broader gait improvement, the approach may need to be rethought, with future designs targeting particular joint problems rather than assuming a single adjustment can improve how the whole foot moves.
A team of researchers studying how the foot works during walking has discovered something counterintuitive about a popular type of shoe insert: making it stiffer helps one joint but hurts another, leaving the overall benefit in question.
The devices in question are carbon fiber deformable foot orthoses—custom-made inserts that can be tuned to different levels of rigidity. The logic behind stiffening them seemed sound. A stiffer insert should make the ball of the foot joint, called the metatarsophalangeal or MTP joint, behave more like a spring, storing and returning energy with each step. Less energy wasted, more spring in your step. Prior studies had supported this idea, showing that increased stiffness did indeed improve how the MTP joint handled energy.
But the researchers wanted to know whether this benefit extended beyond that single joint. What happened to the midtarsal joint, which sits deeper in the foot's arch? What about the ankle itself? Did stiffening the insert improve the mechanics of the entire foot-and-ankle system, or did it only help the MTP joint while leaving the rest of the foot to fend for itself?
They had volunteers walk in orthoses of increasing stiffness and measured what happened at each joint. The results were revealing. The MTP joint did exactly what the theory predicted: it showed less energy loss and more energy return, becoming more spring-like and better at bracing the foot. That part worked.
But upstream, at the midtarsal joint, something else was happening. As the insert got stiffer, that joint began to absorb more energy rather than generate it. It shifted from acting like a motor—a joint that produces power—toward acting like a damper, something that soaks up force. The ankle joint, meanwhile, showed only minor changes in how much power it generated, with minimal shifts in its overall functional role.
The picture that emerged was one of compensation. The energetic gains achieved at the ball of the foot were being offset by increased work at the midtarsal joint, which had to work harder to manage forces that the stiffer insert was now directing its way. In healthy people with normal gait, the system-wide benefit of a stiffer orthosis appeared to be minimal. The insert was solving a problem at one joint by creating extra demand at another.
This finding matters because it suggests that simply making orthoses stiffer may not be the universal solution clinicians have hoped for. The benefits appear to be localized to the MTP joint rather than distributed across the entire ankle-foot complex. For people with specific dysfunction at the ball of the foot, a stiffer insert might still be worthwhile. But for general gait improvement in people without joint-specific problems, the approach may need rethinking. Future clinical applications may need to be more targeted, addressing particular joint problems rather than assuming that one adjustment to insert stiffness will improve how the whole foot works.
Notable Quotes
Stiffness-driven energetic gains at the MTP joint were offset by proximal compensations at the midtarsal— Study findings
The Hearth Conversation Another angle on the story
So the stiffer insert does what it's supposed to do at the ball of the foot. Why isn't that enough?
Because the foot is a system. When you change how one joint behaves, the joints around it have to adapt. The midtarsal joint isn't just sitting there—it's actively managing forces. Make the insert stiffer, and you're essentially telling that joint to work harder.
But couldn't that be a good thing? More work means more strength?
Not necessarily. The midtarsal joint shifted from generating power to absorbing it. That's a functional change, not just a quantitative one. It's doing different work, less efficient work.
So the gains at the ball of the foot are being canceled out?
In healthy people, yes. The energy you save at the MTP joint is being spent at the midtarsal. It's a trade, not a gain.
What about people who actually need these orthoses? People with foot pain or dysfunction?
That's the open question. If someone has a specific problem at the ball of the foot, a stiffer insert might still help them. But you can't assume it helps everyone, and you can't assume it helps the whole foot.
So the next step is figuring out which patients actually benefit?
Exactly. And maybe designing inserts that address specific joints rather than just making everything stiffer.