The title of this article begs the question whether our legs have gears. Before getting into that, the answer is simple: you don’t have to do anything. You have automatic transmission.
A fascinating paper appeared in PNAS, with a provocative title: “Shifting gears to power human walking.” We don’t normally expect to find the words “gears” and “human” in the same sentence of a paper on anatomy and physiology. This paper uses the word “gear” 37 times. And they mean it: our legs have mechanisms that are analogous to gear-shifting and automatic transmission. But where are they? Muscles we know about; bones, ligaments, tendons we know about. I know I haven’t ever seen gears in anatomy drawings. This must be a paper about prosthetic legs for paraplegics or something, right? No. It’s about those of us who have been standing, walking, and running since childhood. The Significance statement for the paper explains,
Human muscles outnumber the minimum needed to actuate our joints, yet the advantage of this redundancy remains unclear. We show that redundancy provides a system of biomechanical “gears” that allow one to effectively shift power production at the ankle between muscles during human walking. This finding offers a compelling perspective on muscle anatomy as a possible adaptation for versatile performance, with implications for understanding locomotor adaptability, improving rehabilitation, and advancing assistive and bioinspired technologies.
An Exquisite Locomotory System
What the research showed may indeed benefit paraplegics someday and may help physical therapists treat injuries. But this paper is about normal walking. The metaphor of gears and gear-shifting helped this team of four at the University of Wisconsin-Madison, led by Jack A. Martin, to communicate the nature of an exquisite locomotory system in our bodies that bears the hallmarks of continuous automatic transmission.
Read that opening paragraph again. The human body is overdesigned. It has redundancy — more elements than needed for mere survival. It uses physics principles. It provides versatile performance. It is so good, engineers should try to copy it.
No, you won’t find toothed wheels or chains in the human leg. You’ll find something more elegant. The paper shows that the large number of muscles allow for changes in the moment arm during locomotion. A moment arm is the distance between a hinge and an applied force. Think of trying to push a door closed by pressing on the door near the hinges. It takes more force. Pushing it closer near the doorknob requires much less force over a larger distance because the moment arm is longer. In an analogous way, the body can shift the moment arm of the muscles attached to the leg and ankle to improve efficiency.
The musculoskeletal system features an abundance of functionally similar muscles, yet the performance benefits of this apparent redundancy remain unclear. Here, we show that muscles with similar function, but distinct moment arms, facilitate a dynamic gear-shifting mechanism in walking.
No Sound from Our Legs
Bike riders know that the gear shifting levers on the handlebar make one setting easier for uphill riding and another setting better for fast riding downhill or on level ground. In a modern car’s automatic transmission, it’s often possible to hear when the gears shift under the hood. Those represent discreet, separate gearing mechanisms. We don’t hear that in our legs, because it is automatic and continuous.
Ankle power generation is shown to transition from large (triceps surae) to small (posterior tibialis, peroneals, and extrinsic toe flexors) moment arm muscles as joint velocity increases during pushoff. Computational models reveal that the sequential power phasing emerges naturally from muscle-tendon dynamics given the underlying differences in muscle geometry and architecture. Tendon kinetic measures during walking align with the model predictions and reveal that small plantarflexors contribute substantially more to powering human walking than previously recognized. These findings provide insights into the functional role of muscle redundancy, and suggest a fundamental principle governing the coordinated recruitment of synergistic muscles in locomotion.
This means that even though human walking involves discreet steps (left, right, left, right), the muscles sequentially adjust to provide optimum power. The gear shifting is so quiet and continuous, we don’t even feel it.
Evolution Must Have Done It
The authors, expectedly, feel compelled to say evolution produced this functional redundancy. “Human evolution has led to anatomical adaptations that enable efficient, robust, and adaptable movement,” they say, assuming that if something is well designed, evolution must have done it. “This behavior has analogies to gearing in mechanical systems, and likely reflects evolved anatomical features and behavior that enhance the energetics of long-duration locomotion.” Does that sound like something within the powers of gradualistic, unguided natural processes?
Of course not. In spite of their commitment to evolution, they know that the engineering of this automatic gear-shifting mechanism is so good it is worth copying. Speaking of how our over-designed leg muscles spread power generation across redundant muscles, the authors conclude,
Interestingly, the importance of redundant actuators in human motion is in contrast with what is typically employed in robotics, where a one-to-one pairing of actuators to degrees of freedom is typical for simplicity in design and control systems. These systems can encounter difficulties in applications with both high torque and high velocity demands, so a more biomimetic approach may be warranted depending on the application.
Engineers can improve their robot designs, in other words, by copying the smooth, robust, efficient, adaptable movement they enjoy when walking to their offices each day.