The Purpose of Toes

By Carl Zimmer|Wednesday, February 01, 1995
What are toes good for? Compared with fingers, which allow us to manipulate tools, toes are usually thought of as inessential digits, good for traction and balance and not much else. But David Carrier, a biologist at Brown University, thinks toes deserve more respect. To him they are the gears in the engine of the human body, letting us walk and run with exquisite efficiency.

Carrier’s analogy between cars and people is surprisingly precise. In a car, the engine is most effective when the pistons are pumping at a certain rate and the crankshaft is spinning in a certain range of revolutions per minute. Above that range, the pistons don’t transmit force as well, and the extra revolutions just waste gas without increasing the car’s speed much. Gears solve this problem by changing the ratio between the speed of the engine and the speed of the wheels. Shift up to a higher gear and the wheels can spin faster while the pistons chug along at peak efficiency. Carrier realized that muscles also have a window of peak performance, beyond which their efficiency drops off. It occurred to him that evolution might have imitated Detroit and incorporated gears into the human body.

To find out, Carrier and his colleagues Kathleen Earls and Norman Heglund videotaped volunteers running over a force-sensitive plate and calculated the magnitude and direction of the forces in each step. You can think of a step as the tilting of a lever. The ankle is the fulcrum, and one of the arms of the lever extends back to the point above the heel where the Achilles tendon attaches. When the foot hits the ground, it decelerates, and in order to accelerate the foot again, the calf muscle contracts. It pulls up on the Achilles tendon, thus tilting down the other arm of the lever, which extends to the point on the sole where the force is applied to the ground. The rotating foot propels the leg and the whole body forward.

A problem soon arises, though. During a step, Carrier says, the center of force moves very rapidly from the heel to the ball of the foot. The force of the contracting calf makes the lever tilt at a faster and faster rate, which the calf muscle has to match by contracting faster. Although Carrier hasn’t actually measured the contraction rates yet, he figures that at some point the muscle will be working beyond its range of peak performance. The walker then faces the same two choices as the driver who’s starting to top out in first gear: he can stop accelerating his foot and give up on moving fast, or he can shift gears. That’s where the toes come in.

Changing gears in a car, Carrier explains, is like changing the length of the arm of a lever. Imagine a boy sitting halfway along a seesaw. If you push down on your end at six inches a second, he will rise three inches a second. But if the boy moves out to the far end of the seesaw, you can raise him at the same speed by pushing down at only three inches a second. By lengthening his end of the lever, you’ve decreased the ratio of engine speed (you) to wheel speed (the boy). In other words, you’ve shifted to a higher gear.

In Carrier’s model, when your weight is on the ball of your foot during a step, you are in low gear. If we didn’t have toes, he says, the center of force would stay under the ball because that’s where your foot would end. But by rolling onto your toes, you lengthen the arm of the lever. Now your calf muscle can contract at a slower, more effective rate and still tilt the end of your foot at a higher speed. Once again, in other words, you’ve just shifted to a higher gear.

Your foot makes that small acceleration during each step of a steady run. When you are beginning to run from a standing start, though, it accelerates as much as four times more. Carrier also had his volunteers run this way, hitting the force plate with their second step. The results will not surprise anyone who has ever run the 100-yard dash--or drag-raced away from a stoplight with a stick shift. In both cases the point is to maximize acceleration--not efficiency--by maximizing the force applied to the ground. The foot does this by keeping that arm of the lever short. During rapid acceleration, the center of force stays under the ball of the foot until almost the very end, says Carrier. It’s as if you’re staying in first gear and never getting to second.
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