1. What Force Hits the Foot in the Split Second a Runner's Heel Lands?
Researchers have measured the force on the feet of people who run by landing on their heels. They reported that as soon as the foot touches the ground, a sudden force of about 1.5 to 3 times body weight is applied within 50 milliseconds (0.05 seconds) [1]. For a child weighing 50 kg (about 110 lbs), this calculation suggests a force equivalent to placing 75 to 150 kg (165 to 330 lbs) on the foot for less than 1/20th of a second. This is an estimate based on the multipliers in the paper, not a direct figure from the study. These values apply to running, not walking.
The same study noted that 75 to 80% of runners who wear modern shoes and run long distances land on their heels [1]. If the heel receives this force with every step, there must be a way to soften it inside. What shape does that protection take?
2. How Is the Fat Heel Pad Under the Skin Built from Two Layers of Chambers?
Just under the skin of the heel is a cushion made of fat, called the heel pad. Researchers in Taiwan found that this fat body is divided into two layers: a shallow layer and a deep layer [2]. The shallow layer consists of small fat chambers (microchambers) and is harder, deforming less when weight is applied than the deep layer. The deep layer is made of large chambers (macrochambers) and deforms significantly under load. Before any weight is applied, the shallow layer is about 30% as thick as the deep layer but about 10 times harder. If the deep layer’s thickness is 10, the shallow layer is 3, making the shallow layer about 23% of the total thickness (3 divided by 13). A hard, thin layer covers a soft, thick layer. However, these numbers come from measurements of the left feet of six healthy young people, so they may not apply to everyone.
The cushion inside the heel may not be enough on its own. The Lieberman study states that while the human heel fat pad softens the impact of landing, it is less effective than the large heel of modern running shoes [1]. So, what bone sits on top of this cushion to support the body weight?
3. What Is the Calcaneus, the Largest Bone Around the Ankle?
In medicine, the heel bone is called the calcaneus. It is the largest of the small bones around the ankle, known as tarsal bones [3]. It supports body weight whether you are standing or walking. The fat cushion softens the shock, while the calcaneus inside supports the weight. The heel is a place where the role of "softening" and the role of "supporting" are arranged vertically. Connected to the back of the calcaneus is something else.
4. How Does the Achilles Tendon Connect the Calf Muscles to the Heel Bone?
The Achilles tendon connects the calf muscles (the gastrocnemius and soleus) to the heel bone. It is said to be the largest tendon in the human body [4]. The MSD Manual also describes it as the tendon connecting the calf muscles to the heel bone, involved in movements like walking, running, and jumping [5]. When the muscles contract, the tendon pulls the heel bone, and this force moves the ankle. It is similar to pressing one end of a rubber band with your finger and pulling the other end, causing the pressed spot to move. However, the tendon is much stronger and less stretchy than rubber. Thus, the heel is both a place that "receives the impact of landing" and a place that "passes calf power to the foot." When did this way of walking begin?
5. Do Footprints from 3.6 Million Years Ago in Laetoli Show Modern Walking?
In Laetoli, Tanzania, footprints from about 3.6 million years ago remain. These are tracks left by early humans walking on wet volcanic ash. The owners are thought to likely be Australopithecus afarensis. The Smithsonian’s Human Origins site introduces them as the tracks of three people [6]. However, researchers have some disagreement over details like the number of people. The shape of the footprints is explained as showing the same "heel-to-toe" walking style as modern humans [6]. A 2017 paper analyzing the Laetoli tracks (at Site S) states in its title that it "confirms human-like bipedal biomechanics" [7]. Because the volcanic ash was wet and hardened, the shape of the footprints remains visible today. Walking on the heel may have a very long history. But does the heel cushion work the same way throughout life?
6. Does the Heel Pad's Shock Absorption Change as People Age?
A study by Shinshu University investigated how heel shock absorption changes with age. In a drop-weight test, the heels of elderly people showed greater deceleration and absorbed less energy than those of younger people [8]. This means that in this test, the energy absorbed by the heel fat pad decreased as age increased. The abstract does not provide specific numbers on how much changes at which ages. We have seen how the two-layered cushion, the calcaneus bone, and the Achilles tendon all gather in one place.
7. How Can You Make Your Own Footprints at Home with Wet Feet and a Towel?
You can try an activity at home. Wet the soles of your feet slightly and walk slowly three steps on a floor covered with a dry towel. Since wet feet can slip, place the towel on a non-slip floor and have an adult hold the ends. Look at the marks left on the towel to see how the heel prints appear. Compare how the prints change if you change your walking speed. Results vary by person, so there is no correct answer; it is just a record of your own walking style. To learn more, look up "Achilles tendon" in a dictionary or encyclopedia to find the description of it as the body's largest tendon. You can also look up "Laetoli footprints" to compare how the tracks are preserved.