Biomechanics of Foot Strikes
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Biomechanical Differences Between Different Foot Strikes


Why do Different Foot Strikes Matter?

Here we focus on the difference between heel striking and forefoot striking (see bottom of page for more on midfoot striking which is often intermediate). In heel striking, the collision of the heel with the ground generates a significant impact transient, a nearly instantaneous, large force. This force sends a shock wave up through the body via the skeletal system. In forefoot striking, the collision of the forefoot with the ground generates a very minimal impact force with no impact transient.

Therefore, quite simply, a runner can avoid experiencing the large impact force by forefoot striking properly.

The explanations below illustrate how and why a large collision is generated when heel striking and why such a small collision is generated when forefoot striking.

Heel Strikes and Ground Reaction Forces

Heel Strike Barefoot

Heel Strike in Running Shoes


Forefoot Strikes and Ground Reaction Force

Forefoot Strike Barefoot

Forefoot Strike in Racing Flats


Forefoot Strike in Standard Running Shoes

To understand these differences, we need to explore the biomechanics of running, which can be divided into two major components: running kinematics, the way in which the body moves, and running kinetics, the relationship between movements and the forces that cause them. To understand the important kinetic differences between different kinds of foot strikes we will first consider key differences in running kinematics. Note that there is a continuum of different kinds of landings from landing on the heel (heel striking), landing simultaneously on the heel and ball of the foot (midfoot striking), and landing on the ball of the foot (forefoot striking). Again, for simplicity, we focus here on heel and forefoot striking, noting that midfoot striking is often intermediate.

Running Kinematics

 

Heel Striking

Forefoot Striking

Moment of Impact Barefoot Heel Strike  Barefoot Forefoot Strike 
Hip and knee are flexed.
Ankle is dorsiflexed (toes point up). Ankle is plantarflexed (toes point slightly down). Foot is usually slightly inverted (the sole is angled inwards).
Land on the middle to outside of the heel just below the ankle joint. Land on outside of the forefoot (the ball of the foot, just below the 4th and 5th metatarsal heads).
As you land, the ankle begins to plantarflex (toes move towards the ground). As you land, the ankle begins to dorsiflex (heel moves towards the groud).
Arch of the foot is not loaded. Arch of the foot is loaded and begins to stretch/flatten.
Impact



Foot Flat
Barefoot Heel Strike

Barefoot Foot Flat
Barefoot Forefoot Strike

Barefoot Foot Flat
Knee and hip flex.

As the ankle plantarflexes, the forefoot comes down.

As the ankle dorsiflexes, the heel comes down under the control of the calf muscles and Achilles tendon, which are stretching.
Foot Flat




Midstance
Barefoot Foot Flat

Barefoot Midstance
Barefoot Foot Flat

Barefoot Midstance
Knee and hip continue to flex.
The ankle dorsiflexes as the lower leg moves forward relative to the foot and the foot everts (rolls inward).
Now that the whole foot is on the ground, the arch begins to stretch/flatten. The arch continues to stretch/flatten.
This combination of eversion, ankle dorsiflexion and arch flattening is called pronation. This combination of eversion, ankle dorsiflexion and arch flattening is called pronation, but occurs in the reverse direction compared to heel striking (from the forefoot to the rearfoot not heel to toe).
Midstance



Toe Off
Barefoot Midstance

Barefoot Toe Off
Barefoot Midstance

Barefoot Toe Off 
Ankle plantarflexes bringing the heel off the ground (calf muscles and Achilles tendon now shorten).
Foot’s arch recoils, and the toes flex.
These actions push the body upwards and forwards for the next stride.


Running Kinetics and Impact Forces

The physics of collisions: The impact of the body with the ground generates an impact force, which equals mass times acceleration
(F = ma, Newton’s 2nd Law). The mass involved in this collision is whatever portion of the body that comes to a dead stop along with the point of impact on the foot (this is called the effective mass). The acceleration is the rate of change of this mass' velocity. Because the impact occurs over a brief period of time, the force times the duration of the collision, called the impulse, is the effective mass times its change in velocity over the duration of the impact. For a detailed explanation of the physics, see equation 1 in Lieberman et al (2010).
 

Heel Striking

Forefoot Striking

Effective Mass at Impact Foot and lower leg come to a dead stop at impact while the rest of the body continues to fall above the knee. Forefoot comes to a dead stop, but the heel and lower leg continue to fall (in a forefoot strike). The ankle flexes (in both forefoot and midfoot strikes).
Effective mass is approximately the foot plus the lower leg, which equals 6.8% of total body mass in the runners measured in Lieberman et al. (2010). Effective mass is the forefoot and some portion of the rearfoot and leg, which equals 1.7% of total body mass in the runners measured in Lieberman et al. (2010).
Change in Velocity at Impact The change in velocity of the effective mass is the difference between the velocity of the falling foot at the instant before contact and the velocity just after contact, which is zero. This change in velocity does not differ significantly between a heel strike and a forefoot strike.
Conversion of Vertical Momentum at Impact Although the ankle may flex a little (plantarflex) during the impact period, the vertical momentum of the lower leg is mostly absorbed by the vertical component of the collision force. Much of the vertical momentum of the rearfoot and lower leg is converted into rotational momentum.
Analogy: It is like dropping a rod straight down on its end: it comes to a sudden, loud stop. Analogy: It is like dropping a rod on its end at an angle: there is a sudden stop at one end of the rod, but it is much less loud because the rest of the rod continues to fall as it topples over.
Impact Force This kind of collision leads to a rapid, high impact transient about 1.5 to as much as 3 times your body weight (depending on your speed) within 50 milliseconds of striking the ground (see graph a below).

This is equivalent to someone hitting you on the heel with a hammer using 1.5 to as much as 3 times your body weight. These impacts add up, since you strike the ground almost 1000 times per mile!

Many running shoes make heel strikes comfortable and less injurious because they slow the rate of loading considerably, reduce the force by about 10% (see graph b below) and spread this force out over a greater area of the foot. But they do not eliminate the impact transient.

This kind of collision produces a very slow rise in force with no distinct impact transient. There is ESSENTIALLY NO IMPACT TRANSIENT in a forefoot strike (see graph c below). The same is true of some (but not all) midfoot strikes.

We have found that even on hard surfaces (a steel force plate) runners who forefoot strike have impact forces that are 7 times lower than shod runners who heel strike. Rates of loading are equal to or less than rates of loading for shod runners.

Figure 1a

Figure 1b

Figure 1c
Note that the peak force at midstance is the same for both kinds of gaits. This peak reflects the ground reaction force when the body's center of mass is at its lowest point. Because peak force at midstance rises slowly, it is probably less related to injury.


A Note on Midfoot Strikes

Some runners land simultaneously on the ball of the foot and the heel. Midfoot strikes represent a continuum between heel strikes and forefoot strikes, depending on where the center of pressure is at impact and how stiff the ankle and knee are during impact. One can land softly in a midfoot strike without much impact transient, but some midfoot strikes can generate impact transients like those of heel strikes. However, these forces are distributed over larger surfaces areas, reducing the stress on the foot.

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FUNDING DISCLAIMER:  Research presented on this site was funded by Harvard University and, in part, by Vibram USA®.

Creative Commons License

Running Barefoot or in Minimal Footwear by Daniel Lieberman et al. is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.
Based on the research published in the scientific journal Nature.
Permissions beyond the scope of this license may be available by contacting Daniel Lieberman.