Free Energy – No Fancy Shoes Required!


Photo credit: Nike

A recent NY Times article investigates whether or not Nike’s claim that their Zoom Vaporfly 4 is faster than other running shoes. As it turns out, based on their data sets, the Nike Zoom Vaporfly 4 does equate to faster race times – so much so that it is an outlier compared to other running shoes.

So what’s the special sauce? The Vaporfly’s have a carbon fiber plate in the sole which adds stiffness, thus storing and releasing more energy. This equates to greater propulsion.

Did you know that your body has structures that also act to store and release energy and more specifically, can be stiffened to increase performance.

While a tad lengthy, below we’ve cut and pasted an excerpt from our Running Certification that discusses Free Energy and how it can make you a faster runner!



Within the body exist structures and movement patterns that allow individuals to significantly reduce muscle activation while maintaining the same, if not greater running performance. The two areas that this certification will focus on in regard to free energy are:

  1. Energy Return
  2. Passive Movement

Passive movement and energy return are interrelated. The genesis of free energy is that when the foot impacts the ground during running, energy is absorbed by the impact. The goal of a runner is to optimize their running form (mechanics) to convert as much of this stored energy as possible into forces that assist in the forward movement.

In regard to improving efficiency and performance, the five areas that this certification will focus on in relation to free energy is hip rotation in the transverse plane, hip flexion, foot plantar flexion, knee flexion/extension and foot arch compression.

By maximizing their free energy contribution, a runner can improve their running economy. By utilizing free energy, the active muscle requirement is reduced while still producing the required amount of force (620).


“Energy cannot be created or destroyed, it can only be changed from one form to another” -Albert Einstein

This quote pertains to a law of physics, the conservation of energy. In regard to the SSC, it applicable as potential elastic energy is converted to kinetic energy.

By definition, the SSC is representative of an eccentric contraction of a muscle followed by a rapid concentric contraction of the same muscle. In respect to running, the quadriceps, obliques and calves represent the muscles most influenced by the SSC.

When the foot hits the ground, the quadriceps and calves eccentrically contract (lengthen) and then rapidly concentrically contract (shorten) during the drive phase to provide forward propulsion (616). The stretch aspect of the muscle and tendon during the SSC is quite small, 6 to 7% (615).

To illustrate what the stretch shortening cycle is, think of a shooting a rubber band. The more you stretch the band, the further the band will travel in the air once released. However if you stretch the band too much, the band could break. Additionally, the more tension the band has (harder to pull back), the further it will go. In respect to the human body, muscles and tendons are the rubber band. To recap, tendons connect bone to muscle. Therefore in the context of the stretch reflex, muscles and tendons are often considered two parts of a working whole, the muscle-tendon unit (524). The variables that affect the degree of elastic return of the SSC are (525):

  1. Length of the stretch
  2. Speed of the stretch (loading)
  3. Stiffness of the muscle and tendon
  4. Time between the stretch and the contraction

From a running perspective, the legs act as springs. The springs compress during the first half of the support phase and rebound during the drive phase.

The stiffer a muscle is, the greater the amount of energy can be stored and released. However to not increase the chance for injury, a muscle must have full mobility (608).


Let’s first examine tendons and their properties. It is important to note that depending on the location of a tendon in the body, they will vary in thickness, shape and length. These variables affect the stiffness of the tendon and thus the capacity for force production. Tendons have elastic properties that allow them to stretch. The stretch of a tendon (or muscle) stores energy and when the stretch is unloaded, the stored energy is released. Utilizing this stored energy properly can greatly minimize the metabolic cost of movement (521).

The optimal stretch for a tendon should be viewed as a modified bell curve – meaning, too little or too much stretch is not optimal. While it is clear why too little of a stretch would not be optimal, why would a large stretch not be recommended? If stretched beyond the end point of a tendon’s range of motion, a tendon could tear completely. However before this point is reached, the tendon could still be overstretched. When this occurs, structural changes occur to the tendon that effectively changes the length of the tendon and thus reduces the stretch reflex (522). The degree of stretch to a tendon that elicits the ideal stretch reflex is called the elastic region. Once the stretch extends past this region, it is called the plastic region. It is at this point that the structure of the tendon changes and therefore changes the tendon length (522).

Figure 7.6 below illustrates the elastic and plastic regions on a curve-based model (Load-Deformation Curve).

Achilles Tendon

In regard to running, the Achilles tendon has a large impact on performance in respect to free energy. A study that examined the link between resistance training and Achilles tendon stiffness found that a 16% increase in tricep surae tendon (gastrocnemius, soleus, Achilles tendon) stiffness via resistance training decreased the rate of oxygen consumption during running by 4%, thus increasing running economy (530). Another study confirmed these findings by noting that differences in Achilles tendon mechanical properties were primarily influenced by muscle strength (531).

Depending on the study, the degree of energy that can be stored by the Achilles tendon and the resulting increase in running economy varies. Therefore the most important thing to keep in mind is that an increase in muscle strength of the tricep surae will increase Achilles tendon stiffness that results in increased running economy.

There appears to be no difference between men and women in regard to the effect of Achilles stiffness on running economy (529).


Not all toes are created equal, at least not in terms of foot stabilization and forward propulsion. During running, the big toe (1st metatarsophalangeal joint) plays a key role in the following:

  • Stabilize the foot
  • Regulate the degree of foot pronation
  • Forward propulsion

The big toe, in relation to the other toes, is responsible for a much greater percent of foot and body stabilization as well as forward propulsion.

Figure 7.2 Windlass Mechanism (foot)

As noted in figure 7.2, the sesamoid bones are two small (pea-size) bones that are embedded into a tendon (flexor hallucis brevis). The bones sit under the ball of the foot, at the big toe joint. The sesamoid bones act as a fulcrum to provide the foot leverage when pushing off the ground (612).

1st Metatarsophalangeal Joint Facts

  • Carries 12 times more weight than the small toe
  • Only toe made up of 2 bones as opposed to 3
  • It has a separate set of control muscles and tendon insertions than the rest of the toes.


Rapid dorsi-flexion (loading) and plantar-flexion (unloading) of the ankle is what is responsible for the spring action of the Achilles tendon. If the ankle is not dorsi-flexed enough, the energy stored in the Achilles tendon will not be as high as it could be. The implication from a mechanical perspective is that the heel of the foot during the drive phase should touch the ground while the hip is extended. If the heel does not touch the ground during this phase, potential energy is reduced and as a result, running economy is decreased as well.

Based on the influence of dorsi-flexion and potential energy of the Achilles, it could be theorized that shoes that have a high heel in relation to the forefoot (large heel to top drop) would reduce the amount of stored energy, as the shoe limits the amount of ankle dorsi-flexion possible. Additionally, poor Achilles tendon range of motion (flexibility) will likely limit the amount of potential energy able to be stored.


In relation to free energy, running economy is not just based on storing energy but also utilizing it. The interface of the foot and the ground influence the amount of energy that can be utilized. The more solid the interface is, the more efficient the runner will be. If the running surface is very soft (e.g., sand) and, or a shoe has a lot of cushioning, this will reduce the amount of energy that can be used by the runner, as a large portion of the energy is being dissipated via absorption of the ground and shoe, respectively.

Some absorption however is likely a positive thing in relation to running economy and decreasing the chance for injury. Studies have shown that there is a higher metabolic cost to running barefoot than with minimal shoes due to the increased muscular demand (532).

Shoe selection plays a large role in the flexibility and range of motion of the Achilles tendon. It is commonly understood that sitting for long periods of time influence an individual’s posture – primarily due to the hamstrings and hip flexors being in a chronically shortened position. Akin to this is wearing shoes that have a high heel in relation to the forefoot, as this places the Achilles tendon and calf muscles in a shortened position. Therefore from the standpoint of increasing Achilles tendon range of motion, spending time walking indoors without shoes on or in shoes with a small heel-to-toe drop is helpful.


Many distance runners do not do perform strength and sprint training due to the perceived lack of specificity. While there are many benefits to distance runners, if they only did sprint training for the purpose of stiffening the Achilles tendon and Longitudinal Arch (noted below) to more efficiently utilize elastic energy return, it would be well worth the effort.


The primary functions of the foot in regard to running are shock absorption and propulsion. The foot plays a large part in respect to free energy, specifically, the longitudinal arch (figure 7.7). The muscles and connective tissue of the foot, including the plantar fascia, which affect the longitudinal arch act as a spring to provide passive energy during the running gait (601). Figure 7.7 denotes how the foot acts as a spring, in relation to the plantar foot muscles and connective tissue.

Figure 7.7 Longitudinal Foot Arch

A common argument for running in bare feet or in minimal type footwear is that since the arch of the foot is supported in conventional running shoes, the elastic energy of the longitudinal arch is negated – thus eliminating and, or greatly reducing an energy source used for forward propulsion.


Like tendons, the stiffer a muscle is, the greater the elastic return will be. Pre-activation of leg muscles when running prepare the body and leg musculature for foot impact. Pre-activation of leg muscles is theorized to decrease stress to leg muscles and increase the cushioning upon landing (520). Think about this for a second – if the leg muscles did not contract before and during landing, the body would collapse upon foot strike.

Leg muscle ‘stiffness’ can be controlled consciously or unconsciously (526). The degree of leg stiffness directly affects the amount of knee flexion. In regard to conscious control of leg muscle stiffness, an individual is able to control their stride rate and stride length (527). Leg stiffness is also influenced by the geometry of the leg at impact. This is because depending on the angles of the leg at foot impact, there will be varying loads on the leg that the muscles must counteract (526). Lastly on the topic of leg stiffness, the type of surface that the foot lands on correlates to the degree of muscle stiffness. When running, the body looks to maintain the same degree of total vertical stiffness (surface stiffness + leg stiffness) at all times. Therefore when running on different surfaces such as pavement and sand, the degree of leg stiffness will change to maintain the same degree total vertical stiffness. As an example, if running on sand, the legs must become stiffer to compensate for the decrease in surface stiffness (528).

It is critical to note that while muscle stiffness elicits a higher SSC, the muscle must have full mobility (range of motion) (523).

Click here to download a course overview of the UESCA Running Coach Certification – inside you’ll find a code for $50 off the price!!

To learn more about UESCA, please visit our website

Rick Prince

Rick Prince

Founder/Director of United Endurance Sports Coaching Academy (UESCA).

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