What the Ground Gives Back: Natural Grass, Artificial Turf, and Energy Exchange

In sports performance, we spend a lot of time talking about how much force an athlete can put into the ground. We measure sprint times, jump height, force production, rate of force development, and change-of-direction ability.

What we probably do not talk about enough is what happens after that force reaches the ground.

The playing surface is not passive.

Every time an athlete accelerates, lands, decelerates, or changes direction, there is an interaction between the athlete, the shoe, and the surface. The athlete applies force into the ground, and the ground responds. Some of that energy is absorbed, some is dissipated, and some is returned back through the athlete.

That interaction changes depending on the surface.

Natural grass and artificial turf are not mechanically the same. They can differ in stiffness, deformation, traction, shock absorption, and energy return. Even within those categories, one natural-grass field may behave very differently from another, just as two artificial-turf systems may have completely different mechanical properties.

This is why I think the conversation needs to move beyond saying that turf is simply “harder” or that grass is automatically “better.”

The better question is this:

What mechanical environment is the athlete being exposed to, and how prepared is the athlete to manage it?

Positive and Negative Mechanical Work

When we talk about “positive energy” and “negative energy,” we need to clarify the language.

From a biomechanics standpoint, energy is not simply entering the body as something positive or negative. What we are really discussing is positive and negative mechanical work.

Positive mechanical work occurs when the athlete is producing force to create or increase movement. Acceleration and propulsion are good examples.

Negative mechanical work occurs when the athlete must absorb energy and reduce movement. Deceleration, landing, and braking are examples of this process.

During change of direction, an athlete has to rapidly move from one to the other.

An athlete may sprint toward an opponent, apply large braking forces, reduce the velocity of the center of mass, plant the foot, redirect the body, and accelerate again. That sequence requires both negative and positive mechanical work within a very short period of time.

Research examining change-of-direction running has shown clear braking phases in which negative mechanical work occurs before the athlete transitions back toward positive work and reacceleration (Zago et al., 2018).

This is why I have always viewed deceleration as more than simply slowing down. The athlete has to absorb and control energy before being able to redirect it.

That matters when we start talking about the ground underneath the athlete.

The Athlete and the Surface Work Together

One of the bigger mistakes in sports performance is looking at the athlete as if movement occurs independently of the environment.

It does not.

The athlete, footwear, and playing surface are all part of the same mechanical system.

Ferris et al. (1998) demonstrated that runners altered leg stiffness when running across surfaces with different stiffness characteristics. The athletes did not simply maintain the same leg mechanics regardless of what was underneath them.

Kerdok et al. (2002) demonstrated something similar. As surface stiffness changed, runners altered leg stiffness to maintain relatively consistent overall mechanics. The authors also showed that surface properties could influence the metabolic cost of running.

That is an important point for coaches.

The body recognizes changes in the environment and adjusts. This means the surface underneath the athlete can influence the way the neuromuscular system organizes movement.

A simple way to think about this is:

Surface → shoe → foot and ankle → tendon → muscle → skeleton

Every part of that chain interacts with the next.

If one part changes, the rest of the system may have to adjust.

The Human Body Is Also an Energy-Return System

The playing surface is not the only structure storing and returning energy.

The human body does this as well.

Tendons and other elastic tissues can store mechanical energy during loading and return some of that energy during subsequent movement. The plantar-flexor muscle-tendon complex, particularly the Achilles tendon, plays an important role in this process during running, walking, and jumping (Lai et al., 2015).

The foot itself also contributes.

Kelly et al. (2019) demonstrated that the intrinsic muscles of the foot contribute to elastic energy storage and return within the longitudinal arch during running.

This matters because the foot and ankle are not simply structures that make contact with the ground. They actively participate in the way forces are absorbed, stored, and transferred.

That is why I do not like looking at running as simply muscles contracting and producing force.

There is a much larger exchange taking place.

The athlete produces force.

The surface deforms.

The shoe deforms.

The foot deforms.

The Achilles tendon and other elastic tissues store energy (in milliseconds)

Muscles absorb and produce force.

Some energy is returned.

Some is lost.

Some has to be actively controlled.

The athlete is constantly managing this interaction.

What Happens When the Foot Hits the Ground?

Every ground contact involves a transfer of mechanical energy.

When the foot contacts the surface, several things can happen at once. The ground may deform, the shoe may compress, the foot and ankle may move, the muscle-tendon system may lengthen, and the center of mass may either accelerate or decelerate.

Some of that mechanical energy is stored temporarily.

Some is dissipated.

Some is returned.

How that happens depends partly on the surface underneath the athlete.

This is one of the reasons FIFA does not simply classify artificial football fields as hard or soft.

The FIFA Quality Programme for Football Turf evaluates several different mechanical characteristics including shock absorption, vertical deformation, energy return, and rotational traction (FIFA, 2024).

That tells us something important.

Surface quality cannot be reduced to one variable.

How much the surface compresses matters.

How much energy it absorbs matters.

How much energy it returns matters.

How the shoe grips and rotates against the surface matters.

All of those variables potentially change how the athlete interacts with the ground.

Artificial Turf Is Not One Surface

There is a tendency to talk about artificial turf as though every artificial field is the same.

That is not accurate.

Artificial turf is an engineered system that can include synthetic fibers, infill material, backing layers, shock pads, and different sub-base systems. Each of those components can influence how the surface behaves.

FIFA has specifically updated its testing standards as artificial-turf systems have changed, including developments in infill, shock pads, fibers, and other system components (FIFA, 2024).

As a result, one artificial field may have very different shock-absorption or energy-return characteristics from another.

Natural grass has the same problem when we try to place every field into one category.

A natural field can change depending on soil composition, moisture, grass density, compaction, weather, field preparation, and maintenance.

A dry and heavily compacted grass field may behave very differently from a well-maintained and properly hydrated field.

This is why statements like “grass is soft” or “turf is hard” are too basic.

Energy Return Is Not Always a Positive Thing

The term energy return sounds like something we should always want more of.

That is not necessarily true.

The demands of soccer are not the same as running straight ahead at a constant speed.

Soccer requires repeated acceleration, deceleration, jumping, landing, cutting, turning, and rotation.

During acceleration, we want the athlete to apply force effectively into the ground and generate propulsion.

During deceleration, the athlete has a completely different task.

The athlete must reduce velocity.

That requires mechanical energy to be absorbed.

During change of direction, the athlete must absorb energy in one direction before producing force in another.

Because of this, the question is not whether energy return is good or bad.

The question is whether the athlete can manage the energy exchange occurring between the body and the ground.

That is a much more useful way to look at it.

Natural Grass and Artificial Turf

Research has identified differences in athlete loading and performance between natural grass and artificial turf.

Ford et al. (2006) examined in-shoe loading patterns during cutting and reported differences in plantar loading between natural grass and synthetic turf.

Sultan et al. (2021) also examined plantar pressure during several athletic movements on natural grass and artificial turf and found surface-related differences in how forces were distributed across the foot.

These findings are important because they show that the surface can change what happens underneath the foot.

There are also physiological differences to consider.

Sassi et al. (2011) found that the metabolic cost of running could differ depending on the playing surface.

More recently, Modric et al. (2023) reported greater match-running demands during elite soccer matches played on artificial turf compared with natural grass in the players they studied.

None of this means that artificial turf is automatically bad.

That would be an overreach.

What it shows is that the playing surface can alter the mechanical and physiological environment the athlete has to perform in.

That is really the point.

Traction May Matter as Much as Surface Stiffness

When people talk about artificial turf, the conversation usually focuses on impact and hardness.

For soccer, I think traction deserves just as much attention.

Soccer involves a tremendous amount of rotational movement.

Players cut, turn, pivot, brake, and reaccelerate constantly.

Villwock et al. (2009) examined shoe-surface interactions and found differences in rotational traction between playing surfaces. The artificial surfaces evaluated in their study produced greater rotational traction than the natural-grass condition.

Athletes need traction.

Too little traction and the foot slips. Force production is lost.

Adequate traction allows the athlete to push into the surface and redirect the body effectively.

But there is another side to that interaction.

If the shoe grips the ground very strongly and the foot is unable to rotate or release as the body continues moving, more rotational demand may have to be absorbed elsewhere in the kinetic chain.

That does not prove that artificial turf causes a particular injury.

It does give us a reasonable biomechanical explanation for why the relationship between footwear, surface traction, and athlete movement deserves attention.

To me, the important question is not simply whether a cleat has good traction.

It is:

How does this cleat interact with this particular surface?

Does Artificial Turf Increase Injury Risk?

This is where the research becomes more complicated.

The evidence does not support the blanket statement that artificial turf always causes more injuries than natural grass.

Ekstrand et al. (2006) examined elite football and reported no overall increase in injury risk on artificial turf compared with natural grass.

Steffen et al. (2007) reported similar overall acute injury risk between artificial turf and natural grass in young female football players.

Fuller et al. (2007) also found similar overall injury incidence between the surfaces in the soccer populations they studied.

Other studies have produced different findings depending on the athlete population, injury type, level of competition, and type of artificial surface being investigated.

That inconsistency is exactly why the conversation has to be more specific.

Artificial turf is not one surface.

Natural grass is not one surface.

The footwear is different.

The athletes are different.

The environmental conditions are different.

The exposure is different.

The game demands may also be different.

So saying “turf causes injuries” without understanding those variables is too simplistic.

At the same time, saying there is no meaningful difference between the two environments ignores the fact that surface mechanics, traction, deformation, and athlete loading can clearly differ.

Both statements miss the bigger picture.

What This Means for Performance Training

For me, this is where the research becomes useful.

The goal is not to decide whether grass or turf wins. The goal is to prepare the athlete for the environment in which they have to perform.

Surface Exposure Matters

If athletes change leg stiffness and running mechanics based on surface conditions, then training exposure to different surfaces likely matters.

Ferris et al. (1998) showed that the neuromuscular system adjusts leg stiffness in response to changes in surface stiffness.

For a soccer player who trains primarily on one surface and suddenly competes on another, that represents a change in the mechanical environment.

We already monitor training volume, sprint exposure, high-speed running, acceleration, and deceleration.

Surface exposure should probably be part of that conversation as well.

Athletes Need to Be Able to Absorb Force

We tend to celebrate force production.

How fast can the athlete sprint?

How high can they jump?

How much can they lift?

Those are important qualities.

But soccer also requires athletes to absorb large forces repeatedly. Deceleration and change of direction involve substantial negative mechanical work (Zago et al., 2018).

If we want athletes to tolerate those demands, we have to train braking and force absorption intentionally. That means progressive deceleration work, eccentric strength, landing control, change-of-direction exposure, and appropriate tissue preparation.

The Foot and Ankle Need Capacity

The foot and ankle are where the body directly interacts with the surface. They cannot be an afterthought. The intrinsic foot muscles contribute to elastic energy storage and return (Kelly et al., 2019).

The Achilles tendon and plantar-flexor muscle-tendon complex play major roles in storing and returning energy during locomotion (Lai et al., 2015).

That means calf strength, tendon capacity, ankle stiffness, foot strength, and landing control are all relevant performance qualities.

They are not simply rehabilitation exercises.

They are part of how athletes interact with the ground.

Footwear Should Match the Surface

The shoe and the field should never be looked at separately. Rotational traction depends on both footwear and playing surface (Villwock et al., 2009).

A cleat that works well on one surface may create a completely different interaction on another.

This becomes especially important when athletes transition between natural grass and different types of artificial turf.

More traction is not automatically better.

The goal is enough traction to apply force without creating unnecessary restriction between the foot and the ground.

References

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