Understanding Impact Forces
The following article is adapted from Emily Splichal’s lecture Understanding Impact Forces.
Each time a body comes in contact with another surface, it encounters impact forces. These impact forces can have an influence on movement efficiency, injury risk, gait and limb health.
In the following article, we’ll be looking specifically at the impact forces that a body encounters through the feet. You’ll learn:
- The magnitude of impact forces the body encounters during common activities like walking, running and jumping
- How a body senses impact forces
- How a body keeps the joints and soft tissue safe when it encounters large impact forces
- The factors that contribute to common impact force injuries like stress fractures and shin splints
You’ll finish with a better understanding of how impact forces work and, more importantly, how to manage them for better health and performance.
About Dr. Emily Splichal
Dr. Emily Splichal is a podiatrist, movement specialist and founder of the Evidence Based Fitness Academy, a Continuing Education Institute created to provide scientific and research-based curriculum for health and fitness professionals.
Impact forces experienced in daily activities
Each time your foot strikes the ground your body encounters impact forces. When walking, these impact forces range from between 1-1.5 times bodyweight. When you start running, this increases to 3-4 times bodyweight.
Typical impact forces encountered during different activities
For more ballistic activities such as jumping or high velocity landings, a body encounters forces 10 times bodyweight or more. Gymnasts regularly encounter up to 18 times bodyweight in impact forces when doing moves like tumbling passes.
As you can see, the magnitude of impact forces even during common daily activities can be quite large. Don’t just think of impact force management as something only for elite athletes. It’s an important topic for the health and performance of anyone and everyone, regardless of age or fitness level.
What happens when your body comes in contact with the ground
When your foot comes into contact with the ground, it encounters an impact force. Now, the body actually perceives these impact forces as vibrations. These vibrations are picked up thousands of nerves in the skin along the bottom of the skin. About 80% of the receptors on the bottom of the foot are actually sensitive to vibration.
About 80% of the nerves on the bottom of the feet are sensitive to vibration
When it comes to managing and preventing injuries that are caused by impact forces, such as stress fractures or shin splits, the frequency of these vibrations becomes a very important factor.
These vibrations have a frequency of between 15-20 Hz, meaning they enter the body in less than 50 milliseconds. A body must therefore be able to load these forces adequately in 50 milliseconds.
How exactly does the body load these impact forces?
It’s a common belief that these vibrations are absorbed in joints or through eccentric contractions. But according to the Muscle Tuning theory introduced by Dr. Benno Nigg, a body actually damps and absorbs vibrations through isometric contractions. When muscles contract isometrically, the intracompartmental pressure of the lower leg and foot increase, which dampen the vibrations.
All muscles and soft tissue vibrate at a certain frequency. Soft tissue does not like to vibrate, so muscles isometrically contract to stop vibrations.
So where does the energy end up? It’s stored within the connective tissue, specifically the fasciae and tendons, as potential energy. This is then released as kinetic energy in the counter-movement.
What can go wrong
Impact force injuries can occur whenever one or a combination of these processes breaks down. Let’s look at each case.
A) Impact forces are perceived incorrectly
Research by Dr. Benno Nigg has found that the body can store and remember information about the impact forces it experiences after just three or four steps. The body is able to remember the characteristics of the walking surface, the walking speed and the magnitude of the impact forces it experiences.
This information helps the body determine the right response for safe and efficient movement.
Since the information comes from the nerves in the skin on the bottom of the feet, footwear can have a big impact on the ability to accurately perceive impact forces. This is why shoes can have such a big impact on the way we walk, run and move.
The more cushioning a shoe has, the thicker the sole, or the more rigid it is, the more it will alter the body’s perception of impact forces. Be careful when selecting footwear or suggesting footwear to clients as this can seriously impact movement efficiency, injury risk and foot health.
Thick or rigid shoes alter the information your feet receive from the ground
Going barefoot provides the body accurate information about the ground
Most people believe that adding more cushioning into a shoe decreases the magnitude of impact forces encountered by the foot. While that may sound correct, the opposite is actually true. Cushioning actually dampens and skews the perception of the receptors on the bottom of the foot and how they detect vibration, leading to higher impact forces. Opt for less cushioning to minimize injury risk.
Inconsistent surfaces can also prevent the body from accurately perceiving incoming impact forces, therefore making very difficult for the body to judge what response to create.
In one of his books, Dr. Benno Nigg recounts the time he was contacted by Cirque du Soleil in Los Angeles. The performers at this particular Cirque du Soleil had all of a sudden started coming down with injuries like achilles tendonitis and plantar fasciitis and the company couldn’t figure out why.
Why were highly skilled, highly conditioned professional performers all of a sudden getting injured doing moves they were used to doing?
Dr. Nigg investigated and eventually found the problem.
The injuries had all started happening after the company brought in a brand new stage at their performance center. Underneath this new stage were a bunch of beams.
These beams changed the vibration frequency of the surface above it, creating an inconsistent surface. Wherever there was a beam, there would be a different vibration frequency. When the performers landed on a beam, they would experience an impact force with a different frequency than if they landed on a part of the stage that was not above the beam.
The stage support beams caused parts of the stage to resonate at different frequencies
Since the beams were underneath the stage, the performers had no idea whether they were landing on a part of the stage with or without the beam. They had no way to accurately perceive or anticipate what vibrations they would receive when landing.
And this was what was causing all the injuries.
Dr. Nigg took out all of the beams and made sure the stage had the same frequency throughout, so whenever the performer landed, they would be able to accurately perceive and anticipate the vibrations.
Once Dr. Nigg did this, the new injuries the performers were experiencing went away.
B) Loading of impact forces doesn’t occur quickly enough
Since impact forces enter the body in less than 50 milliseconds, fast twitch muscle fibers are responsible for creating damping contractions rather than slow twitch muscle fibers. However, fast twitch fibers can sometimes take up to 70 milliseconds to meet their peak maximum contraction.
Impact forces must be anticipated
This means the reaction of fast twitch muscles alone can’t be relied upon, as their response just isn’t quick enough. Impact forces have to be anticipated.
As previously mentioned, the body automatically learns to do this when it encounters a new movement or surface. The body is able to remember information about a new surface and the magnitude of impact forces it experiences after just three or four steps. This helps it create the right anticipatory response.
What this means practically is if you are doing a new movement or using a new surface, give your body time to pick up the information it needs to create the right anticipatory response before doing those movements on a new surface at a higher speed or with more force.
C) Contractions aren’t strong enough to dampen impact forces
If the muscles cannot create enough tension to create the necessary intracompartmental pressure to dampen the impact force vibrations, the vibrations end up travelling into the soft tissue and into bone, causing all sorts of issues. This is primarily where we start to see shin splints and stress fractures.
This can also be caused by fatigue. When we get fatigued, lactic acid builds up in the muscles. This changes the pH of the environment the muscles contract in. The more acidic the environment, the more difficult it becomes for the muscle to contract. This reduces the peak compartmental pressure and the amount of damping, therefore making it more likely for vibrations to travel through to soft tissue and bone.
This is how fatigue can lead to overuse injuries and why fatigue is such an important consideration when it comes to minimizing impact force injuries.
D) Not storing potential energy properly
Whether we’re walking, running, jumping or tumbling, the body must store the energy from impact forces before it can release it as elastic energy on the counter movement.
For example, during walking 50% of the energy returned when the foot strikes the ground actually gets stored and released by the Achilles tendon. The Achilles tendon’s ability to store and release potential energy is directly related to its ability to rapidly stiffen the muscles of the lower leg and the foot.
This elastic recoil is called the catapult effect and is the foundation of movement efficiency. You can recognize this in athletes who can move well and seem to have a ‘spring’ in their steps.
When it comes to the body’s ability to store potential energy, we need to pay attention to connective tissue health. Connective tissue needs to have the perfect balance between stiffness and elasticity.
As we age, we lose the crimps in the fascia, which causes a reduction in elasticity. Fascia can also start to get adhesions.
Health conditions associated with aging, including diabetes and other comorbidities can also alter the health of our tendons and cause them to lose elasticity and become stiffer in a negative way.
Pay attention to connective tissue health. This is where proper nutrition, supplementation, foam rolling, mobility and plyometric training can all help.
The impact of surface type
Another important factor to look at when reducing impact force injuries, is the surface the foot comes in contact with.
Most of us spend most of our time travelling on hard surfaces
Unlike softer ground surfaces like grass, gravel or dirt, hard, artificial surfaces like cement or marble, aren’t good at absorbing force. When the foot strikes a hard surface like cement, the vibrations actually rebound and double up back into the body.
This is why moving on hard surfaces is so hard on the body. Hard surfaces create more stress and fatigue on the connective tissues and bones and contribute towards injury.
Softer surfaces are easier on the body
Unfortunately, in society today people spend most of their time walking on hard, paved surfaces in buildings, on walkways and on roads. Combined with rigid, cushioned footwear, this takes an enormous toll on our bodies.
Since we want to minimize injury risk and take care of connective tissue and bone health, the surface we travel on is definitely one we need to look at.
Learn more ways to minimize impact force injuries in Emily’s full lecture
This article was adapted from Emily Splichal’s lecture called Understanding Impact Forces. In the full lecture, you’ll also learn the strategies she uses to help her clients avoid impact force injuries, including:
- What Emily recommends her patients wear to improve their ability to handle impact forces when they have shin splints or when running on hard surfaces such as concrete
- How to adjust to new surfaces and how long you should give your body to ‘break into’ unfamiliar surfaces
- Supplements for maintaining connective tissue health (vital for storing elastic energy during movement)
- Can’t change your footwear or what surface you train on but still want to minimize the chance of impact force injuries? How to alter your movement to lessen impact forces.
- The simple thing Emily gets her clients to do before they put on their shoes to improve their foot’s ability to get accurate information from the ground.
To listen to Emily Splichal’s full lecture, Understanding Impact Forces, click here.
For more on barefoot training from Emily, click here.
Want more resources to minimize injuries and train smarter?
If you enjoyed Emily’s article and are looking for more ways to create smarter, safer training programs, check out these resources:
Can You Go
Assessments are the first step in creating safer, more effective training programs. However, with so many different assessments available, it can be difficult to know where to start and which to use. In Can You Go, legendary strength coach Dan John provides a simple but effective assessment method that you can use with most clients.
A systematic approach to training reduces injury risk and improves the chance of making consistent progress. In Intervention, Dan John gives away the coaching system that took him over 35 years to develop. Inside you’ll find a comprehensive, step-by-step guide to creating a sustainable, effective and safe training program.
Essentials of Coaching and Training Functional Continuums
To get results safely, you need to know what tool to use, when to use it, and what to use afterward. In Essentials of Coaching and Functional Continuums, Gray Cook and Dan John describe how to categorize training priorities, and when and how to progress or regress certain exercises.
Advances in Functional Training
Having coached top athletes from the MLS, MLB, NHL, NFL and PGA, Michael Boyle knows what it takes to get results without compromising safety. In Advances in Functional Training, Michael Boyle examines the latest athletic training concepts and provides insights on how he gets great results for his athletes with minimal injury risk.
Restoring fundamental movement patterns can go a long way in unlocking performance and reducing injury risk. In Prehab/Rehab 101, Mark Cheng teaches five groundwork progressions based on the neurodevelopmental sequence. These progressions will help you form a solid foundation for more challenging movements and activities.
Applying the FMS Model to Real Life Examples
Restoring and maintaining movement quality can help reduce injury risk and improve performance. The FMS is a popular tool for screening and improving movement quality. In Applying the FMS Model to Real Life Examples, Gray Cook explains how to use the FMS and goes through five case studies to bring the theory to life. If you are interested in using the FMS in your own setting, you’ll find these case studies invaluable.
Tap Into the Brains of Some of the World’s Leading Performance Experts
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