Course:KIN366/ConceptLibrary/Force Absorption

Movement Experiences for Young Children
KIN 366
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Instructor:	Dr Shannon S.D. Bredin
Email:	shannon.bredin@ubc.ca
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Force absorption is the process of decreasing the force of an impact between two objects. Understanding the principles of force absorption is critical for children and youth to move effectively and efficiently in both sport and active play. A primary importance of absorbing force is to reduce the risk of stress and impact-related muscular skeletal injuries. In both the teaching and coaching environments, it is also important that principles of force absorption are incorporated so that children acquire both the knowledge and practical skill set needed to become more advanced movers.

How Is Force Absorbed?

There are three ways force can be dissipated in human movement. These are:

(1) giving with the incoming force. Giving increases the time and distance over which the force can dissipate (slowing the object down).

(2) distributing or spreading the force out over the greatest area possible. This decreases the impact on any single part of the body.

(3) establishing a base of support. Bracing such body parts as the spine and tensing the core, bending the knees, and lowering one's centre of gravity, all increases one's stability.

Giving with the Force

The bare hand has less surface area to dissipate the impact forces.

A critical principle of force absorption is “giving with the force” or allowing one’s body to move with the receiving or opposing surface or object. The purpose of 'giving' is to increase the distance and time over which the kinetic energy (the energy an object has while it is in motion) is dissipated from the impact in a gradual and smooth manner. For example, to catch a ball, the performer will start the movement with slightly bent outstretched arms. The arms will continue to bend at the joints as the ball is brought into the body. This strategy allows for the dissipation of force over a greater distance and time compared to stopping the ball at its initial place of contact.

Dissipating Force Over a Greater Surface Area

A catching glove is a more favourable choice (compared to a bare hand) with increased surface area for absorption.

A second force absorption principle is to spread or dissipate the force over a greater surface area. The larger the area of contact, the greater the dissipation of force and the less impact to any single area. A good example is the catching mitt in baseball. A mitt has a greater surface as well as more overall padding allowing it to distribute the force of the incoming baseball better than the bare hand while catching. This could also be used in scenarios where transferring body weight from one foot to the other which is required in movements such as tackling in football or scrumming in rugby. Another easy way to demonstrate this is a simple fall back. If one falls and contacts the floor with their hands, elbows or tail bone first, this is going to hurt because the surfaces taking initial contact don’t have very much surface area and are pure bone. However taking the impact in the lower back and rolling the length of the back will dissipate the ground force over greater surface area and it will hurt less with increased padding in this region.

Establishing a Base of Support

The last important aspect of being proficient at force absorption is organizing one’s body in a stable position when anticipating a force. In many of the macro- sports in general having a stable position means having a wide base of support in the direction in which the force will be projected at you. This also means having well braced torso and legs so that when the force is received there is torque in the body to help with absorbing the energy. Transferring weight about the two legs again, is another important aspect of this principle in certain scenarios. For example in many sports where one is receiving a ball from the front, a forward- back standing position is popular so that weight can be transferred from the front to back leg. During this extra time and distance of transferring the weight, energy from the ball is absorbed into the system without setting the person receiving the object off balance. To ensure optimal balance and stability throughout the process of taking in the object, it should be directed at the center of gravity as not to throw the organized body off balance.

Application to Sport

The capability to absorb force is an important principle across a wide variety of sports. The purpose of this section is to provide additional applied examples of force absorption principles in sport. A focus is on those sports involving repetitive movements wherein the proper application of force absorption principles is essential for successful performance and for reducing the prevalence of overuse stress-related injuries.

Running

Teaching a child the importance of force absorption and how it works during running is key in reducing musculoskeletal injuries. When it comes to running there is only so much a person can do to reduce the impact force as the foot hits the ground. For even someone correctly using the principle of force absorption to the best of their conscious ability, is still subject to immense ground impact forces. For instance studies have shown that each foot strike produces a force that is equal to about three times one’s body weight. Naturally our lower extremities act like natural human springs during running and force is absorbed at every joint and leg muscles from the foot to the hip. To understand what is happening as we run in terms of force absorption one has to know the running phases. The phases of running are initial contact (ideally ball of the foot contact) followed my mid stance, takeoff, initial swing, mid-swing, terminal swing and then back to initial contact where the process repeats. The absorption of force starts as we make initial contact with the ground. It depends on what type of running one is performing because intensity and speed can vary but nonetheless preferably the ball or mid-foot should take the first brunt of the force. The benefit of having the ball of one’s foot or mid-foot make contact first instead of the heal, is that it takes away the occurrence of an extra unnecessary peak impact force on each strike. With a heal strike, the heal makes first contact, then one rolls to the ball of the foot where they then push off and produce another peak force. Mid-foot or ball of the foot striking also increases the area in which the reaction ground force is distributed about the foot without slowing the runner or wasting energy from the unnecessary braking of the heel on every step. The heel is also a small area so for it to take three times our body weight is more dangerous than striking with more of the ball or mid-foot which have bigger surface areas. Again the more surface of contact the more force is evenly distributed. Now in the mid-stance phase, slight dorsiflexion occurs and the plantar- flexor tendons that make up the arch of our foot collapse and pass the force onto the Achilles tendon. This phase absorbs a considerable amount of the force. From here the force is translated to the quadriceps that absorb the majority of the shock from the ground reaction force. From the quadriceps the hip flexors absorb the rest of the force as we go from mid-stance to take off phase. By this phase most of the force of the impact has been absorbed into the system and the remainder of the phases take place. In these phases the hamstrings and hip flexors transfer the energy through the stride and swing portions of the run. According to a study increasing ones step rate and decreasing ones step length reduces the amount of force absorbed by the knee and patellar tendon. This may be useful for people that have knee problems from running and want to reduce the brunt on the knees.

Aside from being completely in control of our running mechanics in regards to absorbing a lot of the force one puts down each time the foot hits the ground, consciousness of the surface in which one runs on is another way to be smart about decreasing ground reaction forces on our body. Simply choosing to run on soft shoulder of a road, grass fields, gravel or bark mulch trails and cinder or mondo surface tracks are good choices for our bodies and greatly reduce the chance of acquiring a stress related injuries.

Tennis return (forearm)

When it comes to tennis, the biggest contribution a coach or trainer can give a learner is how to appropriately return an opponent’s shot by absorbing the ball and the using its energy to return it back. When anticipating the ball, a player should have their legs wide as to increase their base of support, have bent knees, and center of mass lowered for increased stability. Positioning of the feet should be at first from side to side and then as the opponent hits the ball the stance should change from slightly forward to back so that the base of support is facing the direction of the oncoming force. Just as the ball is about to get to you, create a short and low back-swing while at the same time slightly externally rotating the torso. From here one has a stable, prepared and elastically loaded system ready to make contact and use its energy to return it. Execute the swing from a downward to upward position. Remember that although it’s tempting to produce force from the upper body and the limb with the racket, force for projecting the ball should be generated by the bigger muscles in the leg such as gastrocnemius and quadriceps as well as the core because they are the muscles that have the most stored energy from the preparatory phase. Contact with the ball should happen at a comfortable distance from the body. Out of control arms can throw the whole system out of whack. Keeping the head steady and the body low throughout contact will make sure that one’s center of mass will remain compact and stable throughout the duration of absorbing the balls energy and then for sending it back. The follow through after a swing is very important in tennis for a number of reasons. Mainly, it allows the arm to slow the force generated during the swing and ball contact phase over a longer period of time and distance. Failing to do so might cause injury as one is breaking the arm to quickly and not dissipating the swing force over a long enough period of time. A strain in the triceps or shoulder is quite possible under these conditions.

Soccer (strike)

Soccer is one of the most popular sports among young people in many countries. There are a lot of different forces during play that one is exposed to which require the knowledge of force absorbing principles. The most important skill to teach a younger player in soccer is the principle of force absorption in kick, such as in the case of an on the ground moderate to hard kick.

After the approach and swing phase of a kick the key part with regards to force absorption is the phase right before the player is about to make contact with the ball. At this point the quadriceps and the anterior hip flexors have accelerated the leg into the kick. He/she should now have a braced kicking leg with a bend at the knee, left over from the swing phase. A braced leg means that the muscles and tendons are moderately contracted and flexed, especially the tendons around the knee that are loaded with elastic energy. The hamstrings are also fully active at this point. As well the ankle should be fully planter-flexed so it is ready for contact with the ball. This allows the lower extremities to take the impact as a system rather than just the ankle taking the force of the ball. It also allows for some power in the kick. The principle of use here is spreading the force out among a larger area. Not having a braced leg could lead to an injury when contact is made with the ball, due to the leg being torqued a weird way. The point of contact should be in the middle of the foot because this is the spot with the most torque. Contact at the toes increases risk of ankle impingement or hyper extension. The follow through of a kick is also important. The purpose of the follow through is to maximize the amount of time the leg is in contact with the ball. Increasing the time over which the force is received by the body can help reduce injury. Also after the ball has been projected there is kinetic and elastic energy remaining in the leg from the preparatory and swing phase of the kick. It is the job of the hamstring to act antagonistically to slow this forward and upward movement of the leg down. An inadequate follows through or in other words the immediate breaking or slowing down of the hamstring increases the risk of a hamstring strain.

receiving a pass using the correct principles

Basketball (receiving a chest pass)

In the game of basketball one of the fundamental skill that requires one to absorb a force directed at them is the receiving of a firm pass from another player. To many times when watching children play basketball, they fail to catch a pass because they lack of knowledge and practice in dealing with forces. It can be frustrating for the player and observers. Here’s how to avoid frustration. To begin one has to set up for receiving the ball. That is positioning the feet in the same direction (forward to back) as they will be receiving the ball in, positioning one’s body height so that the ball will make contact in line with ones center of gravity and having a slight bend in the knees as to increase the stability of the stance. The forward to back stride stance should be moderate in length as to increase the size of the base of support. Arms should be extended in front with a bend in them ready for the ball. As the incoming ball is going to make contact with the extended arms the spine should be braced with tension in the core. As the ball is caught bring the arms back or give with the ball to increase the amount of time and distance the ball is decelerated. At the same time in synchrony, one’s body weight should be transferred from the front leg to the back leg, again to increase the amount of time and distance the object is decelerated and the force of the ball is absorbed. Trying to stop the ball on instant contact will not only be uncomfortable to the hands and arms but it's also unreliable and inefficient. Failing to have an adequate base of support and to transfer ones weight from front foot to back may cause one to lose their balance and slow down a potential in game play to beat an opponent. Taking a pass in the above absorption friendly way will greatly reduces the chance of a potential rebound of the ball and or losing possession by an opponent stealing the ball out of ones hands.

Hockey (receiving a pass)

Again there are many essential skills in the sport of hockey that require proficiency early on, most notably being able to skate well but also being able to proficiently handle the puck. If one can handle the puck the whole game of hockey will improve. In particular, receiving a pass from a teammate and managing that pass properly without giving it up is crucial and it’s not easy for learners. For receiving a pass in hockey there are two main components that you want to keep in mind and both have to do with absorbing the force of the ball or puck so it doesn’t deflect off the blade of the stick or jump over it and keep going in the direction it was traveling. The first thing is ones hand position and grip strength on the stick shaft. The second is the position and movement of the blade in order to cushion the pass. For optimal control and leverage have one hand at the top of the shaft and the other a quarter to half way down the shaft or whatever feels more comfortable. Right before the puck or ball gets to the blade loosen your grip so that there is give in the stick system when the puck makes contact. Have the blade perpendicular to the direction of the incoming puck. When the buck is an inch or two away from the blade bring the blade back so that you are giving with the buck or again slowing it down over a larger amount of time and distance. Then when the buck or ball is decelerated enough re-strengthen your grip to regain control and stability so that you are ready for the following move. The same rules apply whether you’re receiving on the back hand or forehand. Failing to use these principles may not cause injury but will undoubtedly effect how the easily and successfully the puck or ball is received. Because the surfaces in this impact are both fairly hard (though rubber balls and pucks have some give), failing to use force absorption principles will almost always cause a frustrating rebound or roll over in an undesired direction.

Advice for coaches, educators or therapists

As kids are developing they learn the majority of what they know in terms of moving by trial and error and doing what feels the most comfortable to them. Keeping this in mind, the one way that an educator can get maximum learning out of the kids of whom he or she is working with, is through meaningful experiences. This is having the kids experience first-hand both the positive and negative aspects of a movement. In this case it’s important for educators to demonstrate the force absorption principles to children and allow them to experience the negative and positive aspects of movement with and without these principles. A simple way to let a kid experience both sides of the coin in force absorption is through a simple two foot jump. First have the child jump with the worst form or the way they are jumping at the current moment in time if they are performing the jump noticeably wrong. This means jumping with an overextended spine, and landing with flat feet, straight legs or in other words not giving with the force of the ground. Directly after, have them perform the same jump but this time demonstrate to them proper form and then let them try it. This means after leaving the ground stabilizing the spine and then landing and contacting the ground with the ball of ones foot while translating into mid-foot stance. At the same time ones feet should be screwing into the ground to create torque stability. This also causes the knees to drive out and not inward which is what you want. The Stine should be neutral, hamstrings and hips should be driven back, the knees should be bent and the shins should be as vertical as possible. It will soon be apparent that by using the principles in the second jump (giving with the force while creating a high stability landing position) discomfort will be kept at a minimum. The child will retain this knowledge by continuing to jump in this form because the negative reinforcement (the pain or discomfort of not giving with the force) would make the feeling during future jumps undesirable if continuously allowed to happen.

Landing of a jump

A few things that should be kept in mind while educating children are possible structural constraints and rate limiters they may have due to the current point in their development. Educators who aren't familiar with the child's learning characteristics (ex visual, Auditory, kinesthetic learner) should ideally also seek information from the guardian in respect of the the best way to teach the child. It's also advisable that for the majority of the teaching session that the educator keeps passive movement of the learner to a minimum unless completely necessary because it can inhibit the child's learning of the skill or principle. Again the more they can experience it themselves the better.

Any of the movement examples in the above principles are also good easy examples that can be used to demonstrate to children the comparisons between use and non-use of the principles, and the implications for not using them.

Implications in failing to use the proper principles

It is true that we as humans need impact forces to increase our bone density and maintain optimal bone health. However if these impact forces happen on a continuous basis at high magnitudes they may create reverse effects and bring about stress related injuries including fractures, Cartilage breakdown, strains and tendinitis. In teaching these skills to children of an early age we are giving them the tools they need as they grow and develop in sports, to reduce their chance of stress and impact related injuries. In sporting movements where injury from non-use of these principles isn't a huge concern, there are still direct negative effects on performance that will make movements look and feel uncoordinated and clumsy such as unwanted rebounds and unstable body positions that are of a disadvantage to performance. Many of these unwanted effects can also be embarrassing and frustrating to the performer and could potentially effect their enjoyment in a game as well as their self-efficacy ( Ones confidence in one’s ability to achieve goals and complete activities). Since kids potentially have a long life ahead of them in terms of sport and other activities, it’s important that they are educated to be in tune with how their bodies move in order to increase their longevity in sports and make them smart and proficient performers in whatever they do.

Protective sports equipment use similar principles

Helmets

Many head injuries that involve impact to the head occur because people weren’t wearing a helmet to help absorb the force they collided with and the brain accelerated and compressed on the inside of one’s skull. Since the brain is suspended in a layer of cerebrospinal fluid any moderate to large compression will cause physical and sometimes permanent damage. Blood vessels are also at risk of getting pinched causing hemorrhages. The purpose of a helmet is to lessen the impact of a force on the head and reduce the acceleration of the brain hitting the skull. As well helmets largely reduce the chance of skull fractures. In ridding and winter sports where falling and hitting one’s head against the ground is a concern, helmets contain mostly high and low density polystyrene foam. The foam compresses and increases the heads stopping time by under a second as it contacts the ground, reducing the chance of life threatening injuries. The thickness of the foam used in helmets varies. The thicker the foam the more time there is for the impact to be absorbed and the less dense the foam is the more it compresses making impacts softer. Helmets designed for lessor impacts such as for rock climbing are made with only a hard polycarbonate shell. These helmets have are ideal for softer sharper blows to the head and don’t have to be replaced unless they crack from an impact.

It is especially important for kids to wear helmets and should be continuously educated about why it’s important as their heads have a full life ahead of them. No one needs permanent brain damage for the rest of their lives. If you have a child it is also very important to monitor any crashes they might have as the foam does not recover after a big blow and the helmet should be replaced. Also never buy a second hand helmet as one has no way of knowing one hundred percent if the function of the helmet has been compromised by any past crashes.

Mouth Guards

Mouth guards should be an essential part of safety equipment to any person competing in contact sports. When force is delivered to the jaw or mouth the energy has to be absorbed by whatever the force contacts. The job of a mouth guard is to help distribute the total degree of force to the entire mouth so that any one tooth or part of the mouth doesn’t take the brunt impact. The national youth sports safety foundation estimates that around 200 000 oral injuries and 3 million missing teeth could be prevented per year with the use of sports mouth guards. Mouth guards are also proven to significantly decrease the occurrence of concussions as they reduce cranial pressure, deformation to the jaw bone and the acceleration to the head when an impact occurs in the chin region.

Its recommended that all children and youth participating in contact and collision sports where custom fitted mouth guards as store bought stock ones are poor fits, usually uncomfortable, inhibit desired speech and breathing and aren’t reliable enough to maintain proper oral health and reduce orofacial injuries. It’s also recommended that educators make parents and kids aware that when mouth guards become physically damaged and altered; they no longer have the protective qualities vital in preventing concussions and injuries to the mouth.