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Biomechanics
of Sport Injury
Introduction
The stresses of athletic competition can be unimaginable.
Football players can collide and the forces calculated can reach that
of a car collision. Amazingly the body can absorb and distribute thousands
of pounds of force. However, small amounts of force applied in the
right manner can damage tissues. If forces reach certain thresholds
within tissues, then injury will result. There are various mechanisms
that account for injury in sport. Many tissues can be affected from
the various mechanisms. The type of injury, degree of injury, and
the tissues affected will be a direct result of the injury mechanism.
For instance bone can sustain a variety of fractures. A fracture will
result from a force applied to the bone. Depending on the amount of
force, the bone may simply fracture and not affect or disrupt tissues
around the fracture. However, if enough force is applied, the bone
could fracture and puncture or lacerate tendons, nerves, muscles,
ligaments, cartilage, blood vessels, and other tissues.
There are several factors that can contribute to or
increase the risk of sport injury such as poor nutrition, poor conditioning,
poor flexibility, improper equipment, playing surface, and many more.
Most of the factors that contribute to injury can be avoided or eliminated.
If an athlete does not maintain a proper athletic diet, then muscles
and tendons may not have adequate nutrition to repair from the damage
occurred during the stresses of sport. This may lead to muscle injury.
Please refer to the Nutrition Section to learn more about how to eat
to prevent injuries. Another example may be that of playing surface.
If the playing surface is slippery, then an athlete could fall which
could influence a variety of injuries.
One of the main goals of a strength and conditioning and sports medicine
program should be to prevent injury. The following examples are just
a few of the many ways these teams can prevent injuries. Injury prevention
can be accomplished by structuring training programs to increase flexibility
and condition tissues to the vigorous stresses of sports (refer to
the Strength & Conditioning Section to learn more about this topic).
The sports medicine team will work with athletic surface crews to
make sure all playing surfaces are safe to play on. In addition, the
sports medicine team will evaluate the athletes and determine if braces,
tape, or wraps should be worn over structures to add support and help
prevent injuries.
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Bone Injuries
The purpose of the skeletal system is to protect internal organs,
provide attachment sites for muscles, and help facilitate movement.
The internal architecture and external geometric make up of bones
allow it to absorb and distribute forces. When these forces can’t
be absorbed or dissipated then injury results. Bone is highly vascular
and has the ability to self-repair. The unique feature of bone is
that it has the ability to alter its properties and internal structure
in response to changes in mechanical demand. When stress is applied
to bone, it causes minute electrical signals that stimulate bone remodeling
and strengthening in the area of the signal. This is done through
a process called Wolf’s Law.
Bone injury depends on its mechanical properties, geometric make
up, the force applied to the bone, the rate of force (force is applied
all at once or gradually over time), and the frequency of loading.
For instance, bone injury can result from the frequency of loading.
Anytime an athlete increases his/her training in large volumes, he/she
will increase the risk of sustaining a fracture. If an athlete were
to suddenly increase training from one hour per day to two hours per
day, then he/she may suffer bone injury. The reason being is that
the athlete’s bones will suffer twice the load they are a custom
to and Wolf’s Law will not have time to respond.
Another variable that may contribute to bone fractures is that of
muscle fatigue. The muscles may not be in shape to handle two hours
of activity. The muscle fatigue that will result during activity will
cause a decrease in ability of the muscles to help absorb force. This
will increase the amount of force that the bones will have to absorb
and dissipate. Thus, muscle fatigue will increase an athletes risk
for suffering a fracture. Therefore, athletes should gradually increase
their training over a period of time.
Bones can be loaded in six different ways. Each way the bone is
loaded, will influence the type of fracture sustained by the bone.
Bone can withstand greater force in certain mechanisms of loading
than in others. The following are the six mechanisms in which a bone
can be loaded.
Bone Fracture Model Goes Here
- Tension -
Tensile loading can cause a pulling apart of the bone. Equal and
opposite forces are applied away from the surface of the structure.
Bone withstands greater stress in tension than in shear.
- Compression - Compressive
loading is a pushing together of the bone. Equal and opposite forces
are applied directly into the surface of the structure. Bone withstands
greater stress in compression than in tension.
- Shear - Shear loading
results from a force that is applied parallel to the surface of
a structure.
- Bending - Bending
causes a bone to bend about an axis. When a bone is loaded in bending,
it is subjected to compression on one side of the axis (concave
side) and tension on the other side (convex side) of the axis.
- Torsion - In torsion
a load is applied that causes the bone to twist about an axis and
torque is produced within the bone.
- Combined Loading -
Combined loading is when two or more loads are applied to
a bone at the same time. In the combined loading example it is compression
and torsion loading the bone.
Fractures
Fractures are considered to be the breaking of bones. Names and classifications
of fractures must consider the location of the fracture relative to
the rest of the bone, the shape, direction, and magnitude of the fracture
lines, as well as the fracture’s onset. Different fractures
will produce different secondary injuries such as severing nerves,
muscles, tendons, arteries, and more.
Exostosis
Wolf’s Law states that a bone will remodel itself in order to
become strong do to forces placed upon it. Exostosis is growth of
extraneous bone. It can occur from a stress reaction to injury or
from irregular forces on the bone. This irregular force on the bone
causes an exostosis to occur at the site of stress. In some cases,
it can be come a mechanical block that disrupts ROM.1
Apophysitis
Apophysitis is an inflammatory condition involving a bones growth
plate. Some doctors refer to it as growing pains. Some of the growth
plates are along some of the attachment sites of the larger, stronger
muscle groups in the body. Tightness of the muscles, or repetitive
forces applied to the bone by these muscles, can result in inflammation
and separation of these areas away from the rest of the bone. This
is why age and maturity is such an important factor in training and
conditioning. Training and conditioning too early in life can cause
harm to the athlete.1
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Muscle
and Tendon Injuries
When a muscle contracts and lengthens at the same time, the muscle
is conducting an eccentric contraction. When a muscle contracts and
shortens, the muscle is conducting a concentric contraction. These
contractions are important because we are constantly moving. In movement,
a joint angle will accelerate and then decelerate. For instance, when
a pitcher pitches a ball, he/she moves the arm forward (concentric
contractions make it go forward). Then, because the agonist muscles
(muscles that produce the movement) are still pulling the arm forward,
the antagonist muscles (muscles being stretched in response to the
antagonist muscles) are lengthening. Now the joint must be slowed
down or else the arm will just dislocate and sling out of socket from
all the force. This is where the eccentric muscle contraction of the
antagonist muscles comes into play. The antagonist muscles of the
pitchers shoulder start to eccentrically contract. These muscles are
producing a force to slow the joint down, but the muscles are lengthening
instead of shortening.
Eccentric contractions are especially important in sports injury because
this is thought of as one of the precursors to muscle damage. When
a muscle eccentrically contracts, it is more prone to microscopic
damage. You can’t move without eccentric contractions. It is
when athletes are moving at maximum speeds and exerting maximum forces
that muscles tear or undergo microscopic damage. Pitchers are usually
sore after a game due to the massive amount of eccentric contractions
that occur throughout a game. Athletic trainers see a lot of hamstring
injuries in athletes such as sprinters, football, basketball, hockey,
and soccer players. This is because the quadriceps are such a strong
group of muscles. Often the strength ratios between the quads and
hamstrings are out of balance. The quads are stronger than the hamstrings.
In this case it is not uncommon to see the hamstrings work extra hard
to help decelerate the body during sprinting. This extra hard work
done by the hamstrings is usually because the quads are so strong.
When the hamstrings have to work hard trying to decelerate the body,
it is usually done eccentrically, and the hamstrings sometimes tear.
Therefore, balancing the dynamics of human motion is important in
keeping an athlete injury free and successful.
Injuries to a muscle belly or tendon adversely affect the muscles
ability to contract fully because of a mechanical insufficiency or
due to the onset of pain. If the musculotendonous (muscle and tendon)
unit has been mechanically altered through partial or complete tears,
then the unit may no longer be able to produce movement. Partial tears
decrease force production while, complete tears do not produce any
motion.
Strains
Strains to the muscle and tendon are caused by excessive stretch or
tension within the muscle. Tensile (pulling apart) forces are produced
when the muscle is stretched beyond its normal range of motion. This
causes the muscle to tear. Muscle can also be traumatized by tension
overload, which occurs when the muscle generates more force than its
fibers can withstand. Depending on severity, strains are ranked from
first degree (limited tearing of the muscle) to third degree (complete
rupture of the muscle).
Tendonitis
Tendonitis is inflammation of the tendon. Tendonitis can result from
a single traumatic force. It may also result from smaller repetitive
forces being placed on the tendon. Tendonitis may also involve the
synovial sheath (a thin membrane that houses synovial fluid) surrounding
the tendon known as tenosynovitis. Tendonitis is a common overuse
injury encountered in most sports.
Myositis Ossificans (Muscle Ossification)
Myositis Ossificans is the formation of bone within the muscle. It
occurs secondary to deep contusions or muscle strains and is usually
related to a fault in the bodies healing process. Following an injury,
osteoblasts and chondroblast (muscle and cartilage builders), form
immature bone within the muscle. This injury is usually found in collision
sports such as football and rugby.1
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Joint and
Ligament Injuries
The most prevalent of the soft tissue injuries are the injuries
to the capsular (capsule that surrounds the joint) and ligamentous
(hold joints together) tissues. These injuries directly affect the
ability of these structures to function in a stable manner during
movement.
Sprains
Sprains occur when a joint is stretched beyond its normal limits.
The result is stretching or tearing of the ligaments (e.g. ACL, MCL,
LCL, UCL, and other ligaments) and or joint capsule. There are three
injury classifications of sprains and they are based on the amount
of laxity produced by the injury relative to the opposite limb (i.e.
one knee may have more laxity in it relative to the other knee because
of injury). The sprains are ranked in order from first degree (little
or no tearing of the ligament) to third degree (complete rupture of
the ligament).
Joint Dislocation
Joint dislocations involve complete dislocation of the joints boney
surfaces. The forces that cause the dislocation are usually sufficient
to rupture the ligaments and capsule surrounding the joint. Dislocations
result in obvious deformity and in some cases the bony structures
may protrude through the skin.
Joint Subluxation
A subluxation involves the partial or complete dislocation of a joint’s
boney surfaces, which may or may not suddenly return to their normal
alignments. In other words the joint may pop out of place and right
back in. The amount of force to cause displacement of bones is often
sufficient to cause soft tissue or bony injury. Tearing of the joint
capsule and ligaments as well as bone fractures will usually be associated
with subluxations.
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Articular
Injuries
The hyaline cartilage lining a bones articular surface is commonly
injured in young athletes. In adult athletes however, damage is a
result of degenerative changes. Most of these injuries are irreversible
and result in chronic joint pain.
Osteoarthritis
Osteoarthritis is the most common type of arthritis found in athletes.
It is the degeneration of a joint surface. The degeneration can lead
to complete destruction of the cartilage and the exposure of the subchondral
bone. Flaking pieces of bone can lead to loose bodies.
Osteochondral Defects
Fractures of bones articular cartilage and the progressive
softening of this cartilage are referred to as OCD’s or osteochondral
defects. The severity of an OCD is based on its depth, where partial-thickness
OCD’s involve the outer layering of the articular cartilage
and the full-thickness OCD’s exposes the underlying bone.1
Osteochondritis Dissecans
A condition characterized by dislodged fragments of bone
in the joint space. Osteochondritis dissecans is a lesion of the bone
and cartilage that results in the delamination of the subchondral
bone. The piece of bone may be stable or free floating in the joint.
1
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Nerve Injuries
Trauma to the neurovascular structures (i.e. nerves, arteries, and
veins) is often a consequence of joint dislocation, bony displacement,
or concussive forces.
Peripheral Nerve Injury
Entrapment of the nerves is common among athletes. The closer to the
CNS (spinal cord and brain) the injury occurs, the greater the symptoms.
Likewise peripheral nerves distal (away from) to the spinal column
have a greater probability of regeneration, than nerves proximal (closest
to) to the column. In some cases a soft tissue may swell, this will
cause a mechanical deformation of the nerve, and cause parathesia
(numbness and tingling in the limb) and muscular weakness. Nerves
may also be injured by tensile forces or stretch injuries. These injuries
are classified into three categories. Neuropraxia is the mildest form
of nerve stretch injury. Axonotmesis involves a disruption of the
axon and myelin sheath of the nerve. Neurotmesis is a complete disruption
of the nerve and is the most severe form of nerve injury.1
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Brain
Injuries and Compartment Syndrome
Brain injuries and compartment syndrome are common among athletes.
However, brain injuries can be catastrophic. Unfortunately as the
body strikes an object and suddenly decelerates or the body suddenly
accelerates, the brain and organs do not respond in a uniform fashion.
For instance, think of your brain like a human in a car accident,
except in this case the human does not have on a seat belt. The car
stops, the human remains in motion and strikes the dash board. This
same principle applies to the brain and organs, when the skull stops
and strikes an object such as the ground, the brain remains in motion.
The brain will then strike the wall of the skull and damage will ensue.
The brain will swell but there is no where for the pressure to go.
Therefore, tissues are manipulated and damage occurs. Compartment
syndrome injuries are similar in that swelling occurs within a compartment
and manipulates tissues. This cuts off neurovascular supply to tissues
distal to the injury site, and further harm to the distal tissues
occurs.
Cranial Injuries
The majority of athletic head injuries are not catastrophic. However,
they are the leading cause of death in sports. The most common head
injury death in sport is the subdural hematoma (hematoma= pooling
of blood). Researchers have indicated that repeated concussions may
predispose an athlete to major head trauma. A subdural hematoma is
a hemorrhage beneath the duramater of the meninges within the cranium.
In other words, it is bleeding between the brain and skull. The hematoma
presses on the neurologic tissues and leads to ischemia (lack of blood
flow) and death if pressure on the brainstem is not relieved. It needs
to be understood that acute subdural hematomas are more susceptible
after repeated minor head trauma, and that other types of cranial
hematomas will not present signs for as up to as long as a week after
injury. These are very dangerous and should be taken with grave caution.
Any concussion should be examined by a medical professional. CAT Scans
and MRI’s are a great way to properly diagnose these injuries.
Compartment Syndrome Injuries
Certain regions of the body contain compartments such as those in
the lower leg. The compartments are housed in fascia or incased in
a cavity. In the lower leg there are four such compartments encased
in fascia. When there is an injury and swelling ensues with in these
compartments there is no where for the swelling to go and the compartments
do not stretch. The swelling causes the occlusion of blood vessels.
Athletes will feel numbness and tingling in the regions below the
injury site. THIS IS A MEDICAL EMERGENCY. It will not seem like much
or look like much, but the blood supply will be cut off do to the
pressure. Soon after the blood is cut off, tissue will start to die.
Get the athlete to the hospital as fast as possible.
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Reference
- Zatsiorsky, V. M. (2002). Kinetics of Human Motion. Human Kinetics:
Champaign, IL
- Starkey, C., & Ryan, J., (1996).Evaluation of Orthopedic
and Athletic Injuries. F. A. Davis Company: Philadelphia
Related Links
National Center For
Injury Prevention and Control
Physician
Sports Medicine Journal: Baseball Injuries
Physician
and Sports Medicine Journal: Female ACL Injuries
Physio Room.com
E Medicine
Knee Injuries
Post
Graduate Medicine: Cold Injuries
Oslo Sports Trauma
Research Center
Pediatrics
Sports Injuries
Mechanisms
of Shoulder Injuries
American Academy
of Family Physicians: Burner Syndrome
National Institute for Sports Science
and Safety
Biomed Central:
Injuries in Taekwondo
BBC
Health: Sports Injuries
Medline
Plus Sports Injuries
Sports-Injuries.com
Hockey Injuries.com
About.com
Running Injuries
Children and
Sports Injuries
EMEDx.com Sports Medicine and Orthopedic
Surgery
Sports Med Web Injury
Clinic
Knee Pain Info
Facial
Sports Injuries
Tennis Elbow
Virtual Sports Injury
Clinic
Running
Injuries
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