Download Overuse injuries in youth sports: biomechanical considerations

APPLIED SCIENCES
Biodynamics
Overuse injuries in youth sports:
biomechanical considerations
DAVID HAWKINS and JEFF METHENY
Exercise Biology Program, Exercise Science Graduate Group, Biomedical Engineering Graduate Group, University of
California, Davis, Davis, CA; and Sutter West Medical Group, Davis, CA
ABSTRACT
HAWKINS, D., and J. METHENY. Overuse injuries in youth sports: biomechanical considerations. Med. Sci. Sports Exerc., Vol. 33,
No. 10, 2001, pp. 1701–1707. Many children today participate in highly organized sports programs that involve regimented year-round
repetitive training. This type of training has led to an increased incidence of overuse musculoskeletal injuries. Sports physicians have
dealt with sports injuries in children for many years and, on the basis of their clinical experience, have developed guidelines to treat
and to try and prevent these injuries. The purpose of this article is to provide a biomechanical perspective of sports injuries in young
athletes and blend ideas from this perspective with more traditional clinical perspectives that dominate the literature relative to this
topic. Basic tissue and gross movement mechanics principles are used to identify growth, morphological, and movement factors that
may predispose a child to an overuse injury. Several biomechanical analyses of simple movement tasks are presented to quantify the
forces developed in various tissues and to illustrate the effects that growth can have on these forces. Guidelines are given for developing
injury prediction models that may be used in the future to establish safe and effective training guidelines for children. Key Words:
MECHANICS, CHILDREN, ATHLETICS, YOUTH, MUSCULOSKELETAL, TRAUMA
A
prevention guidelines. Biomechanical analysis techniques
are used to identify various factors that may contribute to
overuse injuries, to illustrate how injury sites may be predicted, and to describe what is needed to develop reliable
injury prevention guidelines in the future.
large number of children are injured each year as a
result of participation in strenuous physical activities. Treating these injuries is costly and the injuries
can lead to long-term health problems (10,13). Reducing the
incidence of these injuries may be achieved by identifying
factors that may predispose a child to injury and through
proper training, technique, and fitness. Efforts to reduce
these injuries are warranted both to ensure the long-term
health of children and to reduce medical costs. Ideally, we
would like to have a simple set of guidelines that outline
exactly how often a child can exercise, at what intensity, and
for what duration to optimize the physical and psychological
benefits of exercise and to minimize the risk of overuse
injuries. Such guidelines are elusive but may be achievable
through the collective efforts of scientists and clinicians.
Numerous articles have been written over the past 20 yr
discussing overuse injuries in children from a clinical perspective (2,8,11,12,16,28,29). Most of these articles have
been written by physicians on the basis of their clinical
observations and experience. The purpose of this report is to
evaluate overuse injuries in children from a biomechanical
perspective and to provide direction for developing injury
HISTORICAL PERSPECTIVE
Children’s physical activities today are much different
than they were 50 –100 yr ago; 100 yr ago, physical chores,
free play, and sandlot sports dominated the spare time of
American children. Now, children participate in organized
and regimented physical activities. The number of children
participating in organized sports programs has increased
considerably over the past 70 yr from practically zero in
1930 (before the establishment of organized sports programs such as Pop Warner football and Little League baseball) to approximately 50% today (26,27).
There were approximately 60 million children between 5
and 18 yr of age living in the United states in the year 2000
(25). Approximately 38% of high school children (27) and
34% of middle school children (1) will sustain a physical
activity related injury that will be treated by a doctor or
nurse. It is likely that the actual number of injuries incurred
by these children is larger than cited because of the fact that
many injuries go unreported. Although younger children
presumably sustain fewer sports related injuries compared
0195-9131/01/3310-1701/$3.00/0
MEDICINE & SCIENCE IN SPORTS & EXERCISE®
Copyright © 2001 by the American College of Sports Medicine
Submitted for publication September 2000.
Accepted for publication January 2001.
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with older children because of differences in size and
strength, a conservative estimate of the number of children
that will incur and be treated for a sports/physical activity
related injury this year is 18 million (30% of 60 million). It
is estimated that approximately 50% of these injuries (9
million) can be attributed to overuse-type mechanisms (2).
The total cost to treat these potentially preventable overusetype injuries is approximately $1.8 billion (assuming an
average cost for treatment of $200 (1) and 9 million preventable injuries per year). The cost and long-term health
risks associated with overuse injuries in children warrants
developing tools or strategies that can be used to prevent and
treat these injuries. Biomechanical analysis techniques provide one such tool.
BIOMECHANICAL ANALYSIS
Biomechanical analysis techniques provide a way to estimate the forces and deformations experienced by various
tissues during human movement. Understanding the physical principles involved in these techniques is useful for
identifying factors that may contribute to injury mechanisms. The body and individual limb segments move according to basic physical principles. According to Newton’s
second law, the sum of all forces (⌺F) acting on a rigid body
is equal to the product of the body’s mass (m) and its linear
acceleration (a) (i.e., ⌺F ⫽ ma). Similarly, the sum of all
moments (M), or torques, acting on a body is equal to the
product of the body’s moment of inertia (I), which is a
measure of the radial distribution of mass about an axis of
rotation, and its angular acceleration (␣) (i.e., ⌺M ⫽ I␣).
The resultant forces and moments that act on a limb segment
result from external loads applied to the segment, as well as
forces developed in muscle-tendon units, ligaments, and
bones associated with the segment. If the kinematics (e.g.,
the position, velocity, and acceleration) of a segment are
known, then the resultant forces and moments acting on the
segment to cause that motion can be determined. Such an
analysis is called inverse dynamics. Once the resultant
forces and moments are known, then they can be distributed
among individual tissues if the structural properties (i.e.,
mechanical behavior that depends on the properties of the
material constituting the tissue as well as the tissue’s size
and shape) of these tissues are known along with the relative
location and direction of ligament and tendon attachments to
bone. This is referred to as a distribution problem analysis.
A different approach called forward dynamics prescribes the
forces developed by individual muscles and determines how
the body moves on the basis of Newton’s laws and the
forces induced in bones, tendons, and ligaments. Relative to
injury mechanisms, the forces and deformations developed
in the muscles, tendons, ligaments, and bones are of interest
as well as the tolerance these tissues have to such forces and
deformations. The physical principles described above indicate that to understand and predict injury mechanisms in
the growing child requires an understanding of 1) limb
inertial properties (i.e., mass, center of mass location, moments of inertia); 2) muscle strength; 3) bone, ligament, and
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FIGURE 1—Body height growth rate for five different males illustrating the variability in growth rate and timing. These curves were
constructed from data published by Tanner (21).
tendon structure and structural properties; 4) external loading conditions; and 5) how all these factors change with age
and exercise. A review of these relevant topics is given
below.
GROWTH AND DEVELOPMENT
Gross properties. Overall body height and mass increase during childhood, often at sporadic rates. Body mass
increases on average from 18 kg to 55 kg in females and 18
kg to 73 kg in males between 5 and 18 yr of age. Body
height increases from 110 cm for males and females to 160
cm in females and 175 cm in males over this same age
range. Girls tend to reach their peak body height and mass
around 15 yr of age, whereas boys continue to increase their
body mass and height beyond age 18 (9,18,19,21). An
important aspect of growth that may increase the risk of
injury is the growth rate. The growth rate of an individual is
not accurately characterized by average growth curves. The
growth spurt in boys may occur any time between 10 and 18
yr of age (Fig. 1) and may reach a peak value of 8 to 12
cm·yr⫺1. Growth spurts may result in changes in the ratios
of muscle strength to body or limb mass and moments of
inertia, and soft tissues may experience increased stress and
strain (discussed later in this section).
Limb length, mass, and moments of inertia change with
age (Table 1) (4,5,19). On average, limb lengths increase by
a factor of about 1.4 between 6 and 14 yr of age. Limb
masses increase by more than 3-fold over this same age
range. Limb moments of inertia increase with age as a result
of increased limb length and mass. Limb moments of inertia,
TABLE 1. Limb length, mass, and moment of inertia change with age: the ratios of
these quantities at age 14 compared with age 6 are listed.
Lower leg
Thigh
Forearm
Upper arm
Torso
a
Length Ratioa
Mass Ratiob
Itransverse Ratioc
1.4
1.4
1.4
1.4
1.3
3.4
3.8
3.2
3.5
2.7
10.0
12.9
10.0
10.0
6.0
Snyder et al., 1977; b Jensen, 1987; c Jensen and Nassas, 1988.
http://www.acsm-msse.org
FIGURE 2—Biomechanical analysis of a stationary knee extension
task. A free-body diagram of the lower leg is shown on the middle left.
Newton’s equations are shown on the middle right. F, resultant force;
m, lower leg plus foot mass; a, lower leg acceleration (equal to zero in
this example); T, resultant torque; I, moment of inertia of the lower leg
plus foot about its center of mass; ␣, lower leg angular acceleration
(equal to zero in this example); Fx and Fy, horizontal and vertical
components, respectively, of the resultant force acting on the proximal
end of the lower leg; L, distance from the knee joint center to center
of mass of the lower leg plus foot; g, gravitational acceleration; Tm,
muscular torque acting at the knee. The 6-yr-old must generate a knee
extension torque equal to 2.2 Nm to hold the lower leg horizontal,
whereas the 14-yr-old must generate a 10.3-Nm torque.
relative to a transverse axis, increases on average by a factor
of 10 between 6 and 14 yr of age. The rate at which these
quantities change depends on the child. The affect these
changes have on movement performance and muscle
strength requirements can be illustrated by analyzing a simple movement task.
Consider the task of holding the lower leg horizontal
while sitting in a chair. The quadriceps muscles need to
generate 4.7 times (10.3 Nm vs 2.2 Nm) more knee extension torque to hold the limb stationary at age 14 compared
with age 6 (on the basis of a biomechanical analysis, a 140%
increase in leg length, and a 340% increase in limb mass;
Fig. 2 and Table 1). There is an increase in muscle moment
arm during the aging process that allows a 4.7-fold increase
in joint torque to be generated by a 3.9-fold increase in
muscle force. The increase in muscle force required to
accelerate the leg during dynamic actions is even greater
than the 3.9-fold increase required for static conditions.
Therefore, if the same activities are to be performed at age
14 as were performed at age 6 without increased muscle
fatigue or increased stress in tendons and apophyses, then
there must be an increase in muscle strength and a corresponding increase in the strength of muscle-tendon junctions, the tendons, and the apophyses. Quadriceps muscle
BIOMECHANICAL PERSPECTIVES OF SPORTS INJURIES
strength increases on average about 3.3-fold between 6 and
14 yr of age (17). This average strength change is similar to
the change in static strength needed to perform the knee
extension task described above, and suggests that on average muscle strength increases appropriately to match
changes in limb inertial properties in the long term. Relative
to injury mechanisms, it is important to understand how
limb growth affects the muscle forces that must be generated
by a child to perform a movement task, but it is also
important to understand how growth affects the strength of
tendon, apophysis, ligaments, and bone. The acute and
chronic changes experienced by these tissues relative to
each other are important. A muscle group may adapt quickly
to accommodate increased demands created either by
changes in limb inertial properties or changes in physical
activity and generate greater force either by increasing its
size or activating a greater portion of its mass. If the tendons
and apophyses associated with that muscle group adapt
slowly, then the stress induced in the tendons and apophyses
will increase in response to the increased muscle force and
perhaps lead to injury. Unfortunately, data on the relative
strengths and rate of strength changes between muscle,
tendon, ligament, and insertional zones are lacking for both
humans and animals (discussed in the next section).
Although acute limb anthropometry and muscle strength
data are not readily available, a simple biomechanical example illustrates how tissue forces must increase during a
growth spurt to maintain a certain movement performance.
Imagine that a child’s lower leg grows from 26 cm to 30 cm
in length during a growth spurt. If muscles and tendons
lengthen during the growth spurt, but they do not hypertrophy until some time after the growth spurt, then because of
the increased limb mass and moment of inertia the muscles
will need to develop a greater percentage of their maximum
force to produce movements similar to those performed
before the growth spurt. The increased muscle forces will
cause the tendons to experience increased stresses. Using
conservative estimates of the changes in limb mass that
might occur for the lower leg length change stated and
solving the equations of motion for the lower leg (see Fig.
2) indicates that the thigh muscles need to develop about
30% more force after the growth spurt to develop the same
lower leg angular acceleration that might be required during
a kicking movement. The child may or may not be able to
produce this additional force. If the task required a muscular
effort near maximum before the growth spurt, then the child
will not be able to perform the task after the growth spurt.
If the child can produce the needed force, then the tendons
and apophyses will be subjected to greater stresses/strains
until they can adapt to the new loading conditions. Muscle
fatigue and/or overuse injuries may result if this activity is
performed repetitively (e.g., swinging the leg repetitively to
kick a soccer ball).
An interesting concept that has received very little attention relative to injury mechanisms is tissue preload (i.e., the
amount of force sustained by a tissue in a normal relaxed
state of the body). The higher a tissue’s preload, the higher
the acute and chronic force that tissue will apply to other
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tissues. A muscle-tendon unit may experience an increased
preload if the bone grows faster than the muscle-tendon unit.
Consider the 4-cm growth spurt of the lower leg described
previously. The plantar flexor and dorsi flexor muscles and
tendons act in a lengthened position as a result of the bone
growth. Presumably, these muscle-tendon units increase
their length over time to restore their “preferred” preload,
but it is likely that initially these muscle-tendon units act in
a stretched position and experience an increased preload.
This would result in a chronic increase in force applied
through the tendons and apophyses. No studies have been
performed to test this idea, but the incidence of apophysitis
in growing children suggests that increased tendon preload
may be a contributing factor to the injury. A gross measurement that can indicate changes in tissue preload is flexibility. A reduced flexibility may result from an increase in
tissue preload, an increase in tissue stiffness, and/or an
increase in bony and cartilage geometry around the joint.
Interestingly, boys tend to lose back and leg flexibility
between 8 and 13 yr of age (9). This may predispose boys
to lower extremity tendon and apophysis injuries.
Tissue properties. Structural properties describe the
mechanical behavior of the whole tissue, whereas material
properties describe the mechanical behavior of the material
constituting the tissue. The integrity of a functional unit
such as a bone-ligament-bone (BLB) or muscle-tendonbone (MTB) complex depends on the relative structural
properties of the individual tissues. Structural properties are
affected by the tissue’s material properties and the tissue’s
geometry (e.g., length and cross-sectional area). The relative
structural properties of tissues acting in series need to be
matched (e.g., muscle-tendon-insertion) to minimize the
risk of injury. Unfortunately, very little information is available regarding tissue structural properties for the growing
child. There is even less information about the interaction
between exercise, growth and development, and tissue
properties.
Limited human data suggest that muscle and tendon do
not grow proportionally. Elbow flexion (21) and knee extension (17) strength increase on average by a factor of 1.8
between 10 and 15 yr of age. However, hamstring muscle
tendons and biceps brachii tendons increase in cross-sectional area by a factor of about 1.5 during this same time
frame (6). These data suggest that tendon may operate closer
to its failure limits as maturity is reached, or the material
properties of tendons change during maturation, resulting in
an increase in the strength of the material.
Despite the lack of direct information about tissue properties in children, data from animal and cadaver studies
provide insight into the complex interactions between age,
activity, and modes of injury for various functional units
such as BLB and MTB (7,14,15,20,22–24,30 –33). Ligaments and tendons become stiffer and stronger during maturation (31,33). These tissues withstand greater stress and
strain before failure as the loading rate increases (33). BLB
complexes tend to fail by avulsion in immature animals and
by midsubstance tears in mature animals (31). There are
differences in the strain induced in different regions of BLB
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and MTB complexes. The tendon insertion zone appears to
experience twice as much strain as the tendon under moderate stress (20). Tendons become stronger in response to
long-term exercise, but the cross-sectional area of the tendon does not necessarily increase in proportion to the
strength gain (30). This suggests that the material properties
of the tendon change in response to exercise. These studies
highlight the important and asynchronous rates of tissue
changes that likely occur in the growing child.
The examples given above illustrate relative changes in
muscle and tendon size during maturation, but they do not
indicate the absolute stress that a muscle might induce in a
tendon. A biomechanical analysis of a simple movement
task provides some insight in this area. Consider the forcegenerating potential of the triceps surae muscle group of a
6-yr-old girl. The girl generated an 18-Nm plantar flexion
torque during an isometric effort. On the basis of her anatomy, a 1000-N triceps surae force was required to generate
that torque. Her Achilles tendon had a cross-sectional area
of approximately 35 mm2 (on the basis of ultrasound measurements). A 28-MPa stress was induced in the tendon and
insertion zone during the plantar flexion effort, assuming the
tendon-bone insertion cross-sectional area is similar to the
tendon cross-sectional area. The ultimate tendon and insertion failure stresses are estimated to be 50 MPa and 45 MPa,
respectively, on the basis of tissue properties obtained in the
various animal studies described above (33). It should be
recognized that tissue failure stresses vary considerably with
age and loading rate in young children. The plantar flexion
effort would induce 63% and 56% of ultimate stresses in the
bone and tendon, respectively. Tendon tests suggest that
little damage is created for strains less than 3% and stresses
less than 40 to 50% of ultimate stress (6). Thus, the 63% of
ultimate stress calculated in the above example suggests that
if this child were to perform this activity repetitively, she
may be at risk for sustaining an apophysis overuse injury.
INJURY PREVENTION MODEL
The previous discussions highlight only a few of the
factors that may contribute to childhood overuse injuries,
but they provide a foundation and direction for future endeavors that may lead to injury prediction models. There are
many factors that contribute to tissue injury, including the
strength of the tissue, the magnitude of the loads applied to
the tissue, the number of loading cycles, the recovery period
between loading bouts, and the rate of tissue repair. Unfortunately, because of the complex interactions between these
various factors, there do not exist quantitative models for
predicting conditions that lead to injury. Our current injury
prevention guidelines, such as the 10% rule or the limits set
for the number of allowable pitches that can be thrown by
young baseball pitchers (28), are derived from clinical observations and epidemiological data, not an understanding
of basic physiology and mechanics.
If we are to develop injury prediction models for musculoskeletal tissues, then we must 1) characterize tissue structural properties, 2) characterize tissue rates of adaptive rehttp://www.acsm-msse.org
repair rate was insufficient to cause complete recovery between exercise bouts:
DF ⫽ N䡠⑀q/K
FIGURE 3—Illustration of an injury prediction model. This model
estimates the amount of damage that a tissue will incur during an
exercise bout as a function of the number of loading cycles and the level
of strain induced in the tissue during each loading cycle. The number
of loading cycles would be determined from the activity. The strain
induced in the tissue would be determined from biomechanical analysis
techniques and information about the structural and material properties of the tissue. The contour plot shown was derived for cortical
bone.
sponses, 3) predict the loading profiles that tissues are
exposed to during various activities, and 4) develop more
complete injury tracking systems. Efficient methods need to
be developed to characterize the structural properties of
specific tissues constituting functional units (e.g., MTB) and
to track how these properties change with maturation and in
response to exercise. We need to characterize how tissues
respond to various loading parameters (e.g., the load magnitude, duration, and frequency) as well as the repair rates
for these tissues. This will require integrating information
from both clinical observations and basic research. Tissue
properties and the rates at which they adapt to altered
loading conditions can be quantified using animal models
and perhaps human studies. Human studies may involve
noninvasive imaging technology combined with specialized
joint strength testing equipment (3). Musculoskeletal modeling and computer simulation techniques are approaching
the sophistication needed to quantify the forces induced in
individual tissues during prescribed movements. Injury
tracking systems need to become more comprehensive and
include information that may be used to test injury prediction models such as the loading history before an injury.
A simplistic injury prediction model is given in equation
1 and illustrated in Figure 3. This model is derived from
basic fatigue fracture studies of bone, expanded to characterize other tissues. This model considers the number of
loading cycles (N) and the strain (⑀) induced in the tissue
during each loading cycle. The coefficients q and K would
be different for each tissue and likely depend on age. The
model does not consider repair rates or the time between
exercise bouts. Repair could be expressed as a function of
time and damage accumulation could be modeled if the
BIOMECHANICAL PERSPECTIVES OF SPORTS INJURIES
(1)
where DF is a damage factor ranging from 0 to 1 (1 representing complete or 100% damage), N is the number of
loading cycles during a bout of exercise, K is constant
(approximately 6 ⫻ 1019 for cortical bone), q is constant
(5 ⬍ q ⬍ 15), and ⑀ is strain in microstrain.
Experimental studies need to be performed to establish
the coefficients in equation 1, but clinical information is
equally important for establishing the starting point for
experimental studies directed in this area. Clinical observations that identify different modes of injury for different
sporting activities or for different ages may provide the
scientists with clues about how different tissues respond to
different loading stimuli. For example, if children that perform relatively slow loading rate resistance training experience a high incidence of tendonitis injuries and children
involved with kicking or throwing sports (faster loading
rates) experience more apophysitis, then this might suggest
that apophyses are more influenced by loading rate than are
tendons. If the type of injuries experienced by long distance
runners (e.g., stress fractures) and long distance swimmers
(e.g., rotator cuff) are different, then this may provide clues
about the interactions between loading magnitude, the number of loading cycles, and tissue injury. This information can
help define tissue-specific coefficients in injury prediction
models.
SUMMARY
Epidemiology data suggest that 9 million children will
sustain preventable injuries this year. The annual cost to
treat these injuries exceeds $1.8 billion. The long-term
health and fitness ramifications of these injuries are not
known, but reducing the incidence of these injuries is certainly desirable. A variety of biomechanical concepts have
been presented to illustrate how biomechanics can be used
to identify factors that contribute to overuse injuries in
children and to identify information that is needed to develop reliable injury prevention guidelines for our youth.
Limb length, mass, and moment of inertia increase during
growth. These quantities interact but not necessarily in a
linear manner. They affect limb dynamics and the muscle
forces required to perform a movement task. Muscle
strength increases during growth, but it may not increase in
proportion to limb inertial properties. Joint flexibility may
change during growth. This may indicate changes in tissue
preloads. Reduced flexibility appears to be gender specific,
occurring more in boys between 8 and 13 yr of age than in
girls. Tissue loading depends on the dynamics of the movement; greater linear or angular accelerations lead to greater
forces. Each child grows at his or her own rate, and individual responses should be considered when assessing a
child’s risk for injury. Gross limb anthropometry, muscle
strength, and joint flexibility are factors that can contribute
to injury, but tissue mechanics also play an important role.
Medicine & Science in Sports & Exercise姞
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Tissue structural and material properties change with
maturation and training. Tissues become stronger but not
necessarily at the same rate. During early stages of training,
tendon and apophysis properties may lag behind muscle
strength gains, causing asynchronous rates of strength gain.
Tissue can accumulate damage during cyclic activities, but
we do not know how easily different tissues are damaged by
cyclic loading. Clinical evidence suggests that insertion sites
are more easily damaged by cyclic loading compared with
ligaments and tendons.
There is considerable information that must be acquired
and modeling approaches developed to formulate predictive
models of injury mechanisms. This will involve collaborations between basic scientists, clinicians, athletes, and sports
organizations. Ideally, extensive studies of the structural and
material properties of functional units such as BLB and
MTB will be performed to characterize the asynchronous
rates of strength gain within these tissues as a function of
age and altered activity level. Studies will be performed to
characterize the injury response of tendons, bones, and
ligaments to various loading stimuli (e.g., load magnitude,
loading rate, number of loading cycles). Studies will characterize the damage repair rate of these tissues. These data
will then be used to define the coefficients in tissue-specific
injury prediction models. Biomechanical analysis techniques such as inverse dynamics and forward dynamics will
be combined with appropriate musculoskeletal models to
quantify the stresses and strains induced in specific tissues
during various movement tasks. This approach will result in
a powerful tool for identifying growth and movement related injury risk factors, and for prescribing safe movements
and physical fitness training strategies for our youth.
The material presented here represents a summary of material
presented at the 19th Annual Meeting of the Southwest Chapter of
the American College of Sports Medicine, November 12–13, 1999,
San Jose, CA.
Address for correspondence: Dr. David Hawkins, Exercise Biology Program, Room 275 Hickey Gym, University of California, Davis,
Davis, CA 95616; E-mail: [email protected].
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