Cynthia A. Bir, MS, RN, Steven J. Cassatta, MSE, David H. Janda,
MD
From The Institute for Preventative Sports Medicine, Ann Arbor,
Michigan
See the Abstract
World wide soccer is the most popular team sport with an estimated 40 million amateur
participants [3]. In addition, soccer is the fastest growing
team sport in the United States [6]. The increase in the number
of injuries parallels an increase in the popularity of the sport. The Consumer Product
Safety Commission has determined through the National Electronic Injury Surveillance
System (NEISS) 647,368 injuries have occurred from 1989 to 1992 subsequent to soccer [7]. The NEISS is a system in place since 1972 that is based on
accruing information through hospital emergency rooms. Therefore, non-hospitalization
physician visits are not included in this injury figure. The distribution of the injuries
as determined by the Consumer Product Safety Commission reveals 71% of the injuries were
sustained by males and 29% females. Ankle injuries comprised 19.2% of total injuries; knee
injuries 12.7%; head and facial injuries 11.3%; lower leg injuries 7.8%, and foot injuries
7.5%.
Pandolf and Holloszy [5] described sports injuries to be due
to one of the following: collision, contact and noncontact occurrences. During the play of
soccer, collisions are not as common as they are in a sport such as American football.
However, contact is common and can be the etiology of serious injuries due to the higher
speed at which soccer is played [1]. The contact can be player
versus player, player versus ball, player versus goal post or player versus ground. A
player versus player contact commonly occurs when players are trying to kick the ball and
instead impact each other. This scenario can result in soft tissue injuries as well as
fractures to the upper or more commonly to the lower extremities. A study by McCarroll et
al [4] revealed, when reviewing the types of injuries occurring
in a youth soccer league by location, the area of the shin was the third most common area
injured (13.1%). Of the 17 fractures that occurred, four were to the tibia.
The protective ability of shin guards has only been studied to a limited extent [8]. It was the purpose of this study to investigate the
usefulness of an experimental model, a Hybrid III crash dummy, to evaluate the force
attenuating capabilities of various types of protective equipment currently utilized to
prevent injuries to the lower extremity during the play of soccer. An "unprotected
leg" served as the control for this study.
METHODS
Pendulum impact apparatus
Each shin guard in this study was tested on a pendulum impact apparatus (figure 1). The basic setup of this apparatus is a free swinging
pendulum approximately 34 inches in length from the pivot point and 3 inches wide. A
weighted mass was placed at the free swinging end of the pendulum to increase the impact
force to approximately 2300 N. A steel piece of piping 1.5 inches in diameter was also
mounted transversely across this end of the pendulum to represent the "foot".
Although there is little biofedility between the steel pipe and a human foot, the steel
pipe allowed for consistent impact loads to be applied to the shin guards.
By changing the height of the pre-swing and the weight at the end of the pendulum, the
amount of impact force was controlled. Therefore, a consistent impact force was achieved
throughout the entire study by maintaining the same height of pre-swing and weight of the
pendulum. This test setup represents one of the possible configurations of lower extremity
impacts that occur during the play of soccer. The basic kinematics of one person kicking
and the other person with their leg planted is represented by this apparatus.
A 5th-percentile female Hybrid III instrumented leg was utilized to measure the amount
of force transferred to the lower leg with each impact. The 5th-percentile female Hybrid
III anthropometrically represents a 10-year-old boy and provides the most sensitive means
of measuring injury in the adult population. The shin guards tested were fitted to leg
appropriately.
Load cells located at the ankle and knee of the Hybrid III leg allowed for the total
load transferred to the tibia to be calculated. The relationship of the impact and the
resulting force loads can be illustrated by the force diagram depicted in figure 2. The vector equation for this relationship is as follows:
sum(Fx) = 0 or Ftotal - Fknee - Fankle = 0 The load cell at the knee measured the moment
experienced at the knee. This moment about the axis is product of the force times a known
distance. Therefore, the moment data collected was divided by this distance to quantify
the force at the knee. The load cell at the ankle measured the force at the ankle
directly, therefore no calculation was required.
The time of the peak force was determined and the force values for both the ankle and
knee at this time were calculated. These two values were then summed to determine Ftotal:
Ftotal = Fknee + Fankle This force was the total load experienced by the tibia. The leg
was placed so that the impact would occur at the midpoint of the shin when the pendulum
was perpendicular to the horizontal plane. The impact point did not alter the total force
measurement.
Figure 1. Pendulum impact apparatus utilized to test shin guards.
Figure 2. Vector diagram representing the forces seen by the tibia during impact.
Test setup
A control was established by performing five impacts without a shin guard on the Hybrid
III leg. The target range for these impacts was approximately 2300 N. This target range
was determined by maximum impacts performed on the leg by a healthy 26 year old male who
was physically active in the area of athletics. These impacts were performed in the same
manner as those delivered by the pendulum impact apparatus.
After the control was established the height of the pre-swing and weight of the
pendulum remained unchanged throughout the entire study. The control of no pad was also
performed at the end of all testing to insure the accuracy of all tests. During the
impacts, the lower portion of the leg was allowed to move freely with the knee as the
pivot point. After each impact, the leg was returned to its original position. Markings on
the leg and testing apparatus insured that the joints were at the same angle and the leg
in the same position for each test.
Shin guards tested
Twenty-two different commercially available shin guards, priced from $20-$30, were tested
with the pendulum impact apparatus (figure 3). Each shin guard was
impacted five times at three different temperatures: 0°C, 20°C, and 38°C. These
temperatures represent the average and extreme temperatures that might be encountered
during the play of soccer. Each shin guard was placed in an environmental chamber at the
desired temperature for at least three hours prior to testing. The guard was then tested
within a five minutes time period from the time it was removed from the chamber. All of
the guards were tested at each temperature in a random fashion.
Table 1 shows all of the shin guards tested, the manufacturers
and weight. A sock was placed over each shin guard tested as if a player was wearing the
guard. All straps or retention devices were attached to the Hybrid III leg in the manner
in which a player would attach the guard. No further means were used to hold the guard in
place.
Biomechanical responses
Leg impact responses were measured. A Texas Micro Systems 486 computer with a RC
Electronics A-to-D conversion board and software served as the data acquisition system
utilized to record these responses. The sampling rate was 5000 Hz. A total of two channels
of load data was collected. The total load transferred to the leg through the shin guard
was calculated by determining the time of the peak load and adding the loads measured by
two separate load cells; one located near the knee and one located near the ankle.
Statistical methods
Differences in the force attenuating capabilities among the shin guards was statistically
evaluated using an analysis of variance (ANOVA). Further analysis involved a Tukey's
multiple comparison when the ANOVA was significant (p<0.05).
Figure 3. A total of twenty-two different shin guards were tested in this study.
TABLE 1
Shin guards tested and their corresponding weight
Shin Guard Manufacturer Weight (oz)
Body Pro Albion 2.0
Shin Pal Ohio Cellular 2.0
The Protector Ohio Cellular 2.5
Kevlar Adidas 3.0
Rick Patrick Patrick 3.75
Mitre Mitre 4.0
Brine Brine 4.5
Prostyle Brine 3.0
Dunlop Dunlop 4.5
Pro-Pad Quassar 3.0
Real Seville Real 4.5
Trisafe Uhlsport 3.25
Air Shield Uhlsport 4.75
Pro Uhlsport 3.5
Italia Air Lotto 4.75
Flex Guard Lotto 2.5
Italia II Lotto 5.25
Italia III Lotto 3.25
Air Silicone Lotto 5.0
Umbro Umbro 3.75
Diadora Diadora 3.75
Sondico Seton 6.5
RESULTS
Leg impact responses
Table 2 shows the peak impact force for each shin guard at the
three different temperatures. The control of no shin guard demonstrated an average impact
force of 2320.6 N for the five trials performed. Table 3 reveals
that all shin guards demonstrated at least a 40% reduction in the peak impact force over
the control of no shin guard. The greatest reduction (77.1%) was found with the Air Lotto
Italia shin guard at 0°C which had a peak impact force of 531.33 N. This shin guard also
demonstrated the best reduction of force with all factors considered; 74.4% (figure 4). However, there was no statistically significant difference
(p>0.05) among the Air Lotto Italia and the following shin guards (percent reduction of
impact force): Brine (72.8%), Pro-Pad (71.2%), Uhlsport - Air Shield (70.6%), Lotto Air
Silicone (70.2%) and the Lotto Italia II (70.1%). The Uhlsport Trisafe demonstrated the
least amount of force reduction with only a 41.2% reduction. This was still a significant
reduction in the impact force when compared to the control of no shin guard.
There was a significant increase in the amount of force transferred through the shin
guard when the guards were heated to 38°C in comparison to the lower temperatures (table 3). The average force transferred for all shin guards increased
146.05 N over the room temperature measurements and 167.33 N over the cold temperature.
Even at the higher temperatures, the shin guards were found to significantly decrease the
force transferred to the tibia when compared to impacts without a shin guard.
TABLE 2
Total percent reduction of force from control
for all shin guards at varying temperatures
Cold Room Hot
(0°C) (20°C) (38°C)
Shin Guard (%) (%) (%)
Body Pro 59.1 58.6 57.2
Shin Pal 66.3 68.3 64.1
The Protector 68.3 66.0 64.2
Adidas-Kevlar 57.9 5l.6 48.4
Rick Patrick 49.0 50.2 47.2
Mitre 56.0 61.6 53.8
Brine 76.0 75.7 66.4
Brine Prostyle 49.4 45.5 46.5
Dunlop 49.3 47.7 45.3
Pro-Pad 73.8 71.5 68.0
Real Seville 69.8 63.4 56.1
Uhlsport-Trisafe 41.8 41.5 39.3
Uhlsport-Air Shield 72.8 71.2 67.5
Uhlsport-Pro 64.6 62.3 56.6
Air Lotto Italia 77.1 75.5 70.2
Flex Guard 43.8 46.3 40.7
Lotto Italia II 72.3 73.0 64.7
Lotto Italia III 5l.2 55.0 45.7
Lotto Air Silicone 72.9 66.5 70.4
Umbro 68.5 72.2 64.1
Diadora 59.2 57.5 51.3
Sondico 71.4 68.9 63.1
TABLE 3
Total peak impact forces for all shin guards
at varying temperatures
Cold Room Hot
(0°C) (20°C) (38°C)
Shin Guard (n) (n) (n)
Body Pro 949.63 962.30 1035.46
Shin Pal 784.49 735.75 869.57
The Protector 737.23 790.59 865.40
Adidas-Kevlar 979.60 1124.66 1249.25
Rick Patrick 1185.77 1157.27 1276.62
Mitre 1021.89 893.18 1116.91
Brine 557.04 565.06 858.59
Brine Prostyle 1176.63 1266.05 1294.41
Dunlop 1179.27 1215.65 1323.97
Pro-Pad 609.32 662.82 774.23
Real Seville 702.91 850.12 1062.43
Uhlsport-Trisafe 1353.81 1359.50 1469.80
Uhlsport-Air Shield 632.22 669.00 785.48
Uhlsport-Pro 822.17 876.11 1050.07
Air Lotto Italia 531.33 569.90 721.24
Flex Guard 1306.32 1249.25 1435.57
Lotto Italia II 645.08 627.29 853.04
Lotto Italia III 1135.23 1046.84 1314.66
Lotto Air Silicone 629.13 777.77 717.02
Umbro 732.61 645.97 868.18
Diadora 947.88 986.81 1179.05
Sondico 665.81 722.89 891.92
Figure 4. Impact forces transferred through shin guards utilizing pendulum impact
device.
DISCUSSION
This study reinforces the utilization of shin guards to attenuate the forces seen by
lower extremities during the play of soccer. An analysis of our data reveals a
statistically significant reduction of force (40-77.1%) transferred to the lower leg when
the protective guards were tested. A reduction in force of impact could prevent injuries
that would normally occur without the use of a shin guard. Even at varying temperatures
the guards were effective in dissipating the impact forces (Table 2).
The results of this study seem intuitive since the shin guards are made of soft
compliant materials. However, in previous studies it was determined that soft compliant
materials do not always attenuate the forces they encounter [2].
Therefore, the conclusion that soft compliant materials always reduce the amount of impact
force is not indisputable.
In order for the shin guards to prevent lower extremity injuries, they must be utilized
by the players on the fields. Comfort is one factor in whether a player will utilize the
guard and weight of the guard is an important component of comfort of wear. The lighter
the guard the easier it will be for the player to run, kick and compete in the play of the
game. The weight of the guards tested varied from 2.0 to 6.5 ounces per guard (Table 1). The Shin Pal had the highest reduction of force (64.1-68.3%)
in relationship to the lightest weight (2 oz).
Based on this study, the utilization of shin guards would help attenuate the amount of
impact force seen by the tibia and thus may prevent injuries. Like any other type of
protective gear, the guards must be applied properly and worn during the play of the game
in order to reduce the risk of injuries.
ACKNOWLEDGMENTS
The authors wish to express their appreciation to the University of Michigan Men's
Soccer Club for funding this project as well as Mr. Mike Malley for advising us on general
soccer issues. In addition, the authors would like to thank Mr. Don Nelson for his help
and guidance in procuring the shin guards tested. The authors would also like to express
their appreciation to General Motors Biomedical Science Department for the donation of the
Hybrid III leg and instrumentation for testing purposes. Thanks must also go to Mr. Scott
Lukas for his photography skills utilized.
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This article was published as:
"An Analysis and Comparison of Soccer Shin Guards"
Clinical Journal of Sports Medicine
Vol. 5, No. 2, 1995; pp. 95-99
Bir CA, Cassatta SJ, Janda DH
It is possible to order a copy of this article.
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