Preseason strength and flexibility imbalances associated with athletic in female collegiate athletes*

injuries

JOSEPH J. KNAPIK,†‡ MS, ScD, CONNIE L. BAUMAN,§ ATC, MS, BRUCE H. JONES,† MPH, MD, JOHN McA. HARRIS,† MD, AND LINDA

From the

VAUGHAN,§

PhD

tU.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts, and §Wellesley College, Wellesley, Massachusetts

ABSTRACT

Both

military and sports professionals have a long-standing interest in identifying risk factors for injuries associated with intense physical activity and training.&dquo;,&dquo; Injuries to females are of particular concern because of increasing female participation in both military training and civilian athletic programs. Training injuries to women in basic military training are more than twice as frequent as for men.l3

One hundred thirty-eight female collegiate athletes, parin eight weightbearing varsity sports, were administered preseason strength and flexibility tests and followed for injuries during their sports seasons. Strength was measured as the maximal isokinetic torque of the right and left knee flexors and knee extensors at 30 and 180 deg/sec. Flexibility was measured as the active range of motion of several lower body joints. An athletic trainer evaluated and recorded injuries occurring to the athletes in practice or competition. Forty percent of the women suffered one or more injuries. Athletes experienced more lower extremity injuries if they had: 1) a right knee flexor 15% stronger than the left knee flexor at 180 deg/sec; 2) a right hip extensor 15% more flexible than the left hip extensor; 3) a knee flexor/knee extensor ratio of less than 0.75 at 180 deg/sec. There was a trend for higher injury rates to be associated with knee flexor or hip extensor imbalances of 15% or more on either side of the body. These data demonstrate that specific strength and flexibility imbalances are associated with lower extremity injuries in female collegiate athletes.

ticipating

However, the incidence of sports-related injuries are about the same for males and females.7, 20 It has long been assumed that specific strength and flexibility imbalances may be associated with athletic injuries.12 These imbalances may manifest as differences between the right and left leg or as abnormal ratios between antagonistic muscle groups. Previous attempts to find a relationship between muscular imbalances and injuries1, 5,14 have met with limited success,’ probably because small sample sizes and limited numbers of injuries were reported. We performed an extensive preseason screening for strength and flexibility in a group of female collegiate athletes. We then followed them during their athletic seasons, recording all sports-related injuries that occurred. Our purpose was to examine the relationship of imbalances in leg strength and flexibility to likelihood of lower extremity injuries. MATERIALS AND METHODS This study was approved by the Human Use Review Committees of the U.S. Army Research Institute of Environmental Medicine and Wellesley College. Prior to participation in the study, athletes were briefed in large groups regarding the purposes and risks of the study, and those who chose to volunteer gave their written informed consent. This study comprised 138 female student/athletes attending Wellesley College, a 4 year, Division III, liberal arts institution. They were involved in eight weightbearing, in-

*

Portions of this paper were presented at the 36th annual meeting of the Amencan College of Sports Medicine, Baltimore, Maryland, June 2, 1989. The views, opinions, and findngs contamed m this article are those of the authors and should not be construed as an official Department of the Army position, pohcy, or decision unless so designated by other official documentation Human subjects participated m these studies after gmng their free and informed voluntary consent. Investigators adhered to AR 70-25 and USAMRDC Regulation 70-25 on Use of Volunteers in Research z Address correspondence and reprnt requests to Joseph Knapik, CAPT, USA, U S Army Research Institute of Enmronmental Medicine, Natick, MA 01760-5007

76

77

tercollegiate varsity sports including soccer, volleyball, field hockey, tennis, fencing, basketball, squash, and lacrosse. Twenty athletes (15%) participated in two varsity sports but none participated in three or more. Physical characteristics of the subjects were obtained at entry into the study. The mean age (±SD) of the subjects was 18.9 (±1.2) years-range, 16 to 21; mean height was 166.8 cm (±5.7 cm)-range, 152.0 to 178.5 cm; mean weight was 60.6 kg (±6.6 kg)-range, 45.6 to 78.2 kg; and body fat was 23.8% (±4.0)-range, 14.5% to 33.3%. Body fat was obtained from skinfold measurements using the equations of Durnin and

Womersley.’ The study was conducted over a 3 year period. The numbers of athletes tested in the 1st, 2nd, and 3rd years were 78, 30, and 30, respectively. While some athletes were tested in multiple years (N 23), only the athletes’ 1st year of testing was considered for our analysis. There were three testing periods each year: fall (soccer, volleyball, field hockey, tennis), winter (basketball, fencing, squash), and spring (lacrosse). In the 1st year, subjects were tested individually during the day by appointment; in subsequent years, testing was performed in large groups, usually in the evening. =

Testing of a subject was completed in a single session: height, weight, and skinfold measurements were obtained first, followed by flexibility and muscle strength measurements. Flexibility measurements were obtained by methods previously described.4With subjects on a training table, a technician used a goniometer to measure the active range of motion of several lower body joints on both the right and left leg. These joints involved the hip flexors (hamstrings), hip extensors, hip adductors, hip external rotators, knee flexors (quadriceps), and ankle dorsiflexors (gastrocnemius and soleus). Two measurements were obtained on each joint and these two were averaged. Hip extension flexibility was measured with the subject in the prone position, knees flexed at 90°. The goniometer was centered on the greater trochanter with one arm on the lateral midline of the thigh and the other arm parallel to the table. The subject extended the hip while keeping her anterior superior iliac spine on the table. Strength measurements were obtained on the knee extensors (KE) and knee flexors (KF) using a Cybex II dynamometer (Cybex Co., Ronkonkoma, NY) as previously described.19 The subject was aligned and stabilized in the chair by an experienced technician. In a systematically counterbalanced order the subject performed maximal voluntary isokinetic contractions of the right and left KE and KF at both 30 and 180 deg/sec. Only one side and one muscle tested at a time. For each muscle group and each the velocity subject performed two to four submaximal warm-up contractions followed by three to five maximal voluntary contractions. A rest of at least 30 seconds was allowed between contractions. Peak torque was recorded and the three highest values that were within ±10% of each other were averaged for data analysis. An injury was defined as any traumatic event or overuse impairment that occurred during practice or competition for which the athlete or her coach referred the athlete to the group

was

athletic trainer or outside medical care and for which there time lost from practice or competition. The athletic trainer evaluated the injury, then categorized and coded it using the National Athletic Injury/Illness Reporting System (NAIRS).’ She recorded additional information, including which side of the body was injured, and how many days of training or competition were lost. was some

RESULTS At the end of the 3 year

sample) had suffered

period,

one or more

subjects (40% of the injuries. The types and

55

numbers of these are shown in Table 1. There were 19 of a second injury; 5 of these were repeat injuries ankle [2 sprains, 2 muscle strains (adductor and quadriceps), and 1 bicipital tendinitis]. Lower extremity injuries accounted for 80% of all injuries. Muscle strains accounted for 29% of the total injuries, while knee and ankle sprains accounted for 27%. Only first-degree and second-degree strains and sprains were encountered. All lower body muscle strains were first-degree strains. Eight of the nine ankle sprains were first-degree sprains, but five of the six knee occurrences

sprains were second-degree sprains. With respect to time loss, 17 injuries (31% of the total) resulted in the athlete being unable to complete the training session; 33 injuries (60% of the total) resulted in a time loss of 1 or more days (mean ± SD, 5.6 ± 6.2 days; median, 4 days; range, 1 to 30 days); 5 injuries were severe enough to keep the athlete out for the remainder of the season. Ten of the injured athletes (18%) participated in two separate sports. Table 2 shows the descriptive statistics obtained for the flexibility and strength measurements. Ratios between the right and left leg were calculated for each measure. KF/KE ratios were also calculated after the right and left legs were averaged; these values (mean ± SD) were 0.62 ± 0.09 and 0.79 ± 0.13 at 30 and 180 deg/sec, respectively. For each ratio, subjects were separated into quartiles. A 2 x 4 chi square analysis was performed, comparing the number of subjects injured versus those not injured in each quartile. Only the first incidence of a lower extremity injury was considered in this analysis. Statistically significant differences were found for right/left hip extension flexibility (chi square, 11.2; P < 0.001), right/left KF strength at 180 deg/sec (chi square, 7.9; P 0.04), and the KF/KE ratio at 180 deg/sec (chi square, 9.0; P 0.03). The three ratios above were separated into more clinically meaningful groups. For the right/left KF ratio (180 deg/sec) subjects were separated into five groups with ratios ranging from less than 0.85 to greater than 1.15 as shown in Figure 1. Note that the right side of the figure indicates a stronger right leg (values greater than 1.0), while the left side indicates a stronger left leg (values less than 1.0). Examination of Figure 1 suggests that more injuries occurred if the right or left leg was stronger than the opposite leg by 15% or more. Partitioned chi square analysis was used to compare =

=

the three middle groups to the two extreme groups. More

78

TABLE 1 First incidence of injury in

°

Lac, lacrosse; VB, volleyball; BB, basketball; Soc,

soccer;

a

3 year

period

FH, field hockey; Ten, tennis; Squ, squash; Fen, fencing.

TABLE 2

Flexibility and strength

measures

for the

right and left leg

injuries occurred when the right leg was stronger than the left leg by 15% or more (chi square, 9.5; P 0.005); however, if the left leg strength exceeded the right leg by 15% or more, the injury rate was not increased (chi square, 1.15; P 0.28). Table 3 shows the 16 injuries that occurred in the former group of subjects in terms of the right and left leg. For the right/left hip extensor ratio, subjects were also separated into five groups as shown in Figure 2. As with the right/left KF ratio, examination of Figure 2 suggests that more injuries occurred if either the right or left leg had a range of motion that exceeded the opposite leg by 15% or more. Partitioned chi square tests were performed to com=

Figure 1. Percent of subjects injured in various groups of the right/left KF ratio (180 deg/sec). Numbers on top of vertical bars are the number of subjects in each group. The risk ratio comparing values >1.15 to values 0.85 to 1.15 is 2.61.

=

pare these imbalances with the middle three groups. More injuries occurred if the right leg exceeded the left leg by 15% or more (chi square, 10.71; P < 0.001). However, if the left leg exceeded the right leg by 15% or more, the injury rate was not increased (chi square, 1.41; P 0.23). Table 3 shows the 17 injuries that occurred in the former group of subjects. For the KF/KE ratio (180 deg/sec), subjects were sepa-

TABLE 3 Number of injuries (by side of the body) for subjects with KF right/left leg ratios and hip extensor right/left leg ratios >1.15

=

a Quad, quadriceps; Ham, hamstrings; Add, adductors.

79

rated into the five groups shown in Figure 3. More injuries occurred if the KF torque was 75% or less of the KE torque

(chi square, 3.9; P < 0.05). Logistic regression was performed using injuries

versus

injuries as the dependent variable. The right/left KF and hip extensor ratios were converted to categorical variables: either greater or less than 1.15. KF/KE ratios were also converted to categorical variables for values greater than and less than 0.75. Bilateral KF and hip extension ratios were independent predictors of injury with a specificity of 93% and sensitivity of 32%. When hip extensor flexibility imbalances, or right/left KF (180 deg/sec) imbalances on both sides of the body (three categories), were placed in the regression equation, they did not improve the goodness of fit of the model or change the specificity or sensitivity. no

DISCUSSION The present study demonstrated that specific strength and flexibility imbalances were associated with the first incidence of a lower extremity injury in female collegiate athletes. Strength imbalances measured at 180 deg/sec were related to injuries; however, we found no significant relationships at 30 deg/sec, in agreement with Agre and Baxter.’ The higher velocity of 180 deg/sec may be closer to those experienced during athletic events. It is known that in normal gait the leg extends at a rate of about 230 deg/sec25 and is capable of a maximal velocity of about 700 deg/sec.23 For the KF (180 deg/sec), athletes with a right leg stronger than the left leg by 15% or more were 2.6 times more likely to get injured than athletes with imbalances less than 15%. A partial explanation for why the stronger right KF was associated with injuries was offered by examining the distribution of injuries on both sides of the body (Table 3). Strain and sprain types of injuries were much more likely to occur on the weaker left side. A strong force generated by the right leg may have resulted in damage to the left leg if the weaker

Figure 2. Percent of subjects injured in various groups of the right/left hip extension ratio. Numbers on top of vertical bars are number of subjects in each group. The risk ratio comparing values >1.15 to values 0.85 to 1.15 is 2.63.

KF muscle group was operating at a high contractile velocity and was unable to absorb or properly transfer that force. Athletes may tend to favor their right leg at the expense of their left since approximately 90% of all subjects tend to be

right-leg dominant.25 Athletes with the stronger left KF were 1.7 times more to be injured than subjects without this imbalance. This trend was not statistically significant and including this imbalance in the logistic regression did not improve the predictive ability of the model. However, the data did suggest that imbalances in KF strength on either side of the body, but especially on the right, impart a greater risk of injury. Laird&dquo; found no relationship between soccer injuries and right/left KF ratios of 10% or greater. However, he had only 23 subjects and few injuries; there were also major methodologic differences between our study and his in terms of the testing procedure. Burkett5 reported that hamstring strains were more likely to occur if the isometric strength of the right and left KF differed by 10% or more. In the present study only one hamstring strain occurred and this individual’s bilateral KF ratio did not exceed 3%. Athletes whose hip extensors had a 15% greater range of motion on the right side were 2.6 times more likely to get injured than athletes with less than a 15% imbalance. It seemed reasonable to expect that a hip extensor flexibility imbalance on either side of the body may predispose the athlete to injury. Injuries occurred with equal frequency on both the right and left side of the body (Table 3). There was a trend to have more injuries with an imbalance on either side but this was statistically significant only on the right side. Subjects with a greater range of motion on the left side were 1.7 times more likely to experience injuries when compared to subjects with a more balanced range of motion. It should be noted that only about one-half as many subjects had greater than 15% more flexibility in their left legs compared to those with greater than 15% more flexibility in their right legs. Including more subjects with hip flexibility imbalances could strengthen the relationship.

likely

Figure 3. Percent of subjects injured in various groups of the KF/KE ratio (180 deg/sec). Numbers on top of vertical bars represent the number of subjects in each group. The risk ratio for values

Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes.

One hundred thirty-eight female collegiate athletes, participating in eight weightbearing varsity sports, were administered preseason strength and fle...
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