Preseason Eccentric Strength is Not Associated with Hamstring Strain Injury: A Prospective Study in Collegiate Athletes

Abstract

Introduction:

Established risk factors for hamstring strain injuries (HSI) include older age and prior HSI. However, these are non-modifiable and have a limited role in injury prevention. Eccentric hamstring strength is a common component of HSI prevention programs but its association with injury is less clear.

Purpose:

To determine if eccentric hamstring strength was prospectively associated with HSI among collegiate athletes, while controlling for sex, age, and prior HSI. We hypothesized that athletes with lower eccentric hamstring strength or greater between-limb strength asymmetry at preseason would have an increased risk of HSI.

Methods:

Hamstring eccentric strength measures, maximum total force (F_Total) and between-limb asymmetry in maximum force (F_Asym), were measured at preseason on male and female athletes. HSIs were tracked over the subsequent 12-months. Generalized estimating equations were used to identify univariable and multivariable associations between athlete demographics, eccentric hamstring strength, and HSI risk.

Results:

Data for 326 athletes (85 female; 30 track, 43 basketball, 160 American football, 93 soccer) were included and 64 HSIs were observed. Univariable associations between eccentric hamstring strength and subsequent HSI were non-significant (F_Total OR=0.99 [95% CI 0.93, 1.05], p=0.74; F_Asym OR=1.35 [95% CI 0.87, 2.09], p=0.23). No relationship between eccentric hamstring strength and HSI (F_Asym OR=1.32 [95% CI: 0.84, 2.08], p=0.23) was identified after adjusting for confounders including sex, age, and prior HSI.

Conclusions:

No association between preseason eccentric hamstring strength and risk of subsequent HSI was identified after controlling for known risk factors and sex among collegiate athletes. Eccentric hamstring strengthening may continue to serve as a preventative approach to HSI, but it does not provide additional insight into HSI risk beyond factors such as age and prior HSI.

INTRODUCTION

Hamstring strain injuries (HSI) are among the most common injuries in sports involving high speed running(1–3) and are complicated by a high recurrence rate.(1, 2, 4) HSIs result in significant limitations to individual performance and team success.(5, 6) The most established risk factors for HSI are older age and prior HSI,(7) which cannot be altered via injury prevention interventions. A common component of HSI prevention programs is eccentric hamstring strengthening, and although eccentric strength is modifiable, its value in identifying who may go on to sustain an HSI is less clear.

During high-speed running, the terminal swing phase is considered the most likely time of injury due to the hamstrings being active, rapidly lengthening, and absorbing energy to decelerate the advancing limb.(8–10) Thus, quantifying eccentric hamstring strength to assess an individual’s risk of HSI is common practice,(11, 12) with more strength or less between-limb asymmetry in eccentric hamstring strength theoretically reducing HSI risk. Although recent reviews suggest assessment of preseason eccentric hamstring strength provides limited information about the occurrence of a future HSI,(7, 13) findings reported thus far involve males only and are limited to sports such as rugby,(14) soccer,(12, 15) and Australian (11, 16) and Gaelic football.(17) Prospective studies assessing this association and inclusive of females and other sports in which HSI are common, such as track and American football, have not yet been investigated.

The purpose of this investigation was to determine if eccentric hamstring strength was prospectively associated with HSI among male and female collegiate athletes from a variety of sports, while controlling for known risk factors, such as age and prior HSI, as well as sex or sport. An exploratory secondary analysis was conducted to look at the interaction of sport and eccentric strength on subsequent HSI risk. We hypothesized that athletes with lower eccentric hamstring strength or greater between-limb strength asymmetry at preseason would be at greater risk of incurring an HSI during the following year. Furthermore, we hypothesized that this relationship would persist independent of sport.

METHODS

Study Design

This study used three years of routinely collected health and performance data from NCAA Division I athletes in the University of Wisconsin-Madison Badger Athletic Performance database. Injury occurrence was collected as part of a larger, prospective observational study monitoring acute HSIs in collegiate athletes. The records review and prospective injury monitoring were approved by the University’s Health Sciences Institutional Review Board. Athletes were not involved in the design or scientific conduct of this study; however, athletes and coaches received results of individual testing and were regularly informed of important scientific findings from the database.

Hamstring eccentric strength was assessed on male and female athletes participating in American football (male only), soccer, basketball, and track at the start of each season over a three-year period (July 2017-March 2020). Athletes who were not cleared for full athletic participation at the time of preseason testing did not participate in routine performance assessments and thus were not included in the Badger Athletic Performance database. Age, sex, height, total body mass, prior HSI, and prior lower extremity surgery were also recorded at annual preseason testing. Self-reported prior HSIs and surgeries were confirmed via medical records review as available. Data collected from performance assessments done outside of routine preseason testing were excluded.

HSIs that occurred during the 12-month period following preseason assessments were tracked and diagnosed by certified athletic trainers working with each team. An HSI was diagnosed by a member of the respective teams’ sports medicine staff and defined as sudden onset of posterior thigh pain that occurred during a practice or competition resulting in the athlete not being able to return to sport for at least one practice or competition, and the presence of two or more of the following during physical examination: palpable pain along any of the hamstring muscles, posterior thigh pain without radicular symptoms during a passive straight leg raise, weakness with resisted knee flexion, pain with resisted knee flexion, and/or posterior thigh pain.(18) The first HSI an athlete sustained after a given preseason test, within the 12-month follow up period, was considered for analysis.

Data acquisition

Eccentric hamstring strength was measured using the NordBord Hamstring Testing System (Vald Performance, Newstead QLD, Australia) in accordance with a previously described protocol.(19) The instrument’s reliability is moderate to high with an intraclass correlation coefficient of 0.83–0.90 and a coefficient of variation of 5.8–8.5%.(19) The athlete kneeled on a padded platform with each ankle secured in a hook immediately superior to the lateral malleoli. Each hook was in line with a load cell for force measurement. Athletes were instructed to slowly lower themselves, resisting with their hamstrings, with arms across the chest and with shoulders, knees, and hips kept in a straight line. Three warm-up trials were performed, one each at 50%, 75%, and 90% of maximal effort. Following warm-up trials, athletes completed three trials at maximal effort with bodyweight resistance only. Athletes were given a 30 second rest after the warm-up trials and each maximum effort trial. Verbal encouragement was given throughout the trials to incite maximal effort. Trials were accepted if the athlete maintained proper alignment during the trial, a distinct peak in maximal force output followed by a rapid decline was observed, and peaks were within 20% across the three trials. Up to two additional trials were performed if a distinct peak was not reached or the maximum force was more than 20% different from previous trials. Data from all trials were recorded, with the maximum force obtained for each limb out of all trials utilized for analysis.

Sample Size Estimation

An a priori estimate for the necessary sample size was calculated to detect an association between eccentric hamstring strength and HSI risk while controlling for age, prior HSI, and sex. Calculations were based on results of a similar study involving Australian football athletes.(11) We assumed 10% of athletes would experience an HSI each year and conservatively adjusted for 10% attrition due to situations such as non-HSI injury and graduation. Based on these assumptions, n=300 unique athletes would result in 91% power with alpha=0.05 to detect a 2-fold decreased risk of HSI for every N/kg increase in relative eccentric hamstring strength.

Statistical Analysis

Standard descriptive statistics were used to describe the study sample; means and standard deviations (SD) and frequencies and percentages were used to describe continuous and categorical variables, respectively. Measures of eccentric hamstring strength used in this study were maximum total force (F_Total), defined as the sum of the maximum force of each limb (F_RightMax + F_LeftMax) observed across the three trials, and between-limb asymmetry in maximum force (F_Asym), defined as the percent asymmetry relative to the stronger limb using the equation: $\frac{Strongerlimb-Weakerlimb}{Strongerlimb}.$This calculation resulted in all positive asymmetry values. All force values were normalized to body mass prior to analysis.

Univariable generalized estimating equation (GEE) models for binary HSI outcomes with a logit link were built to identify univariable associations between athlete demographics, eccentric hamstring strength, and HSI. Specifically, univariable associations between the following variables and HSI risk were assessed: age, BMI, sex, sport, prior HSI, prior lower extremity surgery, and both eccentric strength measures (F_Total, F_Asym).

A multivariable GEE model was built to determine the association between eccentric hamstring strength and subsequent HSI, while controlling for known risk factors for HSI. The eccentric strength measure included in the multivariable model was chosen as the measure most strongly associated with HSI risk determined by the univariable analyses. Potential confounders included in the multivariable model were sex, age, and prior HSI. Pairwise interactions between eccentric strength and these variables were assessed. Due to complete separation of sex and sport (e.g. no females playing football), both variables could not be simultaneously included in a single multivariable model. Thus, to explore the potential interaction of sport and eccentric strength on HSI, an additional multivariable GEE model was built with eccentric strength, sport, age, and prior HSI. The interaction between sport and eccentric strength was also considered. In both univariable and multivariable models, an exchangeable correlation structure was modeled to account for up to three years of repeated athlete observations. Odds ratios (OR) and 95% confidence intervals (CI) are reported. An OR greater than one demonstrated the increase in likelihood of an HSI for every unit increase in the independent variable, while an OR less than one indicated a decrease in the odds of sustaining an HSI. All statistical analyses were conducted using SAS v9.4 (SAS Institute, Cary, NC).

RESULTS

Data for 326 athletes (85 female, 26%) from four sports (30 track, 9%; 43 basketball, 13%; 160 American football, 49%; 93 soccer, 29%) were included, resulting in a total of 552 preseason strength tests across the three seasons (Figure 1). Of the 326 unique athletes, 153 had one preseason test included in the data set, 120 athletes had strength tests included from two seasons, and 53 athletes had strength tests from all three seasons included in the study. A total of 64 HSIs were observed during the study period, 52 (81%) in males and 12 (19%) in females. Forty-one (64%) of the HSIs occurred in football athletes, 8 (13%) in soccer athletes, and 15 (23%) in track athletes; no HSIs were observed among basketball athletes, thus all basketball athletes were necessarily removed from statistical models involving sport. Due to the pandemic imposed by the outbreak of coronavirus disease 2019, the follow-up period for the third year of data collected on track athletes (3.3% of the study sample) was limited to 3 months. Demographic information is provided in Table 1 (mean age 19.6 (SD=1.4) years; prior HSI n=30, 9%; prior surgery n=53, 16%). Mean (SD) eccentric hamstring strength values across the entire study sample for athletes’ first year in the study were 9.0 (4.1) N/kg for F_Total and 8.2 (6.3)% for F_Asym. Visual representation of maximum eccentric strength among athletes in the study sample by limb and sex with corresponding HSIs are shown in Figure 2. No pattern in injury distribution between left and right limbs or the stronger and weaker limb was observed.

Figure 1.

Flowchart demonstrating the process for records selection of preseason eccentric hamstring strength tests.

Table 1.

Athlete characteristics obtained at first observation.^*

	Overall (N=326)	Male (N=241)	Female (N=85)
Age	19.6 (1.4)	19.7 (1.4)	19.4 (1.2)
Sport
Track	30 (9%)	11 (5%)	19 (22%)
Basketball	43 (13%)	24 (10%)	19 (22%)
Football	160 (49%)	160 (66%)	0
Soccer	93 (29%)	46 (19%)	47 (55%)
Prior HSI	30 (9%)	25 (10%)	5 (6%)
Prior Lower Extremity Surgery	53 (16%)	31 (13%)	22 (26%)
Total Force (N/kg) ‡	9.0 (4.1)	8.8 (2.0)	9.4 (7.3)
Force Asymmetry (%), mean(SD)	8.2% (6.3%)	7.9% (6.1%)	9.1% (6.7%)

^*Values represent means (standard deviation) or frequency (% of sample).

^‡Normalized to body mass

Figure 2.

Maximum eccentric hamstring force observed from each limb and injury outcome for each pre-season test. Each athlete is represented up to three times, once for each year of participation in the study.

Univariable results

Neither of the strength measures (F_Total: OR = 0.99 [95% CI: 0.93, 1.05], p = 0.74; F_Asym: OR = 1.35 [95% CI: 0.87, 2.09], p = 0.23) showed a significant univariable association with HSI risk (Table 2). Significant univariable associations with HSI risk were found with prior HSI (OR = 8.41 [95% CI: 5.18, 13.66], p < 0.01), and sport (p < 0.01), with track athletes having greater odds (OR = 2.59 [95% CI: 1.28, 5.27]) and soccer athletes having lower odds (OR = 0.31 [95% CI: 0.14, 0.66]) of sustaining an HSI compared to football athletes. Younger age (OR = 0.80 [95% CI: 0.68, 1.01], p = 0.06), female sex (OR = 0.60 [95% CI: 0.32, 1.14], p = 0.12), BMI (OR = 1.03 [95% CI: 0.98, 1.08], p = 0.27) and prior surgery (OR = 0.80 [95% CI: 0.38, 1.70], p = 0.57) did not demonstrate a significant univariable association with subsequent HSI (Table 2).

Table 2.

Univariable generalized estimating equation models determining associations with hamstring strain injury (HSI).^*

	OR (95% CI)	p-value
Age	0.83 (0.68, 1.01)	0.06
BMI	1.03 (0.98, 1.08)	0.27
Female Sex	0.60 (0.32, 1.14)	0.12
Sport †		<0.001
Football	REF
Soccer	0.31 (0.14, 0.66)	0.008
Track	2.59 (1.28, 5.27)	0.003
Prior HSI	8.41 (5.18, 13.66)	<0.001
Prior Lower Extremity Surgery	0.80 (0.38, 1.70)	0.57
Total Force (N/kg) ‡	0.99 (0.93, 1.05)	0.74
Force Asymmetry (unit=10%) ^	1.35 (0.87, 2.09)	0.23

^*Univariable models were performed separately for each variable of interest to detect an association with HSI. Models accounted for repeated measures within athlete.

^†Basketball was removed from this model due to no HSI observed during study period

^‡Normalized to body mass

^{^}Unit represents the unit increase used for interpretation of odds ratio and 95% CI. Unit = 1 if not otherwise specified.

Multivariable results

The multivariable model used to determine the association between eccentric hamstring strength and HSI with sex, age, and prior HSI as confounding variables (Table 3) included F_Asym (OR = 1.32 [95% CI: 0.84, 2.08], p = 0.23), female sex (OR = 0.65 [95% CI: 0.36, 1.17], p = 0.15), age (OR = 0.80 [95% CI: 0.65, 0.98], p = 0.03), and prior HSI (OR = 9.12 [95% CI: 5.44, 15.29], p < 0.01). No significant pairwise interactions with F_Asym were detected. Although we did not identify a univariable association with HSI risk for either eccentric strength variable, between-limb asymmetry (F_Asym) was used in the multivariable model as it demonstrated a stronger potential association with HSI risk than did total strength (F_Limb).

Table 3.

Multivariable generalized estimating equation model to determine the association between eccentric strength (between limb maximum force asymmetry) and subsequent hamstring strain injury (HSI), while controlling for sex, age, and prior HSI.^*

	OR (95% CI)	p-value
Force Asymmetry (Unit=10%) ^	1.32 (0.84, 2.08)	0.23
Female Sex	0.65 (0.36, 1.17)	0.15
Age	0.80 (0.65, 0.98)	0.03
Prior HSI	9.12 (5.44, 15.29)	<0.001

^*Models accounted for repeated measures within athlete.

^{^}Unit represents the unit increase used for interpretation of odds ratio and 95% CI. Unit = 1 if not otherwise specified.

The multivariable model with sport, age, and prior HSI as confounding variables (Table 4) included F_Asym (OR = 1.41 [95% CI: 0.91, 2.19], p = 0.19), sport (p < 0.01; soccer OR = 0.43 [95% CI: 0.22, 0.85], p = 0.02; track OR = 1.37 [95% CI: 0.66, 1.01], p = 0.39), age (OR = 0.82 [95% CI: 0.66, 1.01], p = 0.07), and prior HSI (OR = 5.52 [95% CI: 3.06, 9.98], p = 0.03). No significant interaction was detected between F_Asym and sport (p=0.26).

Table 4.

Multivariable generalized estimating equation model to determine the association between eccentric strength (between limb maximum force asymmetry) and subsequent hamstring strain injury (HSI), while controlling for sport, age, and prior HSI.

	OR (95% CI)	p-value
Force Asymmetry (Unit=10%) ^	1.41 (0.91, 2.19)	0.19
Sport		0.005
Football	REF	REF
Soccer	0.43 (0.22, 0.85)	0.02
Track	1.37 (0.66, 2.85)	0.39
Age	0.82 (0.66, 1.01)	0.07
Prior HSI	5.52 (3.06, 9.98)	0.03

Note: Basketball athletes were removed from the model because no injuries were observed among basketball players.

^{^}Unit represents the unit increase used for interpretation of odds ratio and 95% CI. Unit = 1 if not otherwise specified.

DISCUSSION

The purpose of this investigation was to determine if eccentric hamstring strength was prospectively associated with HSI among collegiate athletes, while controlling for known risk factors, such as age and prior HSI, as well as sex or sport. In this study population, no association between eccentric hamstring strength and subsequent HSI risk was identified with univariable or multivariable analyses after controlling for known risk factors for HSI (age, prior HSI) and sex or sport. Despite the well-documented benefit of performing eccentric hamstring exercises for reducing HSI risk (20, 21), measurement of eccentric hamstring strength at preseason was not associated with HSI occurrence. Although initial evidence identified an association of HSI risk and eccentric hamstring strength (11, 12), more recent systematic reviews have failed to confirm this relationship,(7, 13) similar to findings of the present study. This study includes one of the largest collections of HSIs to date and is the first to include male and female collegiate athletes participating in American football, basketball, soccer, and track when assessing the relationship between eccentric strength and HSI.

Prior HSI and older age are well established risk factors for HSI.(7) Analyses in this study also support a strong association between prior HSI as a risk factor for a subsequent HSI, but the influence of age on HSI risk in this specific population is less clear. Prior studies have imposed cutoff thresholds to define the age at which HSI risk increases, with findings ranging from 22 to 25 years old (7, 22–24). Given the limited age range of participants in our study, we opted to treat age as a continuous variable for the analyses. Although not statistically significant, both univariable and multivariable analyses suggested younger collegiate athletes are at greater odds of sustaining an HSI compared to their older counterparts. These unexpected findings may potentially be explained by a limited age range in collegiate athletics, an abrupt change in training load due to the transition from high school to college training programs, or older athletes sustaining other non-HSI injuries.

An overwhelming majority of known evidence supporting HSI risk factors is based on study samples of exclusively male athletes.(11, 12, 14–16) Despite female participants in collegiate sports being at an all-time high (47% of Division I athletes in 2019–2020; NCAA database), a known underrepresentation of female athletes in the sports medicine literature continues to exist.(25–27) To the best of our knowledge, this study is the first to include female athletes when determining the association between eccentric hamstring strength and subsequent HSI. Although males demonstrate a higher rate of HSIs than their female counterparts in sex comparable sports,(28, 29) our data did not show sex to influence the association between eccentric hamstring strength and subsequent HSI risk.

Our study expanded on prior work in Australian Football,(11, 16) soccer,(12, 15) rugby,(14) and Gaelic Football(17) by involving a more diverse sample of American collegiate athletes (American football, soccer, track, and basketball). This allowed us to preliminarily explore whether sport confounds or mediates the relationship between eccentric hamstring strength and HSI. We found that sport demonstrated significant univariable associations with subsequent HSI, with track athletes having greater odds and soccer athletes having lower odds of sustaining an HSI compared to football athletes. This association persisted for soccer and football athletes in the multivariable analysis; however, even after controlling for sport in the multivariable model we failed to identify a significant relationship between eccentric strength and subsequent HSI. Furthermore, no significant interaction was detected between F_Asym and sport. The overall significance of sport (p < 0.001) suggests HSI risk differs by sport, but the lack of interaction between F_Asym and sport indicates that any potential association between eccentric hamstring strength and subsequent HSI does not differ by sport.

Despite our findings and those of others (7, 13) showing that eccentric hamstring strength does not predict future HSI, additional muscle-related factors should be explored. Given that prior HSI does consistently demonstrate a strong association with subsequent HSI, it may be possible that persistent muscle related changes following an index injury predispose an athlete to subsequent HSIs. Potential muscle tissue-level risk factors not included in this analysis are muscle structure (e.g. morphology) and architectural parameters (e.g. fascicle length, pennation angle). For example, shorter biceps femoris long head fascicle lengths have been shown to be associated with greater risk of HSI.(12) Known morphological changes (30, 31) and architectural changes (32, 33) exist after HSI but the prognostic value of these variables for predicting HSI remains unclear. A better understanding of how the hamstring muscle architecture changes after an index HSI may provide greater insight into which athletes are at a greater risk of a subsequent HSI. Thus, future prospective investigations identifying HSI risk factors should aim to include measures of muscle morphology and architecture.

Prior work has also suggested that a history of non-HSI injuries, such as anterior cruciate ligament (ACL) injuries, may predispose athletes to a subsequent HSI, as athletes with a prior ACL injury have a 70% increased risk of HSI.(7) However, our findings failed to demonstrate a significant association of prior lower extremity surgery, regardless of the specific surgery type, with subsequent HSI. Of the lower extremity surgeries recorded in this study population, 40% (n = 21) were ACL reconstruction surgeries. While the present study was not powered to look at ACL reconstruction exclusively, it is possible that the relationship between prior lower extremity surgery and subsequent HSI is limited to those with prior ACL injury or reconstruction.

Although the present study population was composed of male and female collegiate athletes from a variety of sports, these results are only generalizable to athletes of similar levels and sports. Further, the study was not powered to assess associations for males and females separately and there was a larger proportion of male (n = 241, 74%) compared to female (n = 85, 26%) athletes. Moreover, the study was not powered to assess associations by sport, thus those analyses should be interpreted as truly exploratory findings. Additionally, the effects of the pandemic imposed by the coronavirus resulted in a shortened follow up period on track athletes during 2020 (3.3% of the study sample). Finally, eccentric hamstring strength was only assessed at the start of each team’s preseason. However, recent evidence indicates that more frequent measures of eccentric hamstring strength throughout the season does not improve the ability of identifying which players will sustain an HSI. (34) Despite these limitations, findings from this study are similar to recent reviews concluding a lack of association between eccentric hamstring strength measurements and subsequent injury (7, 13) and provide preliminary results for sex and sport associations.

CONCLUSIONS

This study failed to find an association between preseason eccentric hamstring strength and risk of subsequent, acute HSI after controlling for known risk factors such as age and prior HSI, as well as sex or sport. These findings are in support of recent reviews(7, 13) and uniquely involve a sample of male and female collegiate athletes in track, soccer, and American football. Eccentric hamstring strengthening may continue to serve as a preventative approach to HSI but it does not appear to provide additional insight into HSI risk prediction beyond factors such as age and prior HSI. The limited association between preseason eccentric hamstring strength warrants further exploration of the multifactorial nature of HSI.