Research Review By Demetry Assimakopoulos©

Date Posted:

November 2009

Study Title:

The relationship between hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes


Myer GD, Ford KR, Darber Foss KD et al.

Author's Affiliations:

Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio; Sports Medicine Biodynamics Center and Human Performance Laboratory, Cincinnati, Ohio; Department of Kinesiology and Health Promotion, University of Kentucky, Lexington, Kentucky; Center for Epidemiology and Biostatistics, Cincinnati, Ohio

Publication Information:

Clinical Journal of Sport Medicine 2009; 19(1): 3-8.

Background Information:

There is 4-6 fold greater risk for women engaging in high-risk sports to suffer anterior cruciate ligament (ACL) injuries. Research has shown that women are predisposed to injury because of an increase in joint laxity (7) and a lack of neuromuscular control (2). It has been experimentally proven that non-traumatic ACL injury occurs at instances of high dynamic loading of the joint, especially in individuals whose muscles (namely hamstrings and quadriceps) cannot adequately dampen the increased joint load (1). When compensatory muscular action fails, so do the passive ligamentous structures in the knee (5,6). Also, males recruit their hamstrings muscles to a greater degree (2,3) and have greater hamstring strength (4,3) compared to females. The researchers speculate that it is this lack of relative hamstring strength that predisposes women to non-traumatic ACL injuries.

Thus, the investigation sought to determine an association between hamstring and quadriceps isokinetic strength and their contributions to ACL injuries. Their primary hypothesis is that an increase in knee extensor strength without a similar relative increase in flexor strength will be observed in women who subsequently injure their ACLs.

Pertinent Results:

  • The median measurements in hamstrings and quadriceps strength were similar between injured and non-injured female athletes.
  • Female athletes who subsequently suffered ACL injuries had 15% lower hamstrings strength than male athletes (95% CI, P=0.04). However, there were no significant differences in quadriceps strength between injured females and male controls (P=0.14). With this being said, if a practitioner were to take the number of newton metres of the hamstring and divide that number by the number of newton metres produced by the quadriceps you would thus calculate the hamstrings/ quadriceps ratio (H/Q ratio). Given the fact that the injured females had weaker hamstrings and relatively equal quadriceps strength to the males, the injured females yielded a lower H/Q ratio than the males. The bottom line is that the injured females had lop-sided forces about the knee – their hamstrings were weaker than their quadriceps.
  • Female athletes who did not subsequently suffer ACL injury had 10% lower quadriceps strength than male controls (95% CI, P=0.01). However, there were no significant differences in hamstring strength between uninjured females and male controls (P= 0.08). In this case, the H/Q ratio for the uninjured females would be higher than the male controls
  • For the sake of being clear, when you compare the H/Q ratios between injured and uninjured females, the injured females had a lower H/Q ratio than the uninjured females.

Clinical Application & Conclusions:

These results purport that, while controlling for whole body power, females who injure their ACLs have decreased hamstrings strength compared to male controls and similar quadriceps strength to uninjured male athletes. This indicates that a relative decrease in hamstring recruitment and strength may contribute to ACL injury in a high risk female athlete.

The researchers claim that preseason screening of the knee flexors and extensors with comparison to male normative values could identify female athletes who are at greater risk for ACL injury. These values should be monitored throughout the season. Targeted increases in hamstring strength could potentially enhance the stability of the knee joint, minimizing the risk of ACL injury.

Study Methods:

The researchers implemented a matched case control study design, with a total of 132 subjects (22 female subjects with ACL injury and 110 controls). The subject population consisted of high school and collegiate soccer and basketball players. These players were prospectively screened for hamstrings and quadriceps strength before an ACL injury. The athletes were not selected to be injured. Rather, the researchers merely waited for the athletes to suffer ACL injuries and included the subjects into their respective group upon diagnosis.

Prior to the data collection, a warm-up consisting of 5 submaximal knee flexion/extension repetitions was performed at 300 degrees/second.

Kinetic strength of the knee flexors and extensors (both concentrically and eccentrically) was collected for each subject on a seated dynamometer. The test included 10 isokinetic knee flexion/extension repetitions, within the range of 100-0 degrees. From this, peak torques (ft x lbs) were recorded.

The descriptive statistics derived from the data collection showed skewed data. The researchers used a gamut of statistical analytics which allowed them to normalize their data. Additionally, a measurement of vertical jump height was added as a control variable to explain the variability. Vertical height jump testing was performed on an MX1 vertical jump trainer, with a test-retest reliability rating of .993.

Study Strengths / Weaknesses:

  • Injury to the ACL is multifactorial. It is plausible that not all variables that contribute to ACL injury were accounted for.
  • A dynamic non-traumatic injury to the ACL generally occurs in high velocity, closed-chain scenarios. The researchers utilized an open chain method of assessment.
  • The researchers did not elucidate the research methodology effectively or completely, making it difficult to understand their statistical methods. The description of their linear model was unsatisfactory. Because of this, repeating the study will be exceedingly difficult.
  • There were adequate female and male control groups. This allowed the researchers to infer statistical relationships between genders and between injured and non-injured female athletes.

Additional References:

  1. Beynnon BD, Fleming BC. Anterior cruciate ligament strain in-vivo: a review of previous work. J Biomech 1998; 31: 519–525.
  2. Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 2005; 33: 492–501.
  3. Hewett TE, Myer GD, Zazulak BT. Hamstrings to quadriceps peak torque ratios diverge between sexes with increasing isokinetic angular velocity. J Sci Med Sport 2008; 11: 452–459.
  4. Holm I, Vollestad N. Significant effect of gender on hamstring-to quadriceps strength ratio and static balance in prepubescent children from 7 to 12 years of age. Am J Sports Med 2008; 36: 2007–2013.
  5. Li G, Rudy TW, Sakane M, et al. The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL. J Biomech 1999; 32: 395–400.
  6. Markolf KL, Graff-Redford A, Amstutz HC. In vivo knee stability: a quantitative assessment using an instrumented clinical testing apparatus. J Bone Joint Surg 1978; 60A: 664–674.
  7. Myer GD, Ford KR, Paterno MV, et al. The effects of generalized joint laxity on risk of anterior cruciate ligament injury in young female athletes. Am J Sports Med 2008; 36: 1073–1080.