by Jaclyn Chan
March, 2010

Picture a female soccer player who plants with her left foot, and then pivots to her right; but the left cleat stays planted and doesn’t rotate with her entire body.   This is one of most common mechanisms of injury (MOI) in tearing the Anterior Cruciate Ligament (ACL).   The lower left leg was left in external tibial rotation, with a valgus load on the knee, which made the knee collapse into valgus position.  Besides valgus loading in external tibial rotation which is the most likely MOI in grass/turf sports, another common MOI is hyperextension with internal tibial rotation.  This usually occurs during a hard, sudden land from jumping, making it more common in sports such as basketball and gymnastics.

ACL Background
The ACL is one of four major ligaments that holds together and stabilizes the knee joint.  The other three ligaments are the Medial and Lateral Collateral Ligaments (MCL & LCL), and the Posterior Cruciate Ligament (PCL).  The MCL and LCL resist valgus and varus forces on the knee while the PCL and ACL limit knee extension and flexion.  More specifically, the ACL prevents anterior tibial translation on the femur, accepting 75% of anterior forces in full extension, and 85-90% of anterior forces in flexion8.  Other movements limited by the ACL include internal tibial rotation and varus/valgus angulation with external rotation.  Whenever any of these motions occur due to extreme force or speed, an ACL tear is a likely outcome.

ACL Injury Rehabilitation
After an ACL tear, both conservative and nonconservative repair approaches are taken depending on the activity level and age of the patient.  Athletes and the physically active who want to continue in activities that aren’t only in the Sagittal plane, are recommended for surgery.  Older patients or young children who have not yet hit puberty usually follow a more conservative treatment plan.  A conservative treatment plan consists of pain and swelling management, use of a supportive brace during physical activities, cryotherapy and anti-inflammatory medicines.  Also, muscle strengthening and conditioning are important to compensate for the lack of an ACL.  Surgical intervention is performed by replacing the torn ACL with a new ligament/tendon via autograft or allograft procedure.  The two most common forms of the autograft procedure are by bone-patellar tendon-one (BPTB) or hamstring tendon.  Post surgery, return to activity can take more than six months.

The progression of rehab starts with controlling of pain/swelling, then gaining range of motion (ROM), reducing muscle inhibition, strengthening muscle, proprioception, and a progressive return to activity.  Early on, the main goals are to reduce pain/welling while trying to get full extension (180°) and flexion (90°) and to improve the function of the hamstring and quadriceps muscles that severely atrophy in post-operative patients.  First, the patient will be progressed from non-weight bearing (NWB) to full weight bearing once their ROM and gait without a brace have returned to normal.  Hip, knee, and ankle strength are continuously increased while improving proprioception and cardiovascular levels.  Through month 6 the patient will be returned to Sagittal plane running, continue muscle strengthening, and then return to cutting once > 90% of the healthy side’s strength is achieved4.  After 6 months, if strength, proprioception, and pain are at satisfactory levels, the athlete can be returned to full activity.  However, rehab and maintenance of the reconstructed ACL do not stop upon return to activity.  For all patients it is a long recovery process that involves constant care and maintenance.  For many it can take up to 2 years to achieve normal quadriceps function and strength.

Prevalence of ACL Injury
Every year in the US there is a 1/3,000 incidence rate of ACL injury in the US4.  The most susceptible group is 15-25 year olds who participate in sports that involve pivoting and cutting.  Within this age group, many studies have shown that females are at least three times as likely to get an ACL injury as compared to their male counterparts.  Why is this? Why are females more prone to ACL injury when participating in the same activities?  Unfortunately there is not one definite answer that explains this phenomenon.  There have been many explanations for this gender difference including female’s wider pelvis, greater flexibility, narrow femoral notch, genu valgum, joint laxity, limb alignment, and hormones, among others.  The cause of higher incidence rates of ACL injury in active females is multifactorial and an ongoing field of research.

One of the areas that has been studied as a possible explanation for the higher prevalence of ACL injuries amongst female athletes is the difference between male and female muscle recruiting during different types of activities.  The key difference is the ratio of hamstring and quadriceps contraction during knee joint loading.  In two different studies this ratio was observed.  One study, performed at UNC Chapel Hill, looked specifically at male and female soccer athletes during side-step cutting maneuvers, a specific type of knee joint loading1.  Another study performed at the University of Michigan, Ann Arbor looked at hamstring and quadriceps muscle recruitment in abduction knee loading3.

Muscle Recruitment and ACL Injury
The first study was performed based on the belief that differences in muscle activation could be a contributing factor to the higher incidence of ACL injuries in female soccer players. The participants in this study were 20 male and 20 female Division I NCAA soccer players.  During the experiment, they performed two different activities, a running-approach side-step cut and a box-jump side-step cut task.  Contraction of the quadriceps and hamstrings influence the anterior tibial shear force and translation that strains the knee1. Another force believed to put stress at the knee is valgus motion, which is influenced by hip adduction and rotation.  Therefore, the contraction of 6 different muscles (rectus femoris, vastus lateralis, medial hamstrings, lateral hamstrings, gluteus medius, and gluteus maximus) that contract in the previously mentioned motions, were taken by surface electromyographs (EMGs).  The activation of the muscles was measured during two different phases of these tasks, the preparatory and loading phase.  The preparatory phase (PR) was defined as the 50ms time period before contact was made with the ground, while the loading phase (LO) was the initial 50% of stance phase during the cutting step.  The LO phase was chosen because during the period of deceleration directly after ground contact there is a higher chance of ACL injury as seen by previous studies1.

Each participant performed five trials with a 30 second rest period in-between.   The averages for the EMGs were calculated for each of the six muscles that were tracked.  Then, coactivation ratios for the quadriceps and hamstrings (Q:H) were calculated during the two phases of both cutting tasks.  The ratio was the sum of the EMG of the rectus femoris and vastus lateralis compared to the medial and lateral hamstrings.  In general, the female soccer players showed a greater vastus lateralis activation, which equated to a greater Q:H ratio in females than males1. Females use more quadriceps dominant activation when performing cutting motions.  Because previous studies have shown that quadriceps-dominated contraction places a larger load on the ACL and puts it at higher risk of injury, female soccer athletes may be more susceptible to injury.  When the hamstring activation EMGs were similar between the two genders, the Q:H coactivation ration was >1 for the females but <1 for the males1. Because of this increased loading, it seems that females have a neuromuscular activation pattern that increases the load on the ACL during directional changes, thus making the female population more vulnerable to ACL injury.

The second experiment, looked more closely at abduction loads on ACLs in knee valgus strain.  The participants in this experiment were 10 men and 11 women.  Similar to the previous experiment, the contractions of the muscles were measured using EMGs placed on the medial and lateral hamstrings and quadriceps.  From these EMGs, QH co-contraction ratios were taken for the medial muscles as well as the lateral.  Additionally, the peak knee abduction moment was calculated to show the relationship between muscle contractions and knee abduction forces.  The knee abduction moment was defined in this experiment as an external load that moves the knee into an abducted position.  The activity for this experiment was a forward hop, defined as a single leg takeoff and landing.  This motion was used to simulate the rapid deceleration that happens in cutting sports3.

The co-contraction ratio for both the lateral and medial muscles was lower in the female participants as compared to the males.  In recruiting of both the quadriceps and hamstrings, females activated their medial muscles less than the lateral muscle3.  This may limit the ability to resist abduction loads thus increasing the strain on the ACL.  Additionally, the peak external abduction moments were larger in the female participants than the males3.  This finding when combined with the lower activation of medial hamstring and quadriceps muscles provides for condition that is more favorable to ACL injury in females.

This study showed that compared to males, female athletes create a larger abduction load.  Additionally, there was evidence that females use a selective activation strategy favoring abduction loading, thus increasing their risk of ACL injury.  Because the quadriceps and hamstrings have the ability to support 100% of applied abduction-adduction loads when co-activated in the proper ratio these muscles can take away some of the load on the ACL2.  Due to the difference in female Q:H ratio as compared to men, this neuromuscular control may attribute to their increase rates of ACL injury.

Both of these experiments use neuromuscular control as an explanation for the increased rate of ACL injury in females.  While the first explains that females do not have an equivalent Q:H ratio, the second explains that females do not equally activate their medial and lateral muscles.  In both cases, this in-equivalence of muscle recruitment leads to increased stresses placed on the ACL.  A combination of lack of medial muscle contraction compared to lateral muscle contraction along with a more dominant quadriceps versus hamstring contraction puts greater valgus and tibial shear force on the knee.  Because these are two of the forces that put the most strain on the ACL, the neuromuscular approach taken by females should be a target of preventative measures.  Strengthening of medial hamstring muscles and education to actively contract these muscles during deceleration and cutting movement could potentially decrease the number of ACL injuries in females.  Activation of the medial hamstring would balance the Q:H ratio as well as reduce the valgus force on the knee during abduction.

Kinematics and ACL Injury
Another gender difference that may affect ACL injury involves the kinematics of the knee joint in external and internal rotation and valgus/varus movements caused by torsional forces.  The experiments performed to analyze this are based on the laxity/stiffness and joint motion of the knee.  The first experiment looked at knee joint stiffness due to different amounts of varus/valgus, and internal/external torsional stress 5.  A second experiment looked at the motion of the tibiofemoral joint 5.

Joint stiffness is the deformation of the soft tissues connecting bone to bone during a loading/torque force.  The forces at the knee are varus, valgus, internal, and external.  In this study, 20 university students were used and the stiffness in both their knees was evaluated in non-weightbearing (NWB) and weightbearing (WB) positions using a joint testing device.  0-10 Nm of force were applied in all 4 motions.  To make the measurements, electromagnetic position sensors were attached to the lateral thigh and tibial shaft7.

In all scenarios, besides external torque in WB and varus torque in NWB, females’ joint stiffness increased with an increase in torque magnitude.  However, male stiffness values did not change.  Additionally, for low torques (<5 Nm) of valgus and varus forces, stiffness was about 35% less in females.  Also, from 2-3Nm and 9-10Nm, the stiffness increased in magnitude for females while there was no change for males when a valgus force was applied7.  During internal and external torques, females had lower stiffness values for 0-1Nm of force but greater values when 3-5Nm of torque was applied7. This relationship is important to explaining the biomechanics behind the load placed on the ACL switching from NWB to WB.

The material characteristics of the knee ligaments and the geometry of the tibiofemoral joint may explain this observed difference.  During cutting and jumping exercises, when an athlete transitions quickly from a NWB to WB motion, there is anterior translation of the tibia on the femur, which is restricted by the ACL.  When the knee joint has increased laxity, more stress is placed on the ACL.  Therefore, because females are less stiff at lower torques, their ACL is more at risk for injury.

Another experiment examined rotational kinematics in 12 male and 12 females around 30 years old.  Their knees were analyzed by performing single leg lunges while their knee was imaged and then reproduced using images and bony models. Magnetic Resonance Images (MRIs) were taken of each knee to construct the aforementioned 3D bony models.  To examine knee flexion, extension, and internal and external rotation, points along several axes of the knee were selected.  In general, females had a greater range of tibial rotation in comparison to male knees.  More specifically, females had greater external tibial rotation at 0° of flexion but smaller internal rotation at 30° of flexion7.  This finding is significant because passive joint laxity has been found to increase the risk of ACL injury in females.  Because external rotation of the tibia in slight flexion is a common MOI, the larger external tibial rotation in females makes their ACL more susceptible to injury.

Due to the larger ROM at the tibiofemoral joint combined with the decreased stiffness at low torques, noncontact ACL injuries are more likely in females than males.  The larger ROM puts more stress on the ACL while the increased laxity also places more stress.  These two factors combine to put much higher stress on a female versus a male ACL.  Because these two factors are anatomical, they cannot be changed.  However, female athletes can alter the how they move and their muscular recruitment to counterbalance these issues.

Hormones and ACL Injury
Another component to the multifactorial cause of ACL injury is the effect of the menstrual cycle and its related hormones.  Because of the presence of different levels of hormones in the male and female body, many believe this is partially responsible for the higher rate of ACL injury in female athletes. It has been established that the hormones involved in the menstrual cycle, estrogen and progesterone effect ligaments and other soft tissues.  Specifically, there is a link between estrogen and collagen metabolism. Therefore, when the concentrations of these hormones vary during the menstrual cycle, there are times when the knee is more vulnerable to ACL injury.  One experiment evaluated only female participants and analyzed which phase of the patient’s menstrual cycle the injury occured9.  A second experiment compared females and males based on hormone levels and anterior knee joint laxity across the menstrual cycle6.

The menstrual cycle consists of three different phases, follicular (1-9), ovulatory (10-14), and luteal (15-end).  In each of these phases, the levels of estrogen and progesterone change.  During the follicular phase, the concentration of both hormones is low followed by a surge of estrogen just before the ovulatory phase, with a peak at day 12.  During the luteal phase, progesterone levels rise, peaking at day 21.

In the first experiment, 28 women were selected who had recently torn their ACL and who had a regular menstrual cycle. There were no tests performed in this study, all of the data came from a detailed questionnaire.   A majority of the patients reported that they believed that during their cycle, their athletic performance was hindered9.  This was important because it showed that estrogen levels may not only affect the physical structure of the ACL but also the physiological ability of the athlete thus making them more susceptible to injury.

There were more injuries seen during the ovulatory phase, when levels of estrogen are high, than expected.  The expected number of injuries was based on the percentage of days that the particular phase of that cycle covered.  Also, fewer injuries occurred during the follicular phase when all hormone levels are low.  Although it has not been tested on human ligaments, it has been shown in the laboratory that estrogen can decrease total collagen content and collagen synthesis in rats as well as reduces the force to ligament failure ratio in rabbits9.   In human connective tissue from females, there have been hormone receptors for estrogen and progesterone found9.  This supports the hypothesized relationship between hormone levels of the menstrual cycle and ligament laxity.  The major drawback to this study was that all of the data collected was based on participant memory and recall, not actual hormone measurements9.

Another experiment performed to study the effect of the menstrual cycle looked primarily at knee joint laxity.  In this study, serum hormone levels and anterior knee joint laxity/stiffness in male and female participants were analyzed6.  There were 22 females tested daily across a complete menstrual cycle and 20 males were tested once a week for four weeks.  During the testing, serum levels of estradiol, progesterone, and testosterone were taken from blood samples and anterior knee joint laxity/stiffness was measured by a KT 2000 knee arthrometer while the participant was in a supine position6.

In the males, there was no difference in hormone levels or knee laxity over the 4 test days.  In knee laxity, females had more laxity on day 5 of menses, 3-5 of the initial estrogen rise, days 1-4 of the early luteal phase, and days 1-5 of the late luteal phase.  However, knee stiffness didn’t change.  For estrogen, the highest values were found 1-5 of the menses, which represents the initial estrogen rise near ovulation.  The progesterone hormone was also the highest at day 5 of the menses just before ovulation.  Testosterone was greater in males all through the cycle, however there was a spike in the level for females during the early luteal phase6.  These findings show knee laxity was based on the time during the menstrual cycle.  An increase in laxity in females coincided with days when estrogen and progesterone were elevated as compared to males.   Because laxity is measured by the amount of anterior tibial translation on the femur due to a given force, the movement limited specifically by the ACL, the hormone levels may affect the function of this ligament.

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These two studies show that due to the increased levels of estrogen in females during their menstrual cycle, there is increased knee joint laxity in anterior tibial translation.  This is not only compared to males, but within the same person over their menstrual cycle.  Due to receptors for the hormones on the ligaments of the females, estrogen and progesterone directly affect the physical composition of the ligaments.  This increased laxity and possible weakness of the ACL during times of elevated estrogen and progesterone shows that females are more susceptible to ACL injury than males.  More studies based on taking hormone samples during times of injury need to be made in order to better understand this relationship.  Additionally, women who take birth control and medications to control the levels of their hormones should also be included in future studies so as to understand what hormonal control does to knee joint laxity and thus the ACL.

Preventative measures are being made in order to better educate female athletes on how to protect themselves from ACL injury.  Because anatomical differences between the two genders are not controllable, neuromuscular control and techniques in jumping, cutting, pivoting, etc are the focus of this education.  Teaching female athletes to recruit more medial quadriceps muscle during these activities so as to mimic male activation patterns may help to reduce the higher rate of ACL injuries in females.  Neuromuscular control is just one of the factors in explaining the gender difference in ACL injuries.  As a multifactorial problem, the previously discussed factors as well as abnormal posture, misalignment of the lower limbs, large Q-angle, and others all play a role in higher rates of female ACL injuries.


1. Hanson, A., Padua, Da., Blackburn, J., Prentice, W., Hirth, C. (2008). Muscle Activation During Side-Step Cutting Maneuvers in Male and Female Soccer Athletes. Journal of Athletic Training, 43(2), 133-143.

2. Lloyd, D., Buchanan, T., Besier, T. (2005) Neuromuscular Biomechanical Modeling to Understand Knee Ligament Loading. Medical Science Sports Exercises, 37(11), 1939-1947.

3. Palmierei-Smith, R., Mclean, S., Ashton-Miller, J., Wojtys, E. (2009) Association of Quadriceps and Hamstrings Cocontraction Patterns with Knee Joint Loading. Journal of Athletic Training, 44(3), 256-263.

4. Prentice, W. (2003) Arenheim’s Principles of Athletic Training a Competency-Based Approach (11th edition). San Francisco: McGraw Custom Publishing, pp. 583-599.

5. Schmitz, R., Ficklin, T., Shimokochi, Y., Nguyen, A-D., Beynnon, B., Perrin, D., Schultz, S. (2008) Varus/Valgus and Internal/External Torsional Knee Stiffness Differs Between Sexes. American Journal Sports Medicine, 36(7), 1380-1388.

6. Shultz, S., Sander, T., Kirk, S., Perrin, D. (2005) Sex Difference in Knee Joint Laxity Change Across the Female Menstrual Cycle. Journal of Sports Medicine and Physical Fitness, 45(4), 594-603.

7. Varadarajan, K., Gill, T., Frieberg, A., Rubash, H., Li, G. (2009) Gender Differences in Trochlear Groove Orientation and Rotational Kinematics of Human Knees. Journal of Orthopedic Research, 27(7), 871-878.

8. Whiting, W., Zernicke, R. (2008) Biomechanics of Musculoskeletal Injury (2nd edition) Human Kinetics, p. 170-178.

9. Wojtys, E., Huston, L., Lindenfeld, T., Hewett, T., Greenfield, M. (1998) Association Between the Menstrual Cycle and Anterior Cruciate Ligament Injuries in Female Athletes. American Journal of Sports Medicine, 26(5), 614-619.

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