How ACLs Tear

Anterior cruciate ligament (ACL) injury is the number one reason for time missed in sport with 100,000 -250,000 occurring every year in the U.S. alone (Hewett, 2016.) These injuries carry huge financial ramifications and increases rate of post-traumatic arthritis. Fortunately, risk of ACL injury can be quite accurately predicted and prevention strategies are shown to work well at reducing the incidence of injury and re-injury this.

–an important note: this risk is in reference to populations and not individuals; it is impossible to say exactly who will tear an ACL, but you can estimate risk within populations–

About 80% of ACL ruptures are non-contact injuries, with the athlete’s own movement strategies and muscle recruitment patterns causing the injury. Neuromuscular control programs indicate that at least 1/2 of non-contact ACL injuries can be prevented with neuromuscular training oriented at altering movement strategies (Gagnier, 2013)(Hewett, 2016.) These neuromuscular training programs are especially beneficial for at-risk females, as females are known to have altered neuromuscular control which places them at higher risk of ACL injury than men.

Non-contatct ACL injuries happen with rapid deceleration movements such as which happens with change of direction cutting, pivoting, and landing movements, especially when occurring in a single leg stance position. The ACL primarily restricts anterior translation of the tibia on the femur but also helps prevent hyperextension of the knee and gives rotational stability of the knee restricting tibial internal rotation (IR) more so than external rotation (ER.)

The amount of force required to rupture an ACL will vary individual to individual based on size of the ACL (larger individuals have larger ACLs) and likely genetic components. A study by Woo et al. showed younger cadaveric subjects, aged 22-35, had higher average strengths of the ACL with failure occurring around 2200 N of force as compared to 1500N and 660N for subjects aged 40-50 and 60-97, respectively. To put this into perspective 2200N (newton) is the force of about 220 kilograms with the accerleration due to gravity (roughly 10m/s^2.) Which is approximately the force of 485 lbs. This may seems like a lot of force, but consider that straight-line running can create ground reaction forces up to 3x bodyweight. Which means that a 200-lb male would create sufficient force to rupture an ACL with every step if it were to be transmitted into the ACL. Furthermore, the ground reaction forces within sporting events can be much greater than this. When these forces are directed into the ACL, it tears. Video analysis, computer modeling, and cadaveric studies have given us pretty solid evidence on how the ACL ruptures.

There is extensive evidence that now widely supports a triplanar ACL mechanism with knee valgus the common denominator. A 2014 study by Carmen Quatman revealed that loading of the ACL was greatest with triplanar motion involving tibial internal rotation, tibial abduction, and anterior tibial shear, though loads in her simulated falls using cadaver limbs were not sufficient to strain the MCL (Quatman, 2014.) This helps explain why only 4-17% of ACL ruptures occur with a concomitant MCL injury despite the MCL being the primary restraint to tibial abduction.

Video analysis of ACL injuries reveal they nearly always occur at knee flexion positions of around 20-30 degrees, which is actually quite a straight knee when viewed in the context of athletic performance (Koga, 2010.) It is not coincidental that this is the same angle we perform Lachman testing as ACL tension is greater in these positions of lesser knee flexion angles than when the knee is flexed more. An ACL rupture has been found to occur within 40 milliseconds of ground contact (Koga, 201o.) This is incredibly fast. About 3-4 times faster than the average reaction time for auditory and visual stimulus, and therefore, faster than what would be possible to sense and correct the risky position. This reveals the importance of appropriate preparatory movement and motor recruitment patterning. Observations of ACL injuries show the tibia internally rotating quickly in the first 40 milliseconds but then subsequently externally rotating. This change from tibial internal rotation to tibial external rotation is hypothesized to occur because of the loss of the ACL. A loss in the ACL’s ability to resist tibial anterior translation would cause the medial femoral condyle to shift backwards relative to the tibia with concomitant tibial external rotation (Koga, 2010.)

Muscle recruitment patterns can also help identify increased risk of ACL injury. Hamstring to quadriceps ratios (H:Q) have been used in the past as it is acknowledged that the co-contraction of the hamstring muscles create a posterior shear which offsets the anterior shear of the quadriceps and can decrease risk of knee injury, especially to the ACL. A quadriceps dominant landing strategy has been linked to increases in ACL injury (Zebis, 2009) Landing with a quadriceps dominant landing strategy not only increases the anterior shear of the tibia on the femur because of the line of pull of the quads, but it also is a strategy associated with landing on a stiffer leg and thus less knee flexion, which we also understand to be correlated with increased ACL loading and injury. Stiffer leg landing strategies are associated with greater relative quadriceps to hamstrings ratio and less overall hamstrings recruitment (Boling, 2013.)

Historically, hamstring to quad ratios (H:Q) have been used in terms of isometric strength but it may be more appropriate to consider the rate of torque development of the hamstrings as it is noted above and the quickness of ACL tear in response to GRF. It is observed that high peak concentric H:Q tend to be correlated to rate of torque development H:Q but the relationship is not always consistent(Greco, 2012.)

Side-cutting strategies have also been linked with increase rates of ACL rupture. Strategies of cutting in a wide stance and with trunk lean over stance leg show an increase in knee abduction moments and thus increase risk to the ACL (Kristianslund, 2014.) When the trunk flexes laterally over a plant leg the laterally displacement of the COM causes the ground reaction force, which is directed towards the COM, to pass now lateral to the knee which causes an increased knee abduction moment and predisposes to valgus collapse. Furthermore, landing with the center of mass located further from the base of support in the sagittal plane and with a greater limb angle (limb further from the vertical) is associated with increases in ACL rupture (Sheehan, 2012.)

It has been hypothesized that ACL tears could come from tissue fatigue failure, different from a traditional view of fatigue, whereby continual stress applied to tissue may cause eventual failure at the same amount of load which previously caused no harm. Fatigue failure has been demonstrated in MCLs of rabbits(Zek, 2010) and human extensor digitorum longus tendons (Schechtman, 1997.) A study in 2015 out of AJSM by Melanie Beaulieu suggests that ACLs can also fail from tissue fatigue which increases with limited femoral internal rotation with repeated landings (Beaulieu, 2015.) If the femur is not allowed to internally rotate then the relative internal rotation of the tibia on the femur will be much greater. Typically we want the amount of tibial rotation to be matched with the amount of femoral internal rotation. When the tibia accelerates into internal rotation relative to the femur then we increase loading of the ACL. The large valgus collapses we see in ACL injuries likely happen after the ACL has torn and partly because it has torn.

The best screening strategy would be do identify individuals using these movement patterns during dynamic activities. Four variables we are looking for during screening were summarized by Dr. Tim Hewett as ligament dominance, quadriceps dominance, trunk dominance, and leg dominance. Ligament dominance can be observed with inappropriate absorption of GRFs using a stiff legged landing and landings that are observed to coincide with increases in valgus collapse of the knee as mentioned above. Quad dominance is often observed with stiff legged landing strategies but can also be assessed with muscle testing and potentially with observation of movement patterns. Quad dominant individuals may display reduction in ability to hip hinge especially of note when the individual does not posterior shift their center of mass with athletic positions, but instead simply flexes the knee to achieve a lower COM. Trunk dominance, also termed core dysfunction, can be observed with single leg balance and cutting tasks. Tendencies for individuals to maintain balance on a single limb by laterally flexing their spine can cause ground reaction forces to pass lateral of the knee creating a knee abduction moment which can cause valgus collapse. When implementing return to sport/activity tests for patients returning from ACL reconstruction it is important to know how the ACL is torn and positions and movements which could load the ACL so we can appropriately screen for elevated risk. Keep these ideas in mind when rehabbing ACL patients.



Beaulieu ML, Wojtys EM, Ashton-miller JA. Risk of anterior cruciate ligament fatigue failure is increased by limited internal femoral rotation during in vitro repeated pivot landings. Am J Sports Med. 2015;43(9):2233-41.

Boling M, Padua D. Relationship between hip strength and trunk, hip, and knee kinematics during a jump-landing task in individuals with patellofemoral pain. Int J Sports Phys Ther. 2013;8(5):661-9.

Gagnier JJ, Morgenstern H, Chess L. Interventions designed to prevent anterior cruciate ligament injuries in adolescents and adults: a systematic review and meta-analysis. Am J Sports Med. 2013;41(8):1952-1962.

Greco CC, Da silva WL, Camarda SR, Denadai BS. Rapid hamstrings/quadriceps strength capacity in professional soccer players with different conventional isokinetic muscle strength ratios. J Sports Sci Med. 2012;11(3):418-22.

Hewett TE, Ford KR, Hoogenboom BJ, Myer GD. Understanding and preventing acl injuries: current biomechanical and epidemiologic considerations – update 2010. N Am J Sports Phys Ther. 2010;5(4):234-51.

Hewett TE, Myer GD, Ford KR, Paterno MV, Quatman CE. Mechanisms, prediction, and prevention of ACL injuries: Cut risk with three sharpened and validated tools. J Orthop Res. 2016;34(11):1843-1855.

Koga H, Nakamae A, Shima Y, et al. Mechanisms for noncontact anterior cruciate ligament injuries: knee joint kinematics in 10 injury situations from female team handball and basketball. Am J Sports Med. 2010; 38(11):2218–2225. [PubMed: 20595545]

Kristianslund E, Faul O, Bahr R, Myklebust G, Krosshaug T. Sidestep cutting technique and knee abduction loading: implications for ACL prevention exercises. Br J Sports Med. 2014;48(9):779-83.

Schechtman H, Bader DL. In vitro fatigue of human tendons. J Biomech. 1997; 30(8):829–835. [PubMed: 9239568]

Sheehan FT, Sipprell WH, Boden BP. Dynamic sagittal plane trunk control during anterior cruciate ligament injury. Am J Sports Med. 2012;40(5):1068-74.

Woo SL, Hollis JM, Adams DJ, Lyon RM, Takai S. Tensile properties of the human femur-anterior cruciate ligament-tibia complex. The effects of specimen age and orientation. Am J Sports Med. 1991;19(3):217-25.

Zebis MK, Bencke J, Andersen LL, et al. Acute fatigue impairs neuromuscular activity of anterior cruciate ligament-agonist muscles in female team handball players. Scand J Med Sci Sports. 2011;21(6):833-40.

Zec ML, Thistlethwaite P, Frank CB, Shrive NG. Characterization of the fatigue behavior of the medial collateral ligament utilizing traditional and novel mechanical variables for the assessment of damage accumulation. J Biomech Eng. 2010; 132(1):011001. [PubMed: 20524739]


Ankle mobility deficit and SI pain during squatting tasks

I have often observed that when ankle mobility is limited unilaterally, compensations occur during bilateral squat exercises which may result in a pelvic obliquity and in my opinion may be associated with onset or history of SI joint pain and or lumbar/ lumbosacral facet irritation. When one ankle is limited in its dorsiflexion range of motion and you descend into a depth which would would require this range several things happen. First, the unrestricted ankle is fine to allow the tibia to continue to rotate forward in front of the foot but the restricted ankle can no longer allow this forward inclination of the tibia, as a result the knee stops translating forward.  In this situation experienced squatters will not allow the heel to elevate away from the floor largely because the area of pressure on the foot is an area of attention and perception which squatters will try to preserve throughout a squat. In inexperienced squatters, however, you may observe heel elevation. What does typically happen is that with the knee will no longer allowed to translate forward with further knee flexion any further knee flexion occurs with concomitant hip flexion but the hip is also forced to sit farther back due to its connection to the knee via the femur. A hip that must flex in a more posteriorly oriented position in the sagittal plane will undergo more hip flexion relative to the hip on the side of unrestricted ankle dorsiflexion simply because at a given angle of trunk inclination a hip sitting further back will make a more acute angle and require greater hip flexion.  So what then results is a significant discprency in hip flexion with potential for pelvic obliquity towards the side of limited ankle dorsiflexion. The pelvic obliquity can occur because it can be difficult to maintain normal pelvic position under conditions of increased demand for end range hip flexion. Under load this pelvic obliquity may predispose one for irritation of the weight bearing structures or the structures providing stability at the SI and lumbopelvic areas. You may identify this pattern first by observing pelvic obliquity either from behind with a drop of the iliac crest or from the front as noted with a lower position of the ischial tuberosity and glute on the affected side.

The image is an example I collected with 3D motion capture and shows a large discrepancy in hip flexion likely due to decreased ankle dorsiflexion on that side. (I should now explain that those dorsiflexion values seem very large because weight bearing dorsiflexion is larger than non-weight bearing dorsiflexion values.)


Deficit of ankle dorsiflexion increases injury risk

Decreased dorsiflexion during landing tasks are associated with kinetics and kinematics that increase the risk of ACL injury according to a 2015 study (Malloy, 2015.) The study was conducted on 23 female collegiate soccer players. The study utilized a 3D motion capture system analyzing kinetic and kinematic data during a drop vertical jump. This data was then correlated to dorsiflexion flexibility measured using traditional goniometry in a knee extended position. It was found that significant negative correlations exists between dorsiflexion flexibility and peak knee abduction moments and knee flexion. This means that an ankle with more dorsiflexion range will experience greater knee flexion range of motion during landing and less knee abduction moment, which is the moment which would accelerate knee valgus. We know that landing mechanics that utilize a stiff leg strategy as well as those that increase knee abduction moments are correlated with elevated injury risk to the ACL. This study corroborates with previous studies showing similar findings. This information furthers the importance of acquiring dorsiflexion mobility in at-risk populations for ACL injuries especially those rehabbing from ACL rupture.


Malloy P, Morgan A, Meinerz C, Geiser C, Kipp K. The association of dorsiflexion flexibility on knee kinematics and kinetics during a drop vertical jump in healthy female athletes. Knee Surg Sports Traumatol Arthrosc. 2015;23(12):3550-5.

Caution against sidebending during single leg stance.

Watch out for individuals who stabilize in single leg stance by a laterally flexing their trunk. This trunk compensation occurs when an individual side bends at their spine to achieve a position of center of mass (COM) over their base of support. Preferably there would be more contribution from concomitant hip adduction and posterior shift of COM with slight hip flexion. The hip flexion position is important because it loads the hamstrings. There is significant evidence showing that stiff leg balance strategies and poor hamstring activation relative to quads to be predictors of injury particularly of the ACL. A laterally flexed trunk position over a single support is a risky position in sport as the laterally flexed trunk shifts the COM over the stance limb which directs corresponding ground reaction forces directly at the COM. With a laterally flexed trunk, these forces are lateral to the knee and thus create a external knee abduction moment (a force creating valgus collapse.) If you see your patient/ client balancing with a nearly straight knee and be leaning their body over the stance limb then this must be corrected and coached and should be observed to see if any maladaptive carryover has occurred in sport or practice.

Treating the “I feel tight” patient.

We have all had patients present to us describing that some muscle “just feels tight.”  Often a perplexing finding on these patients is the lack of correlation to this sensation of being “tight” and loss of motion.  Some patients who show no perception of tightness show large losses of range of motion while some individuals who feel tight show normal range.  What is going on here?  It may very well be a protective neural mechanism creating a sense of tightness to constrain a perceived threat during dynamic activity. One of these threats to the CNS may occur when a muscle is impaired in production of muscular force at greater muscle lengths.  Notably, these individuals have terrible abilities to eccentrically lengthen their muscle to the same degree to which you can stretch them passively (perhaps relating to gamma motor neuron activity.. a thought for another day.) So let me give the example of the patient who presents with a sense of “tight” hamstrings.  These individuals never seem to be able to appropriately hip hinge to the same degree of hip flexion as you can passively take them in the analogous position of a supine hamstring stretch.  What I believe could be occurring in these individuals is that the perceived “tightness” is actually protective stiffness created from a subconscious response to perceived threat.  This threat may arise as a shift away from the optimal actin-myosin overlap represented in the plateau section of the length tension relationship (see image below.) As you move further right on the graph there is less available distance to elongate before potential fibril damage may occur…an understandable “threat.”

Length Tension Relationship, from:
What is required in these individuals is a shift of the plateau of the length tension relationship towards greater muscle elongation.  To do this there are two practical tools available: stretching and eccentric exercise.  Stretching has gotten a bad rap lately.  Acutely, stretching improves range of motion but typically for only a very short period of time (<60 minutes) with the most likely mechanism simply an increase in tolerance to stretch rather than any biomechanical effects(See study.)  Furthermore, stretching has come under scrutiny due to extensive literature demonstrating no improvement in injury rates and a decrease in muscle performance following stretching.  However, a recent literature review by Behm, Blazevich, Kay, and McHugh (See study) found that while static and PNF stretching did result in small (-3.7 to -4.4%) change in muscle performance this change is both dose dependent and able to be avoided.  Stretches held less than 60 seconds resulted in only a 1.1% decrease in performance with greater than 60 seconds resulting in a 4.6% decrease.  Additionally, performance decrease only occurs if the muscle is tested immediately after stretching with deficits in muscle performance effectively ameliorated with dynamic activity before exercise.  Chronically, stretching programs can cause lasting improvement in range of motion(See study) likely, in part, from the serial addition of sarcomeres, termed sarcomerogenesis, observed in several animal studies(See studySee study.)

So despite lackluster effects with acute bouts of stretching, stretching programs do appear to have a place in rehabilitation, though eccentric exercise may prove more beneficial in improving range of motion through sarcomerogenesis.  A 2012 review by Kieran O’Sullivan (See study) demonstrated that eccentric exercise programs are effective at increasing both range of motion and serial addition of sarcomeres.  Eccentric training allows muscular adaptation which can decrease injury risk and improve force production at greater degrees of muscle elongation (See studySee study.)  I always attempt to modulate threat perception using active muscle contraction at various joint ranges which is, in my opinion, why PNF techniques work so nicely at improving motion.  So while eccentric and stretching programs may both produce improvements in muscle length and flexibility (See study), it would make sense that eccentric exercise should be included with its ability to directly promote the ability to generate eccentric force at greater muscle length and for possible threat inoculation.  Keep in mind  this is about one factor that may contribute to threat, there are a multitude of others including constraining movement at a nearby body segment that are certainly as or more plausible.  In any case, if you don’t utilize eccentric training for range of motion improvement, for that, you should consider incorporating it into your repertoire.


Movement Patterns and SI Joint Pain

I have written previously about my belief in the error in practicing motion palpation surrounding the sacroiliac (SI) joint (see: Time to Let Motion Palpation Die?.)  A new paradigm of assessing motion of the SI joint was first introduced to me in the work of Richard Jackson, PT, OCS and Kris Porter, PT, DPT, OCS in the Pelvis and Sacroiliac Joint section of the Current Concepts of Orthopedic Physical Therapy guide.  Jackson and Porter describe that in light of the unreliable nature of assessing SI motion with palpation another manner of mobility assessment is desired.  One test suggested, while admittedly empirical, is a modified version of the Stork Test.  Traditionally, the stork test is performed by palpating the tested innominate’s PSIS and just medial to the PSIS of the same side.  The patient then flexes the hip of the test leg to 90 degrees while the pracitioner palpates for SI motion, with a normal finding said to be a inferior displacement of the PSIS.  As referenced in my article linked above these motion palpation tests are both woefully unreliable and invalid.  However, the modified Stork test, while not yet researched presents to me with better face validity.  Aberrant gross movement patterns are easier to identify and the evident, anatomical rationale implicates possible SI joint mobility restriction/ SI dysfunction.  During the modified Stork test, instead of placing the thumb on the PSIS you simply place your hands on each hip over the innominate bones.  The patient then again flexes the hip, allowing the knee to remain relaxed and flex as well.  A normal, or negative, test would show concomitant hip flexion with innominant posterior rotation, sacral extension, lumbar flexion, and slight spinal rotation towards the flexed leg.  With the negative test the hands will follow the pelvis into this normal posterior rotation without any compensations not listed above.  A positive test, however, will show a ipsilateral hip “hike” as the hip flexes as a hypomobile SI joint will not allow the posterior innominate rotation.  This presents as potentially more reliable as the “hike” is more easily observable than trying to feel with the thumbs the tiny movement at the SI joint.

Assessment of lumbopelvic rhythm is also suggested as a method of movement pattern observation lending some insight into sacroiliac joint dysfunction.  A normal lumbopelvic rhythm is traditionally said to occur with 120 degrees of motion, 60 from hip flexion and 60 from lumbar flexion.  Jackson and Porter site van Wingerden et al(2) who found individuals with pelvic pain had impaired forward bending.  Therefore, the observation may imply that those with pelvic pain will utilize a more spine dominant movement pattern and/or limited hip flexion during forward bending than those without pain.  It has also been demonstrated that individuals with pelvic pain will exhibit anterior rotation relative to the sacrum on the stance leg during single leg stance(Jackson and Porter.)  This has been described as a faulty movement strategy implicating faulty stabilization for load transfer.

I am note fully espousing these presented tests, however, I am suggesting that they show more promise than the antiquated and invalid idea of assessing and correcting postural flaws (rotations, upslips, downslips.)  The notion of using movement patterns to assess dysfunction is more encouraging as we know these gross movement patterns are more easily observed, although I fully disclose that these methods of assessing SI joint pain have not been validated.  It should be clear that these tests can reveal asymmetry in motor control and local and global muscular dysfunctions which can then implicate the tests as being, to an extent, both diagnostic and prescriptive.  If a patient shows movement pattern aberrancy during the modified stork test then we could conclude that exercise programming should eventually lead the patient towards the ability to stabilize the lumbopelvic girdle to promote the dysfunction in hip flexion.  While the exercises to accomplish this may be a bit more obscure and nuanced than this suggests, it still provides principle to treatment philosophy.


1.  Jackson, Richard, PT, OCS, and Kris Porter, PT,DPT, OCS. “The Pelvis and Sacroiliac Joint: Physical Therapy Patient Management Utilizing Current Evidence.” Current Concepts of Orthopaedic Physical Therapy. 3rd ed. APTA.

2.  van Wingerden JP, Vleeming A, Ronchetti I. Differences in standing and forward bending in women with chronic low back or pelvic girdle pain: indications for physical compensation strategies. Spine. 2008.

Case for the Turkish Get Up in Rehab

As rehab professionals we are all well aware at how many (most) individuals, especially those in pain/ chronic pain, lose the ability to perform movement patterns that toddlers so easily perform.  Watching the movements of a toddler will show efficient and seamless  positional transitions including to and from prone, supine, quadruped, half-kneeling, reaching, and effortless full squatting.  These motor patterns remain ingrained in the adult motor-sensory brain maps but they are often shrouded in movement dysfunction as years of motor-sensory neglect compounded on top of concomitant structural changes.  This is why when you try to have the average activity-naive adult perform a full squat it often looks terribly awkward, a far cry from the aesthetic perfection of the toddler squat.  Had the individual continued to reinforce and practice the motor-sensory pathways for their squat during the time periods of structural change their patterns would have adapted to allow for efficient squat despite bodily changes.  As a physical therapist one of my favorite treatments for poor movers and those in pain is to expose them to developmental positions and movement strategies.  For reteaching proper trunk and upper extremity integration I use the Turkish get up exercise, or at least a partial variation of it.  The full Turkish get up can be thought of as the most efficient way to stand up with a weight held overhead in one hand.  There are several small idiosyncrasies in the form used but generally the full movement is observed as in the video below.

So what is going on at the beginning of the movement that I like is the shoulder flexion, linked to scapular protraction, linked to contralateral thoracic rotation.  This is the perfect set up for practicing reach and upper extremity/ trunk integration in a developmental position.  Think of a baby on its back with some object in front of him, just out of reach.  The most efficient way for that baby to try to reach that object is the initial portion of the Turkish get-up, that is shoulder flexion, scapular protraction, and contralateral thoracic rotation.  For rehab purposes, I tend to have patients end the movement when they are in the pseudo-oblique sitting position with arm overhead and weight through the contralateral elbow.  Give it a try!