The human knee joint acts as a conduit for transmitting forces and moments produced by muscle activity and ligamentous constraints of the lower extremity. Joint stability and mobility occur through the coordinated interaction of the intrinsic joint surface geometry, contractile muscle activity, and noncontractile tissues, including the cruciate ligaments and menisci. The relationship between these factors is essential to the smooth, controlled rolling and gliding motion of the knee when performing a variety of simple and complex weight-bearing (WB) and nonweight-bearing (NWB) activities. Alterations in these relationships may have significant effects on joint function and on an individual's overall functional capability, irrespective of age or gender. Kinematics, defined as the study of the geometry of motion without reference to the cause of motion, is a geometric description of a body's motion using its displacement, velocity, acceleration, and time of movement. Kinetics, defined as the study of forces acting on a body relative to its mass and motion, is typically used to predict motion caused by a given set of forces or to determine the forces required to produce a given motion.1 Intrinsic knee joint kinematics are described by the concave-convex principle2 as rolling and gliding motions at opposing joint surfaces. The concave-convex principle asserts that when a concave surface (e.g., the tibial plateau) rotates about a convex surface (e.g., the femoral condyle), rolling and gliding occur in the same directions. Conversely, rolling and gliding occur in opposite directions when a convex surface rotates about a concave surface. This principle is supported by in vivo3 and in vitro4 studies of knee motion demonstrating that during NWB knee flexion the tibia's concave surface both rolls and glides in the same posterior direction; whereas during WB knee flexion the convex femoral condyles roll posteriorly while concurrently gliding anteriorly on the tibia. During sagittal-plane flexion and extension, the radii of curvature of the femoral condyles and tibial plateau in the human knee joint differ. This results in a change in the location of the knee's instant center of rotation (ICR), defined as an instantaneous point of zero linear velocity about which rotation of one segment occurs relative to another. The pattern of change in location of the ICR is referred to as the pathway of instant centers of rotation (PICR). When a greater amount of rolling relative to gliding occurs between the joint surfaces at any instant during joint motion, the ICR moves closer to the point of contact between the two surfaces.5 Conversely, when less rolling relative to gliding occurs, the ICR location moves further from the contact point. Therefore, at any instant during knee joint motion in which combined rolling and gliding occurs, the ICR should lie between the center of joint rotation and the femoral joint surface. Mathematically, the relative proportion of rolling to gliding can be estimated provided that the segments' radii of curvature and ICR locations are known. The slip ratio quantifies the rolling/gliding relationship6 and is calculated by the equation: slip ratio = Sm/Sf where Sm is the displacement (arc length) from an initial contact point to the next contact point on the convex surface and Sf is the displacement (arc length) from an initial contact point to the next contact point on the concave surface (Figure 1). Pure rolling at the joint surface is indicated by a slip ratio of 1, and pure gliding is indicated by a slip ratio of zero (Figures 1 and 2). When pure rolling occurs at the joint surface, the slip ratio equals 1 (Figure 2A). During rotation without translation, the slip ratio is undefined (infinite). In this case, the ICR remains coincident with the center of the radius of curvature (i.e., rotation in place about a fixed axis) (Figure 2B). When there is no rotation but translation is present, pure gliding occurs (Figure 2C). In this case, the convex contact point is stationary while the flat or concave contact point undergoes displacement (i.e., it is greater than zero). This results in a slip ratio equal to zero. PICR patterns of unimpaired knees during NWB knee flexion remain to be clarified. While some researchers have demonstrated PICR patterns that suggest an equal relationship between rolling and gliding at the joint surfaces,5 other PICR patterns have suggested that the relative proportion of rolling to gliding varies with the degree of knee flexion.7 There is also evidence that knee joint kinematics may change with the different mechanical demands of WB movement due to the effects of superincumbent body weight and increased agonist/antagonist muscle activity as measured with electromyography. EMG activity levels of relevant muscles have been studied to provide insight about variations in muscle force and their contributions to joint compression force.8 A feature of intrinsic joint kinematics is that as joint compression increases, joint surface geometry appears to regulate relative joint surface rolling and gliding due to the constraint of the osseous geometry. In contrast, as joint compression decreases, the articular ligaments appear to play a greater role in regulating joint surface rolling and gliding.9 There is a paucity of literature describing the effect of gender differences or pathologic conditions such as anterior cruciate ligament deficiency on the PICR. Moreover, no published studies to date have addressed muscle activation patterns and their relationship to PICR patterns or intrinsic joint kinematics of rolling and gliding. However, there is evidence that abnormal rolling/gliding kinematics may have a deleterious effect on the long-term function of the knee joint. One study, using a knee sagittal-plane computer model, demonstrated that excessive rolling was associated with increased joint traction10 whereas Blunn et al11 found that excessive gliding was associated with loss of joint stability. These studies suggest that the ability to measure intrinsic joint kinematics may enable prediction of future negative consequences of abnormal joint motion. More important, the effect of therapeutic interventions (e.g., muscle training, bracing, ligament reconstruction) on rolling/gliding kinematics may be objectively quantified. The effect of gender The literature regarding gender differences in lower extremity functional activities is limited. Most studies evaluating gender effects have focused on the sport performance characteristics of men and women as possible etiologic factors in knee injuries, especially ACL tears. Reports suggest that the ACL injury rate is two to eight times higher in women than in men participating in the same athletic activities.16 Speculation on the possible etiology of ACL injury in women is considerable. Briefly, these risk factors are characterized as either intrinsic or extrinsic to the knee joint.17 Intrinsic factors may include differences in the quadriceps angle (Q angle), femoral notch size, joint laxity, and hormonal influences. Extrinsic factors may include differences in muscle strength, neuromuscular activation patterns, passive and active knee stiffness, and sport performance characteristics. The relationship between passive joint laxity, dynamic anterior tibial displacement, and joint injury is not clear. While some investigators have reported that knee joint laxity correlates with ACL injury,18,19 other investigators have suggested that ACL injury cannot be predicted by knee joint laxity.20 Nevertheless, it is well understood that increased anterior tibial displacement is correlated with increased ACL strain and may subject the ACL to greater risk of injury. During dynamic knee extension, anterior tibial displacement occurs when there is greater joint surface gliding relative to rolling at the tibiofemoral joint surface. Therefore, tibiofemoral joint surface gliding measured under dynamic conditions may be more indicative of ACL strain and the potential for sustaining an ACL injury than passive laxity (i.e., knee arthrometer) measures. Moreover, quadriceps and hamstring activation patterns may also influence joint surface rolling/gliding kinematics and dynamic knee joint stability.21 In an attempt to clarify these issues, we studied the differences in knee joint surface rolling/gliding kinematics between men and women during NWB (open kinetic chain) and WB (closed kinetic chain) knee extension movement conditions.22 We also evaluated muscle activity differences between men and women in an attempt to establish a relationship between muscle activity and rolling/gliding kinematics. In this study, 11 healthy adults (six men and five women) with an average age of 24 years underwent kinematic testing using videographic motion analysis during two movement conditions: a seated, OKC knee extension and a CKC sit-to-stand knee extension. Participants performed the movements through a range of approximately 100 degrees of knee flexion to terminal knee extension. This method of ICR measurement, based on hinge joint model calculations, yields a mean error rate of <1 mm,23 and a reliability coefficient >0.8 when applied to the human knee joint.24 Electromyographic signals were collected with bipolar surface electrodes from the quadriceps (vastus lateralis) and medial hamstrings (semitendinosus) during the testing conditions and normalized to maximal voluntary isometric contractions. The resultant PICR patterns of male and female subjects performing the OKC and CKC movements are illustrated in Figure 3. On visual analysis, we observed that the PICR in the female subjects during both movement conditions is located further from the joint surface than in the men, particularly as the knee approaches terminal extension in CKC knee extension. The female PICR patterns suggest that knee joint surface kinematics are characterized by greater joint surface gliding than occurs in men. Comparison of rolling/gliding kinematics as a function of movement condition showed that OKC knee extension did not differ significantly between male and female subjects. However, during CKC knee extension, rolling/gliding kinematics did differ significantly between genders. During terminal extension, the greater gliding than rolling in women indicates the occurrence of anterior tibial translation; the greater rolling than gliding in men indicates a lack of tibial translation. Male and female participants demonstrated similar magnitudes and activation patterns of quadriceps EMG within the OKC and CKC movement conditions (Figure 4). While men and women also demonstrated similar magnitudes and activation patterns of hamstring EMG during OKC knee extension, women did tend to perform CKC knee extension with a lower magnitude of hamstring EMG activation than men. Thus, female participants tended to experience anterior tibial translation during terminal knee extension in the CKC movement condition that was associated with relatively less hamstring activity for them than for their male counterparts. Implications for knee injury Our results suggest that a key difference in knee biomechanics between men and women is that women tend to demonstrate more joint surface gliding than rolling, indicating the occurrence of anterior tibial translation that is concomitant with relatively less hamstring activity during CKC knee extension, particularly as the knee approaches terminal extension. The association of anterior tibial translation and decreased hamstring activity may have implications regarding the increased incidence of ACL injury in female athletes. Unopposed quadriceps activity, particularly at terminal knee extension, causes increased anterior tibial displacement and subsequent ACL strain.25 Hamstring activity, on the other hand, works in sync with the ACL to prevent anterior tibial displacement, thereby reducing ACL strain.25 Huston and Wojtys26 reported that female athletes preferentially recruited their quadriceps as an initial response to a sudden anterior tibial displacement; whereas male athletes preferentially recruited their hamstrings. These authors postulated that this muscle response in women was an activation imbalance that could result in increased ACL strain, potentially rendering the ligament more susceptible to injury. Results of our study seem to concur with those of Huston and Wojtys. Collectively, these studies suggest that activities intended to limit anterior tibial displacement and joint surface gliding might be key to preventing ACL strain and, ultimately, ACL injury. From a biomechanical perspective, knee joint laxity, muscle stiffness (possibly related to muscle endurance), and sport performance characteristics may also contribute to knee injuries in female athletes. Knee joint laxity tends to be greater in women than men,27 although the relationship between joint laxity and joint injury is not fully understood. Muscle stiffness may counteract joint laxity and possibly contribute to knee stability and injury prevention by increasing joint compression forces and minimizing tibiofemoral translations. Markolf et al28 reported that a two- to fourfold increase in knee stiffness produced with isometric quadriceps and hamstring cocontraction yielded a 25% to 50% reduction in knee joint laxity. McNair and Marshall29 reported that active hamstring stiffness was positively correlated with functional knee scores. In neither investigation was gender studied. There is evidence, however, that female athletes have lower quadriceps and hamstring stiffness values around the knee than male athletes, which may contribute to the potential for joint injury.30 Differences in sport performance characteristics may also predispose female athletes to knee joint injury. Malinzak et al31 reported that female subjects, when landing from a jump, performed the maneuver with significantly less knee and hip flexion, greater knee valgus, and with less hamstring activation than male subjects. Wahi et al32 found that female athletes, during a crossover cut activity, exhibited greater internal tibial rotation during midstance and greater abduction moments in late stance compared to male athletes. The kinematic differences during sport performance reported in these studies, in addition to joint laxity and muscle stiffness differences, may predispose female athletes to ACL injury. Our study further suggests that biomechanical differences between genders do occur during knee extension. First, as the knee approaches terminal extension in the CKC movement condition, significantly greater knee joint surface gliding occurs in female subjects compared to male subjects. During CKC knee extension, men tend to roll into terminal extension whereas women tend to glide into extension, indicating that in female subjects terminal knee extension, in particular, is associated with greater anterior tibial displacement and, potentially, ACL strain. Second, in addition to the altered joint surface kinematics observed between genders, female athletes demonstrated less relative hamstring EMG activity during CKC knee extension. There appears to be a fundamental difference in knee joint surface rolling/gliding kinematics between male and female subjects during CKC knee extension-particularly at terminal extension-that is associated with a difference in hamstring activation. We believe that the higher proportion of joint surface gliding in female athletes compared to male athletes allows increased anterior tibial translation that probably increases ACL strain during CKC knee extension. This kinematic difference may make the ACL more susceptible to injury and may explain, in part, the higher incidence of noncontact ACL injuries observed in female athletes. Source: BioMechanics
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