The effect of hamstring muscle compensation for anterior laxity in the ACL-deficient knee during gait

The hamstring muscles have been recognized as an important element in compensating for the loss of stability in the ACL-deficient knee, but it is still not clear whether the hamstring muscle force can completely compensate for the loss of ACL, and the consequences of increased hamstring muscle force. A two-dimensional anatomical knee model in the sagittal plane was developed to examine the effect of various levels of hamstring muscle activation on restraining anterior tibial translation in the ACL-deficient knee during level walking. The model included the tibiofemoral and patellofemoral joints, four major ligaments, the medial capsule, and five muscle units surrounding the knee. Simulations were conducted to determine anterior tibial translation and internal joint loading at a single selected position when the knee was under a peak external flexion moment during early stance phase of gait. Incremental hamstring muscle forces were applied to the modeled normal and the ACL-deficient knees. Results of simulations showed that the ACL injury increased the anterior tibial translation by 11.8mm, while 56% of the maximal hamstring muscle force could reduce the anterior translation of the tibia to a normal level during the stance phase of gait. The consequences of increased hamstring muscle force included increased quadriceps muscle force and joint contact force.     

Differences in tibial rotation during walking in ACL reconstructed and healthy contralateral knees

This study tested the hypotheses that in patients with a successful anterior cruciate ligament (ACL) reconstruction, the internal–external rotation, varus–valgus, and knee flexion position of reconstructed knees would be different from uninjured contralateral knees during walking. Twenty-six subjects with unilateral ACL reconstructions (avg 31 years, 1.7 m, 68 kg, 15 female, 24 months past reconstruction) and no other history of serious lower limb injury walked at a self-selected speed in the gait laboratory, with the uninjured contralateral knee as a matched control. Kinematic measurements of tibiofemoral motion were made using a previously-described point-cluster technique. Repeated-measures ANOVA (α=0.017) was used to compare ACL-reconstructed knees to their contralateral knees at four distinct points during the stance phase of walking. An offset towards external tibial rotation in ACL-reconstructed knees was maintained over all time points (95%CI 2.3±1.3°). Twenty-two out of twenty-six individuals experienced an average external tibial rotation offset throughout stance phase. Varus–valgus rotation and knee flexion were not significantly different between reconstructed and contralateral knees. These findings show that differences in tibial rotation during walking exist in ACL reconstructed knees compared to healthy contralateral knees, providing a potential explanation why these patients are at higher risk of knee osteoarthritis in the long-term.     

Interactions between kinematics and loading during walking for the normal and ACL deficient knee

The relationships between extrinsic forces acting at the knee and knee kinematics were examined with the purpose of identifying specific phases of the walking cycle that could cause abnormal kinematics in the anterior cruciate ligament (ACL) deficient knee. Intersegmental forces and moments in directions that would produce anterior-posterior (AP) translation, internal-external (IE) rotation and flexion-extension (FE) at the knee were compared with the respective translation and rotations of the tibia relative to the femur during four selected phases (heel strike, weight acceptance, terminal extension and swing) of the walking cycle. The kinematic changes associated with loss of the ACL occurred primarily during the terminal portion of swing phase of the walking cycle where, for the ACL deficient knee, the tibia had reduced external rotation and anterior translation as the knee extended prior to heel strike. The kinematic changes during swing phase were associated with a rotational offset relative to the contralateral knee in the average position of the tibia towards internal rotation. The offset was maintained through the entire gait cycle. The abnormal offsets in the rotational position were correlated with the magnitude of the flexion moment (balanced by a net quadriceps moment) during weight acceptance. These results suggest that adaptations to the patterns of muscle firing during walking can compensate for kinematic changes associated with the loss of the ACL. The altered rotational position would cause changes in tibiofemoral contact during walking that could cause the type of degenerative changes reported in the meniscus and the articular cartilage following ACL injury.     

Comparison of kinematics in the healthy and ACL injured knee using MRI

Magnetic Resonance Imaging (MRI) was used to examine the characteristics of abnormal motion in the injured knee by mapping tibiofemoral contact. Eleven healthy subjects and 20 subjects with a unilateral ACL injury performed a leg-press against resistance. MRI scans of both knees at 15 degrees intervals from 0 degrees to 90 degrees of flexion were used to record the tibiofemoral contact pattern. The tibiofemoral contact pattern of the injured knees was more posterior on the tibial plateau than the healthy knees, particularly in the lateral compartment. The tibiofemoral contact pattern of the loaded knees did not differ from the unloaded knees. The difference in the tibiofemoral contact pattern in the ACL injured knee was associated with more severe knee symptoms, irrespective of the passive anterior laxity of the knee.     

Comparison of shear forces and ligament loading in the healthy and ACL-deficient knee during gait

The purpose of this study was to predict and explain the pattern of shear force and ligament loading in the ACL-deficient knee during walking, and to compare these results to similar calculations for the healthy knee. Musculoskeletal modeling and computer simulation were combined to calculate ligament forces in the ACL-deficient knee during walking. Joint angles, ground-reaction forces, and the corresponding lower-extremity muscle forces obtained from a whole-body dynamic optimization simulation of walking were input into a second three-dimensional model of the lower extremity that represented the knee as a six degree-of-freedom spatial joint. Anterior tibial translation (ATT) increased throughout the stance phase of gait when the model ACL was removed. The medial collateral ligament (MCL) was the primary restraint to ATT in the ACL-deficient knee. Peak force in the MCL was three times greater in the ACL-deficient knee than in the ACL-intact knee; however, peak force sustained by the MCL in the ACL-deficient knee was limited by the magnitude of the total anterior shear force applied to the tibia. A decrease in anterior tibial shear force was brought about by a decrease in the patellar tendon angle resulting from the increase in ATT. These results suggest that while the MCL acts as the primary restraint to ATT in the ACL-deficient knee, changes in patellar tendon angle reduce total anterior shear force at the knee.     

Can altered neuromuscular coordination restore soft tissue loading patterns in anterior cruciate ligament and menisci deficient knees during walking?

Injuries to the anterior cruciate ligament (ACL) and menisci commonly lead to early onset osteoarthritis. Treatments that can restore normative cartilage loading patterns may mitigate the risk of osteoarthritis, though it is unclear whether such a goal is achievable through conservative rehabilitation. We used musculoskeletal simulation to predict cartilage and ligament loading patterns during walking in intact, ACL deficient, menisci deficient, and ACL-menisci deficient knees. Stochastic simulations with varying coordination strategies were then used to test whether neuromuscular control could be modulated to restore normative knee mechanics in the pathologic conditions. During early stance, a 3 mm increase in anterior tibial translation was predicted in the ACL deficient knee. Mean cartilage contact pressure increased by 18% and 24% on the medial and lateral plateaus, respectively, in the menisci deficient knee. Variations in neuromuscular coordination were insufficient to restore normative cartilage contact patterns in either the ACL or menisci deficient knees. Elevated cartilage contact pressures in the pathologic knees were observed in regions where cartilage wear patterns have previously been reported. These results suggest that altered cartilage tissue loading during gait may contribute to region-specific degeneration patterns, and that varying neuromuscular coordination in isolation is unlikely to restore normative knee mechanics.     

Coupled motions under compressive load in intact and ACL-deficient knees: a cadaveric study

Knowledge of the coupled motions, which develop under compressive loading of the knee, is useful to determine which degrees of freedom should be included in the study of tibiofemoral contact and also to understand the role of the anterior cruciate ligament (ACL) in coupled motions. The objectives of this study were to measure the coupled motions of the intact knee and ACL-deficient knee under compression and to compare the coupled motions of the ACL-deficient knee with those of the intact knee. Ten intact cadaveric knees were tested by applying a 1600 N compressive load and measuring coupled internal-external and varus-valgus rotations and anterior-posterior and medial-lateral translations at 0 deg, 15 deg, and 30 deg of flexion. Compressive loads were applied along the functional axis of axial rotation, which coincides approximately with the mechanical axis of the tibia. The ACL was excised and the knees were tested again. In the intact knee, the peak coupled motions were 3.8 deg internal rotation at 0 deg flexion changing to -4.9 deg external rotation at 30 deg of flexion, 1.4 deg of varus rotation at 0 deg flexion changing to -1.9 deg valgus rotation at 30 deg of flexion, 1.4 mm of medial translation at 0 deg flexion increasing to 2.3 mm at 30 deg of flexion, and 5.3 mm of anterior translation at 0 deg flexion increasing to 10.2 mm at 30 deg of flexion. All changes in the peak coupled motions from 0 deg to 30 deg flexion were statistically significant (p<0.05). In ACL-deficient knees, there was a strong trend (marginally not significant, p=0.07) toward greater anterior translation (12.7 mm) than that in intact knees (8.0 mm), whereas coupled motions in the other degrees of freedom were comparable. Because the coupled motions in all four degrees of freedom in the intact knee and ACL-deficient knee are sufficiently large to substantially affect the tibiofemoral contact area, all degrees of freedom should be included when either developing mathematical models or designing mechanical testing equipment for study of tibiofemoral contact. The increase in coupled anterior translation in ACL-deficient knees indicates the important role played by the ACL in constraining anterior translation during compressive loading.     

Forces in anterior cruciate ligament during simulated weight-bearing flexion with anterior and internal rotational tibial load

This study determined in-vitro anterior cruciate ligament (ACL) force patterns and investigated the effect of external tibial loads on the ACL force patterns during simulated weight-bearing knee flexions. Nine human cadaveric knee specimens were mounted on a dynamic knee simulator, and weight-bearing knee flexions with a 100N of ground reaction force were simulated; while a robotic/universal force sensor (UFS) system was used to provide external tibial loads during the movement. Three external tibial loading conditions were simulated, including no external tibial load (termed BW only), a 50N anterior tibial force (ATF), and a 5Nm internal rotation tibial torque (ITT). The tibial and femoral kinematics was measured with an ultrasonic motion capture system. These movement paths were then accurately reproduced on a robotic testing system, and the in-situ force in the ACL was determined via the principle of superposition. The results showed that the ATF significantly increased the in-situ ACL force by up to 60% during 0-55 degrees of flexion, while the ITT did not. The magnitude of ACL forces decreased with increasing flexion angle for all loading conditions. The tibial anterior translation was not affected by the application of ATF, whereas the tibial internal rotation was significantly increased by the application of ITT. These data indicate that, in a weight-bearing knee flexion, ACL provides substantial resistance to the externally applied ATF but not to the ITT.     

Extent and distribution of tibial osteochondral disruption during simulated landing impact with axial tibial rotation restraint

Post-traumatic knee osteochondral injuries are often coupled with anterior cruciate ligament (ACL) injury mechanisms during landing. However, it is not well understood whether restraining axial tibial rotation during landing would influence the extent and distribution of osteochondral disruption. Using ski landing as an example, this study subjected knee specimens to simulated landing impact without and with axial tibial rotation restraint, and investigated the extent and distribution of osteochondral disruption at the tibial plateau. Twenty-one porcine knee specimens were randomly divided into three test conditions, namely: (1) control, (2) impact only (I), and 3) impact with restraint (IR). Simulated landing impact was applied to the specimens based on a single 10 Hz haversine. Osteochondral explants were obtained from anterior, middle and posterior regions of medial and lateral tibial compartments. The extent of cartilage and trabecular disruption in these explants was examined based on histology, SEM and microCT. Only specimens in unrestrained condition incurred ACL failure upon impact. Restraining axial tibial rotation during simulated impact generally inflicted cartilage damage and deformation, and further caused trabecular disruption. Axial tibial rotation restraint did not necessarily restrict anterior tibial translation, as indicated by the presence of relative posterior femoral translation and osteochondral disruption at anterior–posterior tibial regions. While the results obtained in the current study may not be completely translatable to human models, there is likelihood that restraining axial tibial rotation during landing may help to prevent ACL failure, but will also induce osteochondral disruption in most tibial regions.     

Role of gastrocnemius activation in knee joint biomechanics: gastrocnemius acts as an ACL antagonist

Gastrocnemius is a premier muscle crossing the knee, but its role in knee biomechanics and on the anterior cruciate ligament (ACL) remains less clear when compared to hamstrings and quadriceps. The effect of changes in gastrocnemius force at late stance when it peaks on the knee joint response and ACL force was initially investigated using a lower extremity musculoskeletal model driven by gait kinematics—kinetics. The tibiofemoral joint under isolated isometric contraction of gastrocnemius was subsequently analyzed at different force levels and flexion angles (0°–90°). Changes in gastrocnemius force at late stance markedly influenced hamstrings forces. Gastrocnemius acted as ACL antagonist by substantially increasing its force. Simulations under isolated contraction of gastrocnemius confirmed this role at all flexion angles, in particular, at extreme knee flexion angles (0° and 90°). Constraint on varus/valgus rotations substantially decreased this effect. Although hamstrings and gastrocnemius are both knee joint flexors, they play opposite roles in respectively protecting or loading ACL. Although the quadriceps is also recognized as antagonist of ACL, at larger joint flexion and in contrast to quadriceps, activity in gastrocnemius substantially increased ACL forces (anteromedial bundle). The fact that gastrocnemius is an antagonist of ACL should help in effective prevention and management of ACL injuries.     

In vivo knee moments and shear after total knee arthroplasty

Tibiofemoral loading is very important in cartilage degeneration as well as in component survivorship after total knee arthroplasty. We have previously reported the axial knee forces in vivo. In this study, a second-generation force-sensing device that measured all six components of tibial forces was implanted in a 74-kg, 83-year-old male. Video motion analysis, ground reaction forces, and knee forces were measured during walking, stair climbing, chair-rise, and squat activities. Peak total force was 2.3 times body weight (BW) during walking, 2.5 x BW during chair rise, 3.0 x BW during stair climbing, and 2.1 x BW during squatting. Peak anterior shear force at the tibial tray was 0.30 x BW during walking, 0.17 x BW during chair rise, 0.26 x BW during stair climbing, and 0.15 x BW during squatting. Peak flexion moment at the tray was 1.9% BW x Ht (percentage of body weight multiplied by height) for chair-rise activity and 1.7% BW x Ht for squat activity. Peak adduction moment at the tray was -1.1% BW x Ht during chair-rise, -1.3% BW x Ht during squatting. External knee flexion and adduction moments were substantially greater than flexion and adduction moments at the tray. The axial component of forces predominated especially during the stance phase of walking. Shear forces and moments at the tray were very modest compared to total knee forces. These findings indicate that the soft tissues around the knee absorbed most of the external shear forces. Our results highlight the importance of direct measurements of knee forces.     

Pattern of anterior cruciate ligament force in normal walking

The goal of this study was to calculate and explain the pattern of anterior cruciate ligament (ACL) loading during normal level walking. Knee-ligament forces were obtained by a two-step procedure. First, a three-dimensional (3D) model of the whole body was used together with dynamic optimization theory to calculate body-segmental motions, ground reaction forces, and leg-muscle forces for one cycle of gait. Joint angles, ground reaction forces, and muscle forces obtained from the gait simulation were then input into a musculoskeletal model of the lower limb that incorporated a 3D model of the knee. The relative positions of the femur, tibia, and patella and the forces induced in the knee ligaments were found by solving a static equilibrium problem at each instant during the simulated gait cycle. The model simulation predicted that the ACL bears load throughout stance. Peak force in the ACL (303 N) occurred at the beginning of single-leg stance (i.e., contralateral toe off). The pattern of ACL force was explained by the shear forces acting at the knee. The balance of muscle forces, ground reaction forces, and joint contact forces applied to the leg determined the magnitude and direction of the total shear force acting at the knee. The ACL was loaded whenever the total shear force pointed anteriorly. In early stance, the anterior shear force from the patellar tendon dominated the total shear force applied to the leg, and so maximum force was transmitted to the ACL at this time. ACL force was small in late stance because the anterior shear forces supplied by the patellar tendon, gastrocnemius, and tibiofemoral contact were nearly balanced by the posterior component of the ground reaction.     

Analysis of three-dimensional knee kinematics during stair descent two decades post-ACL rupture – Data revisited using statistical parametric mapping

Changes in movement patterns following knee injuries have generally used analyses of pre-defined discrete event-related variables, whereas Statistical Parametric Mapping (SPM) assesses continuous data over time. We applied SPM to test differences for knee trajectories during stair descent between participants with past anterior cruciate ligament (ACL) rupture who underwent reconstruction or only physical therapy compared to healthy controls. Three-dimensional knee joint kinematics during stair descent were registered for 31 subjects with ACL reconstruction (ACL_R), 36 subjects with ACL rupture managed with physical therapy only (ACL_PT) (∼23 years post-injury), and 32 uninjured controls. SPM was used to assess differences between groups for the entire three-component knee trajectory. A significant difference between the three groups was found for the first ∼10% of stance phase. Post-hoc analyses showed between-group differences when comparing the ACL_PT to the control groups. Analyses of ACL_PT versus control groups for individual vector components suggested a combination of less flexion at initial foot contact, and less adduction during weight acceptance (∼40% of stance). Altered knee kinematics were confirmed during weight acceptance of stair descent for the ACL_PT group compared to controls, but not for ACL_R group. Further exploration of the use of SPM and agreement with clinical gait assessment is warranted.     

The importance of quadriceps and hamstring muscle loading on knee kinematics and in-situ forces in the ACL

This study investigated the effect of hamstring co-contraction with quadriceps on the kinematics of the human knee joint and the in-situ forces in the anterior cruciate ligament (ACL) during a simulated isometric extension motion of the knee. Cadaveric human knee specimens (n = 10) were tested using the robotic universal force moment sensor (UFS) system and measurements of knee kinematics and in-situ forces in the ACL were based on reference positions on the path of passive flexion/extension motion of the knee. With an isolated 200 N quadriceps load, the knee underwent anterior and lateral tibial translation as well as internal tibial rotation with respect to the femur. Both translation and rotation increased when the knee was flexed from full extension to 30 of flexion; with further flexion, these motion decreased. The addition of 80 N antagonistic hamstrings load significantly reduced both anterior and lateral tibial translation as well as internal tibial rotation at knee flexion angles tested except at full extension. At 30 of flexion, the anterior tibial translation, lateral tibial translation, and internal tibial rotation were significantly reduced by 18, 46, and 30%, respectively (p<0.05). The in-situ forces in the ACL under the quadriceps load were found to increase from 27.8+/-9.3 N at full extension to a maximum of 44.9+/-13.8 N at 15 of flexion and then decrease to 10 N beyond 60 of flexion. The in-situ force at 15 was significantly higher than that at other flexion angles (p<0.05). The addition of the hamstring load of 80 N significantly reduced the in-situ forces in the ACL at 15, 30 and 60 of flexion by 30, 43, and 44%, respectively (p<0.05). These data demonstrate that maximum knee motion may not necessarily correspond to the highest in-situ forces in the ACL. The data also suggest that hamstring co-contraction with quadriceps is effective in reducing excessive forces in the ACL particularly between 15 and 60 of knee flexion.     

Combined measurement and modeling of specimen-specific knee mechanics for healthy and ACL-deficient conditions

Quantifying the mechanical environment at the knee is crucial for developing successful rehabilitation and surgical protocols. Computational models have been developed to complement in vitro studies, but are typically created to represent healthy conditions, and may not be useful in modeling pathology and repair. Thus, the objective of this study was to create finite element (FE) models of the natural knee, including specimen-specific tibiofemoral (TF) and patellofemoral (PF) soft tissue structures, and to evaluate joint mechanics in intact and ACL-deficient conditions. Simulated gait in a whole joint knee simulator was performed on two cadaveric specimens in an intact state and subsequently repeated following ACL resection. Simulated gait was performed using motor-actuated quadriceps, and loads at the hip and ankle. Specimen-specific FE models of these experiments were developed in both intact and ACL-deficient states. Model simulations compared kinematics and loading of the experimental TF and PF joints, with average RMS differences [max] of 3.0° [8.2°] and 2.1° [8.4°] in rotations, and 1.7 [3.0] and 2.5 [5.1] mm in translations, for intact and ACL-deficient states, respectively. The timing of peak quadriceps force during stance and swing phase of gait was accurately replicated within 2° of knee flexion and with an average error of 16.7% across specimens and pathology. Ligament recruitment patterns were unique in each specimen; recruitment variability was likely influenced by variations in ligament attachment locations. ACL resections demonstrated contrasting joint mechanics in the two specimens with altered knee motion shown in one specimen (up to 5 mm anterior tibial translation) while increased TF joint loading was shown in the other (up to 400 N).     

Correlation between translational and rotational kinematic abnormalities and osteoarthritis-like damage in two in vivo sheep injury models

The relations between kinematic abnormalities and post traumatic osteoarthritis have not yet been clearly elucidated. This study was conducted to determine the finite helical axes parameters and the tibiofemoral translation vector in the knee joints of two surgically induced injury sheep models: anterior cruciate ligament and medial collateral ligament transection (ACL/MCL Tx) (n = 5) and lateral meniscectomy (n = 5). We hypothesized that morphological damage in the experimental joints would be correlated to alterations in these kinematic variables. There was no strong evidence that morphological damage to the joints 20 weeks post ACL/MCL transection or meniscectomy was correlated with alterations in the finite helical axes variables. Nevertheless, significant correlations were found between the morphological damage to the joints and the magnitude of the change in the translation vectors after ACL/MCL transection (significant correlations (p = 0.005) during stance and trends (p < 0.1) at all points analyzed during swing). It can be concluded that: (1) osteoarthritic-like morphological damage after ACL/MCL transection is more critically correlated to the absolute tibiofemoral translational change and (2) alterations in analyzed kinematic variables cannot solely define osteoarthritis risk after meniscal injuries. From a clinical perspective, our results suggest that the magnitude of the change in the translation vector, which is independent of the coordinate system and combines the effects of the three translational degrees of freedom, i.e. medial–lateral, anterior-posterior and inferior-superior, would be an osteoarthritis risk factor after ligament injury, and requires validation in humans.     

The effects of femoral graft placement on in vivo knee kinematics after anterior cruciate ligament reconstruction

Achieving anatomical graft placement remains a concern in Anterior Cruciate Ligament (ACL) reconstruction. The purpose of this study was to quantify the effect of femoral graft placement on the ability of ACL reconstruction to restore normal knee kinematics under in vivo loading conditions. Two different groups of patients were studied: one in which the femoral tunnel was placed near the anterior and proximal border of the ACL (anteroproximal group, n=12) and another where the femoral tunnel was placed near the center of the ACL (anatomic group, n=10) MR imaging and biplanar fluoroscopy were used to measure in vivo kinematics in these patients during a quasi-static lunge. Patients with anteroproximal graft placement had up to 3.4mm more anterior tibial translation, 1.1mm more medial tibial translation and 3.7° more internal tibial rotation compared to the contralateral side. Patients with anatomic graft placement had motion that more closely replicated that of the intact knee, with anterior tibial translation within 0.8mm, medial tibial translation within 0.5mm, and internal tibial rotation within 1°. Grafts placed anteroproximally on the femur likely provide insufficient restraint to these motions due to a more vertical orientation. Anatomical femoral placement of the graft is more likely to reproduce normal ACL orientation, resulting in a more stable knee. Therefore, achieving anatomical graft placement on the femur is crucial to restoring normal knee function and may decrease the rates of joint degeneration after ACL reconstruction.     

Excessive compression of the human tibio-femoral joint causes ACL rupture

The knee is one of the most frequently injured joints in the human body. A recent study suggests that axial compressive loads on the knee may play a role in injury to the anterior cruciate ligament (ACL) for the flexed knee, because of an approximate 10 degrees posterior tilt in the tibial plateau (J. Orthop. Res. 16 (1998) 122-127). The hypothesis of the current study was that excessive axial compressive loads in the human tibio-femoral (TF) joint would cause relative displacement and rotation of the tibia with respect to the femur, and result in isolated injury to the ACL when the knee is flexed to 60 degrees , 90 degrees or 120 degrees . Sixteen isolated knees from eleven fresh cadaver donors (74.3+/-10.5 yr) were exposed to repetitive TF compressive loads increasing in intensity until catastrophic injury. ACL rupture was documented in 14/16 cases. The maximum TF joint compressive force for ACL failure was 5.1+/-2.1 kN for all flexion angles combined. For the 90 degrees flexed knee, the injury occurred with a relative anterior displacement of 5.4+/-3.8mm, a lateral displacement of 4.1+/-1.4mm, and a 7.8+/-7.0 degrees internal rotation of the tibia with respect to the femur.     

Steeper posterior tibial slope markedly increases ACL force in both active gait and passive knee joint under compression

The role of the posterior tibial slope (PTS) in anterior cruciate ligament (ACL) risk of injury has been supported by many imaging studies but refuted by some in vitro works. The current investigation was carried out to compute the effect of ±5^o change in PTS on knee joint biomechanics in general and ACL force/strain in particular. Two validated finite element (FE) models of the knee joint were employed; one active lower extremity musculoskeletal model including a complex FE model of the knee joint driven by in vivo kinematics/kinetics collected in gait of asymptomatic subjects, and the other its isolated unconstrained passive tibiofemoral (TF) joint considered under 1400 N compression at four different knee flexion angles (0°–45°). In the TF model, the compression force was applied at the joint mechanical balance point causing no rotations in sagittal and frontal planes.     

Frequency content asymmetry of the isokinetic curve between ACL deficient and healthy knee

The torque-time curve patterns of concentric isokinetic knee extension in anterior cruciate ligament (ACL) deficient patients usually present mid-range irregularities associated with the level of anterior tibial translation. The purpose of this study was to compare the smoothness in isokinetic torque production between the ACL deficient and the healthy knee. Thirty ACL deficient soccer players performed bilaterally five trials of maximum concentric knee extension-flexion at 60 degrees /s on a Biodex dynamometer. The three middle trials (a total of six curves) were retained and submitted to further data processing. Maximum frequency values contained within the 90%, 95% and 99% level of the signal power were calculated for each extension and flexion curve. The frequency content of the ACL deficient side proved to be statistically higher compared to the intact side at all levels of the power spectrum. The percentage differences in the frequency content were 18.8%, 10.6% and 40.0% for knee extension, and 49.5%, 24.5% and 16.3% for knee flexion, for the respective power levels. This indicated higher oscillations and, therefore, more unstable mechanical output of the injured knee. An overall biological interpretation of the present results is based on the notion that disturbed motion is generally connected to poor level of joint functionality.