Strategies of muscular support of varus and valgus isometric loads at the human knee

In this paper we studied how subjects activate their muscles in response to static varus and valgus loads at the knee. The muscles' contributions to the external moments were estimated using an EMG driven biomechanical model of the knee. The individual muscle activation and loading patterns were examined to identify the strategies that the nervous system uses to support varus and valgus knee moments. It was found that the (1) co-contraction of the hamstrings and quadriceps, and (2) activation of the gracilis and tensor fascia lata increased with the increasing magnitude of the varus and valgus moments. These 2 activation patterns provided positive support of valgus and varus loads at the knee The sartorius appears to be activated to provide positive support of valgus loads at the knee, whereas during varus moments this muscle increases the varus load on the knee, i.e. provides negative support. Generally, the hamstrings and quadriceps co-contraction contributed to most of the muscular support of the varus and valgus moments. In addition, co-contraction supported 11-14% of the external moment in pure varus and pure valgus respectively. It appears that there are activation strategies with the specific purpose to support varus and valgus moments, albeit small, which suggest dual goals of the neuromotor system during the support of varus and valgus moments.     

The effect of valgus/varus malalignment on load distribution in total knee replacements

Valgus or varus malpositioning of the tibial component of a total knee implant may cause increased propensity for loosening or implant wear and eventually may lead to revision surgery. The aim of this study was to determine the effect of valgus/varus malalignment on tibio-femoral mechanics during surgical trial reduction and simulated gait loading. In seven cadaver legs, posterior cruciate sparing total knee replacements were implanted and tibial inserts representing a neutral alignment and 3 degrees and 5 degrees varus and valgus alignments were sequentially inserted. Each knee with each insert was loaded in a manner representative of a trial reduction performed during knee surgery and loaded in a physiological knee simulator. Simulated gait performed on the simulator demonstrated that internal/external and adduction/abduction rotations showed statistical changes with some of the angled inserts at different points in the walking cycle. Neither medial/lateral nor anterior/posterior translations changed statistically during simulated walking. The pressure distribution and total load in the medial and lateral compartments of the tibial component changed significantly with as little as a 3 degrees variation in angulation when loaded in a manner representative of a trial reduction or with a knee simulator. These results support the need for precise surgical reconstruction of the mechanical axis of the knee and proper alignment of the tibial component. These results further demonstrate that tibial contact pressures measured during a trial reduction method may be predictive of contact mechanics at the higher loading seen in the knee simulator.     

The influence of knee varus and valgus on quadriceps muscle activity changes induced by stretching and kneeling

This study aimed to determine the influence of knee varus (VARUS) and valgus (VALGUS) on the differences in individual quadriceps muscle (QM) activity during knee extension maximum voluntary isometric contractions (MVICs) and sit/stand transitions and on the changes in individual QM activity during sit/stand transitions after QM stretching and kneeling. Ten young healthy males each with VARUS and VALGUS were included. The electromyography signals of the vastus medialis (VM), vastus lateralis, and rectus femoris were recorded during sit/stand transitions before and after rest, stretching, and kneeling and during knee extension MVICs after rest. The individual muscle-to-total muscle activity ratio was assessed. The VARUS group exhibited a significantly higher VM muscle activity ratio in the sit-to-stand and stand-to-sit tasks than in knee extension MVICs (p = 0.004 and p = 0.044, respectively) and a tendency that the VM muscle activity ratio increased in the sit-to-stand task after stretching (p = 0.051), whereas the VALGUS group exhibited no significance. Individuals with VARUS required high VM muscle activity ratios during sit/stand transitions. Future studies should be conducted to determine whether habitual sit-to-stand exercises after QM stretching are effective in preventing knee medial osteoarthritis development in individuals with VARUS.     

Rotational flexibility of the human knee due to varus/valgus and axial moments in vivo

Knee ligamentous injuries persist in the sport of Alpine skiing. To better understand the load mechanisms which lead to injury, pure varus/valgus and pure axial moments were applied both singly and in combination to the right knees of six human test subjects. The corresponding relative knee rotations in three degrees of freedom were measured. Knee flexion angles for each test subject were 15 and 60 degrees for the individual moments and 60 degrees for the combination moments. For both knee flexion angles the hip flexion angle was 0 degrees. Leg muscles were quiescent and axial force was minimal during all tests. Tables of data include sample statistics for each of four flexibility parameters in each loading direction. Data were analyzed statistically to test for significant differences in flexibility parameters between the test conditions. In flexing the knee from 15 to 60 degrees, the resulting knee rotations under single moments depended upon flexion angle with varus, valgus, and internal rotations increasing significantly. Also, rotations were different depending on load direction; varus rotation was significantly different and greater than valgus rotation at both flexion angles. Also external rotation was significantly different and greater than internal at 15 degrees flexion, but not at 60 degrees flexion. Coupled rotations under single moments were also observed. Applying pure varus/valgus moments resulted in coupled external/internal rotations which were inconsistent and hence not significant. Applying pure axial moments resulted in consistent and hence significant varus/valgus rotations; an external axial moment induced varus rotation and an internal axial moment induced valgus rotation. For combination moments, varus/valgus rotations decreased significantly from those rotations at similar load levels in the single moment studies. Also, a varus moment significantly increased external rotation and a valgus moment significantly decreased internal rotation. These differences indicate significant interaction between corresponding load combinations. These results suggest that load interaction is a potentially important phenomenon in knee injury mechanics.     

Contribution of the musculature to rotatory laxity and torsional stiffness at the knee

The relationships between the mean rectified EMG from two muscle groups crossing the knee joint and the rotational stiffness and laxity about the longitudinal axis of the lower leg were investigated. The EMG signals from three of the quadricep muscle group and two of the hamstring muscle group were monitored using surface electrodes. Each subject sustained self-induced muscle activity from specific muscle combinations while the foot was twisted internally and externally by the researcher. Joint rotation was measured using an electrogoniometer. Analyses of the data showed increased joint stiffness with increased numbers of active muscles. The stiffness measurements ranged from 0.16 to 2.54 Nm degree-1 depending upon the combination of active muscles. The stiffness measured in different tests were very repeatable with standard deviations ranging from 0.02 to 0.25 Nm degree-1. Increases in joint stiffness of over 400% by activation of these muscles were measured.     

Effect of joint laxity on polyethylene wear in total knee replacement

Experimental simulator studies are frequently performed to evaluate wear behavior in total knee replacement. It is vital that the simulation conditions match the physiological situation as closely as possible. To date, few experimental wear studies have examined the effects of joint laxity on wear and joint kinematics and the absence of the anterior cruciate ligament has not been sufficiently taken into account in simulator wear studies.The aim of this study was to investigate different ligament and soft tissue models with respect to wear and kinematics.A virtual soft tissue control system was used to simulate different motion restraints in a force-controlled knee wear simulator.The application of more realistic and sophisticated ligament models that considered the absence of anterior cruciate ligament lead to a significant increase in polyethylene wear (p=0.02) and joint kinematics (p<0.01). We recommend the use of more complex ligament models to appropriately simulate the function of the human knee joint and to evaluate the wear behavior of total knee replacements. A feasible simulation model is presented.     

Effect of varus/valgus malalignment on bone strains in the proximal tibia after TKR: an explicit finite element study

Malalignment is the main cause of tibial component loosening. Implants that migrate rapidly in the first two post-operative years are likely to present aseptic loosening. It has been suggested that cancellous bone stresses can be correlated with tibial component migration. A recent study has shown that patient-specific finite element (FE) models have the power to predict the short-term behavior of tibial trays. The stresses generated within the implanted tibia are dependent on the kinematics of the joint; however, previous studies have ignored the kinematics and only applied static loads. Using explicit FE, it is possible to simultaneously predict the kinematics and stresses during a gait cycle. The aim of this study was to examine the cancellous bone strains during the stance phase of the gait cycle, for varying degrees of varus/valgus eccentric loading using explicit FE. A patient-specific model of a proximal tibia was created from CT scan images, including heterogeneous bone properties. The proximal tibia was implanted with a commercial total knee replacement (TKR) model. The stance phase of gait was simulated and the applied loads and boundary conditions were based on those used for the Stanmore knee simulator. Eccentric loading was simulated. As well as examining the tibial bone strains (minimum and maximum principal strain), the kinematics of the bone-implant construct are also reported. The maximum anterior-posterior displacements and internal-external rotations were produced by the model with 20 mm offset. The peak minimum and maximum principal strain values increased as the load was shifted laterally, reaching a maximum magnitude for -20 mm offset. This suggests that when in varus, the load transferred to the bone is shifted medially, and as the bone supporting this load is stiffer, the resulting peak bone strains are lower than when the load is shifted laterally (valgus). For this particular patient, the TKR design analyzed produced the highest cancellous bone strains when in valgus. This study has provided an insight in the variations produced in bone strain distribution when the axial load is applied eccentrically. To the authors' knowledge, this is the first time that the bone strain distribution of a proximal implanted tibia has been examined, also accounting for the kinematics of the tibio-femoral joint as part of the simulation. This approach gives greater insight into the overall performance of TKR.     

Muscular resistance to varus and valgus loads at the elbow.

Although the contributions of passive structures to stability of the elbow have been well documented, the role of active muscular resistance of varus and valgus loads at the elbow remains unclear. We hypothesized that muscles: (1) can produce substantial varus and valgus moments about the elbow, and (2) are activated in response to sustained varus and valgus loading of the elbow. To test the first hypothesis, we developed a detailed musculoskeletal model to estimate the varus and valgus moment-generating capacity of the muscles about the elbow. To test the second hypothesis, we measured EMGs from 11 muscles in four subjects during a series of isometric tasks that included flexion, extension, varus, and valgus moments about the elbow. The EMG recordings were used as inputs to the elbow model to estimate the contributions of individual muscles to flexion-extension and varus-valgus moments. Analysis of the model revealed that nearly all of the muscles that cross the elbow are capable of producing varus or valgus moments; the capacity of the muscles to produce varus moment (34 Nm) and valgus moment (35 Nm) is roughly half of the maximum flexion moment (70 Nm). Analysis of the measured EMGs showed that the anconeus was the most significant contributor to valgus moments and the pronator teres was the largest contributor to varus moments. Although our results show that muscles were activated in response to static varus and valgus loads, their activations were modest and were not sufficient to balance the applied load.     

Effect of changes in cruciate ligaments pretensions on knee joint laxity and ligament forces

The knee joint cruciate ligaments are reconstructed with the rationale to avoid joint instability, recurrent injury, damage to soft tissues and osteoarthritis. Wide range of procedures with different stiffness, pretension, orientation and insertion locations have been proposed with the primary goal to restore the joint laxity. Apart from the general lack of success in preservation of force in the reconstructed ligament itself, the concern, not yet addressed, arises as to the effect of such perturbation on the other intact cruciate ligament. The interaction between cruciate ligament forces is hypothesized in this work. Using a 3-D nonlinear finite element model of the tibiofemoral joint, we examined this hypothesis by quantifying the extent of coupling between cruciate ligaments while varying the prestrain in each ligament under flexion with and without anterior-posterior (A-P) loads. A remarkable coupling was predicted between cruciate ligament forces in flexion thus confirming the hypothesis; forces in both cruciate ligaments increased as initial strain or pretension in one of them increased whereas they both diminished as one of them became slack. Moreover, changes in laxity and in ligament forces as a cruciate ligament prestrained or pretensioned varied with flexion angle and external loads. These findings have important consequences in joint functional biomechanics following a ligament injury or replacement surgery and in selection of laxity matched or ligament force matched pretensioning protocols.     

Effects of hand and knee positions on muscular activity during trunk extension exercise with the Roman chair

This experimental study was performed to investigate the effects of hand and knee positions on muscular activity during back extension exercises with the Roman chair. Eighteen asymptomatic male amateur athletes performed four prone back extension exercises with two hand positions (crossed-arms and behind-the-head), and two knee positions (extended knee and 90° flexed knee). Surface electromyography (sEMG) was performed to collect data from the lower trapezius (LT), latissimus dorsi (LD), erector spinae in the T12 paraspinal region (ES-T12), erector spinae at the L3 level (ES-L3), gluteus maximus (GM), and biceps femoris (BF). Two-way repeated analysis of variance with two within-subject factors (two hand positions and two knee positions) was used to determine the significance of differences between the exercise conditions, and which hand and knee positions resulted in greater activation with exercise variation. The root mean square sEMG values were normalized using the maximum voluntary isometric contraction (MVIC) and represented as the % of the maximum EMG (%mEMG). There was no significant interaction between knee and hand positions in the %mEMG data. The results showed that the hand position affected the normalized activation of LT; the behind-the-head position resulted in significantly greater muscle activation than the crossed-arms hand position (P < 0.05). The activations of the LD, ES-T10, ES-L4, and GM were greater in the 90° flexed-knee position compared to the extended-knee position (P < 0.05). Although back extension exercise using the Roman chair has been shown to effectively activate the extensor musculature, our results indicated that changing the knee and hand positions could activate specific muscles differently. To achieve greater activation of trunk extensor muscle during extension exercise with the Roman chair, the flexed-knee position is a useful means of increasing resistance.     

Effect of boot shaft stiffness on stability joint energy and muscular co-contraction during walking on uneven surface

Increased boot shaft stiffness may have a noticeable impact on the range of motion of the ankle joint. Therefore, the ability of the ankle joint to generate power for propulsion might be impaired. This might result in compensatory changes at the knee and hip joint. Besides, adaptability of the subtalar joint to uneven surface might be reduced, which could in turn affect stability. The aim of the study was therefore to investigate the influence of boot shaft stiffness on biomechanical gait parameters.Fifteen healthy young adults walked over coarse gravel wearing two different hiking boots that differed by 50% in passive shaft stiffness. Leg kinematics, kinetics and electromyography were measured. Gait velocity and indicators for stability were not different when walking with the hard and soft boot shaft over the gravel surface. However, the hard boot shaft decreased the ankle range of motion as well as the eccentric energy absorbed at the ankle joint. As a consequence, compensatory changes at the knee joint were observed. Co-contraction was increased, and greater eccentric energy was absorbed. Therefore, the efficiency of gait with hard boots might be decreased and joint loading at the knee might be increased, which might cause early fatigue of knee muscles during walking or hiking. The results of this study suggest that stiffness and blocking of joint motion at the ankle should not be equated with safety. A trade-off between lateral stiffness and free natural motion of the ankle joint complex might be preferable.     

The effect of the frontal plane tibiofemoral angle and varus knee moment on the contact stress and strain at the knee cartilage

Subject-specific models were developed and finite element analysis was performed to observe the effect of the frontal plane tibiofemoral angle on the normal stress, Tresca shear stress and normal strain at the surface of the knee cartilage. Finite element models were created for three subjects with different tibiofemoral angle and physiological loading conditions were defined from motion analysis and muscle force mathematical models to simulate static single-leg stance. The results showed that the greatest magnitude of the normal stress, Tresca shear stress and normal strain at the medial compartment was for the varus aligned individual. Considering the lateral knee compartment, the individual with valgus alignment had the largest stress and strain at the cartilage. The present investigation is the first known attempt to analyze the effects of tibiofemoral alignment during single-leg support on the contact variables of the cartilage at the knee joint. The method could be potentially used to help identify individuals most susceptible to osteoarthritis and to prescribe preventive measures.     

The conflicting requirements of laxity and conformity in total knee replacement

Bearing surfaces of total condylar knees which are designed with a high degree of conformity to produce low stresses in the polyethylene tibial insert may be overconstrained. This study determines femoral and tibial bearing surface geometries which will induce the least destructive fatigue mechanisms in the polyethylene whilst conserving the laxity of the natural knee. Sixteen knee designs were generated by varying four parameters systematically to cover the range of contemporary knee designs. The parameters were the femoral frontal radius (30 or 70 mm), the difference between the femoral and tibial frontal radii (2 or 10 mm), the tibial sagittal radius (56 or 80 mm) and the posterior-distal transition angle (-8 or -20 degrees), which is the angle at which the small posterior arc of the sagittal profile transfers to the larger distal arc. Rigid body analyses determined the anterior-posterior and rotational motions as well as the contact points during the stance phase of gait for the different designs. In addition, a damage function which accumulated the fluctuating maximum shear stresses was used to predict the susceptibility to delamination wear of the polyethylene (damage score). This study predicted that of the 16 designs, the knee with a frontal radius of 70 mm, a difference in femoral and tibial frontal radii of 2 mm, a tibial sagittal radius of 80 mm and a posterior distal transition angle of -20 degrees would satisfy the conflicting needs of both resistance to delamination wear and natural kinematics.     

Effect of muscular load on the invertase activity of the small intestine]

Acute experiments were conducted on male albino rats; a study was made of the invertase activity of the proximal, middle and distal portions of the small intestine after a 2-, 4- and 10-hour muscular load in the form of swimming in water at a temperature of 35 +/- 1 degree C. After 2 hours of swimming the invertase activity in the first two portions showed a rather sharp fall; it was restored to the initial level in 48--72 hours. This reduction was much less in the distal portion. Both the 4- and the 10-hour swimming led to an insignificant elevation of the enzymatic activity in all the three portions during 24 hours, with a subsequent decrease in the first two portions and a marked elevation in the distal portion after 48 hours. It is supposed that these changes were realized by means of the hypothalamo-hypophysio-adrenal system by the principle of the common nonspecific adaptive syndrome.     

Influence of a valgus knee brace on muscle activation and co-contraction in patients with medial knee osteoarthritis

The purpose of this study was to analyse the effect of a valgus knee orthosis designed for patients with knee osteoarthritis on the electromyographic activity (EMG) of seven muscles of the lower limb during gait. Twelve patients with medial knee osteoarthritis walked on a treadmill in three different conditions: without orthosis, with a knee orthosis in 4° valgus adjustment and with an orthosis in a neutral flexible adjustment. Root-mean-square (RMS) was analysed in each condition during a 150 ms pre-activation phase and during the stance phase of gait, which was divided in four sub-phases. In addition, co-contraction ratios (CCRs) were calculated between extensor/flexor, medial/lateral muscles and between agonist and antagonist muscle pairs. Significant decreases in muscle activity and CCRs were observed with the use of the knee orthosis in both adjustments compared to the condition without orthosis. Using the valgus brace, medial/lateral CCR decreased significantly during the late stance and the flexor/extensor CCR decreased significantly during the loading phase and late stance. Decreases of muscle pairs CCRs were observed with the neutral flexible adjustment. The results support the theory of a possible beneficial effect of knee braces in reducing knee loading by decreasing muscle activation and co-contraction levels, which could contribute to decelerate disease progression in patients with knee osteoarthritis.