Qualitative visual assessment of the J-sign demonstrates high inter-rater reliability.

OBJECTIVES: To assess the inter- and intra-rater reliability of the classification of the J-sign as "large" versus "small or none" as compared to another two-level system ("present" versus "absent") and a three-level system ("large," "small," or "none") and to identify anatomical and patient factors associated with the presence of a large J-sign. METHODS: Forty patients (40 knees) with recurrent patellar instability were prospectively enrolled and recorded on video actively extending their knee while seating. Four raters classified patellar tracking on two separate occasions using three systems: 1) two groups: J-sign versus no J-sign; 2) three groups: large J-sign, small J-sign, or no J-sign; and 3) two groups: large J-sign versus small or no J-sign. The intra- and inter-rater reliability of each system was assessed using kappa statistics. Anatomical (trochlear dysplasia, tibial tubercle-trochlear groove (TT-TG) distance, patellar height) and patient (Beighton score) factors as well as Knee Injury and Osteoarthritis Outcome Score (KOOS) subscales were compared between patients with a large J-sign and patients with a small or no J-sign. RESULTS: Inter- and intra-rater reliability were found to be highest with the two-level classification system of a large J-sign versus a small or no J-sign (inter-rater kappa â€‹= â€‹0.76, intra-rater kappa â€‹= â€‹0.75). Patients with a large J-sign had more severe trochlear dysplasia as assessed with the sulcus angle (p â€‹= â€‹0.042) and were more likely to have a tight lateral retinaculum (p â€‹= â€‹0.032) and an elevated Beighton score (p â€‹= â€‹0.009). No significant differences in KOOS subscales were noted based on the presence of a large J-sign versus a small J-sign or no J-sign. CONCLUSION: Qualitative visual assessment of patellar tracking with the J-sign demonstrates substantial inter- and intra-rater reliability, particularly when utilizing a two-group classification system to identify knees with a large J-sign. Patients with a large J-sign demonstrate an increased incidence of a tight lateral retinaculum, generalized ligamentous laxity, and trochlear dysplasia. LEVEL OF EVIDENCE: Level III - cross-sectional study.

Elevated Posterior Tibial Slope Is Associated With Anterior Cruciate Ligament Reconstruction Failures: A Systematic Review and Meta-analysis.

PURPOSE: To evaluate the association of posterior tibial slope (PTS) with anterior cruciate ligament (ACL) reinjury following primary ACL reconstruction. METHODS: PubMed, Scopus, Embase, and CINAHL databases were searched from inception through March 1, 2021, to retrieve relevant studies. Comparative studies reporting PTS measurements in a cohort of patients experiencing ACL graft failure versus patients with intact primary ACL reconstruction or studies comparing patients undergoing revision ACL reconstruction versus primary ACL reconstruction were included for analysis. A random-effects model was used to calculate the overall standardized mean difference (SMD) between groups. The following inclusion criteria were used: English language; full text available; Level I, II, or III evidence; studies in humans; and skeletally mature patients. RESULTS: After we systematically screened 1,912 studies, 15 studies met the inclusion/exclusion criteria. Radiographic measurements were used in 6 studies reporting medial PTS in 411 ACL failures versus 2808 controls. Patients with ACL failure had significantly greater medial PTS compared with controls (SMD 0.50; 95% confidence interval [CI] 0.23-0.77; P < .001). Magnetic resonance imaging (MRI) was used in 9 studies reporting lateral PTS measurements in 641 patients with a failed ACL reconstruction compared with 705 controls. Seven of the MRI studies also measured medial PTS in 552 failures versus 641 controls. Patients with ACL failure had significantly greater lateral PTS on MRI (SMD 0.58; 95% CI 0.13-1.03; P = .012) and medial PTS on MRI (SMD 0.59; 95% CI 0.23-0.96; P = .001) compared with controls. CONCLUSIONS: The present meta-analysis demonstrated that patients with elevated PTS on radiographs and MRI are at increased risk for ACL graft failure after primary ACL reconstruction. LEVEL OF EVIDENCE: Level III, meta-analysis of Level III studies.

Association of Elevated Posterior Tibial Slope With Revision Anterior Cruciate Ligament Graft Failure in a Matched Cohort Analysis.

BACKGROUND: Elevated posterior tibial slope (PTS) has been identified as an important risk factor in anterior cruciate ligament (ACL) injuries and ACL graft failures. The cutoff value to recommend treatment with slope-reducing osteotomy remains unclear and is based on expert opinion and small case series. PURPOSE: (1) To determine whether there is a difference in PTS shown on lateral knee radiographs and magnetic resonance imaging (MRI) scans in a group of patients who experienced revision ACL graft failure versus a control group of patients who underwent successful revision ACL reconstruction, (2) to identify cutoff values of PTS measurements that predict risk of revision ACL graft failure, and (3) to examine whether there is a correlation between radiographic and MRI measurements of PTS. STUDY DESIGN: Case-control study; Level of evidence, 3. METHODS: A total of 38 patients who experienced revision ACL graft failure were identified from a revision ACL database. These patients were matched 1:1 by age, sex, and graft type to a group of 38 control patients who underwent revision ACL reconstruction with no evidence of graft failure at a minimum 2 years of follow-up. Medial and lateral PTS were measured by lateral knee radiographs and MRI scans of the affected limb. Demographics, surgical characteristics, and PTS were compared between the groups. The optimal cutoff values of medial and lateral PTS per radiographs and MRI scans for predicting increased risk of revision ACL graft failure were determined by receiver operating characteristic curves. Conditional multivariable logistic regression was used to assess the relative contribution of PTS cutoff values as a predictor of revision graft failure. RESULTS: The mean PTS values in the failure group were significantly higher than those in the control group on radiographs (medial, 13.2°± 2.9° vs 10.3°± 2.9°; P < .001; lateral, 12.9°± 3.0° vs 9.8°± 2.8°; P < .001) and MRI scans (medial, 7.2°± 3.1° vs 4.8°± 2.9°; P < .001; lateral, 8.4 ± 3.1° vs 5.9 ± 3.0°; P < .001). A radiographic medial PTS ≥14° had the highest increased risk of revision ACL graft failure with sensitivity equal to 50% and specificity to 92.1% (odds ratio, 18.71; 95% CI, 2.0-174.9; P = .01). CONCLUSION: Elevated PTS was a significant risk factor for revision ACL graft failure. Patients with radiographic medial PTS ≥14° had 18.7-times increased risk of revision ACL failure.

Is modular control related to functional outcomes in individuals with knee osteoarthritis and following total knee arthroplasty?

BACKGROUND: Individuals who undergo total knee arthroplasty (TKA) for treatment of knee osteoarthritis often experience suboptimal outcomes. Investigation of neuromuscular control strategies in these individuals may reveal factors that contribute to these functional deficits. The purpose of this pilot study was to determine the relationship between patient function and modular control during gait before and after TKA. METHODS: Electromyography data from 36 participants (38 knees) were collected from 8 lower extremity muscles on the TKA-involved limb during ≥5 over-ground walking trials before (n = 30), 6-months after (n = 26), and 24-months after (n = 13) surgery. Muscle modules were estimated using non-negative matrix factorization. The number of modules was determined from 500 resampled trials. RESULTS: A higher number of modules was related to better performance-based and patient-reported function before and 6-months after surgery. Participants with organization similar to healthy, age-matched controls trended toward better function 24-months after surgery, though these results were not statistically significant. We also observed plasticity in the participants' modular control strategies, with 100% of participants who were present before and 24-months after surgery (10/10) demonstrating changes in the number of modules and/or organization of at least 1 module. CONCLUSIONS: This pilot work suggests that functional improvements following TKA may initially present as increases in the number of modules recruited during gait. Subsequent improvements in function may present as improved module organization. NOTEWORTHY: This work is the first to characterize motor modules in TKA both before and after surgery and to demonstrate changes in the number and organization of modules over the time course of recovery, which may be related to changes in patient function. The plasticity of modular control following TKA is a key finding which has not been previously documented and may be useful in predicting or improving surgical outcomes through novel rehabilitation protocols.

Discover your potential: The influence of kinematics on a muscle's ability to contribute to the sit-to-stand transfer.

Existing methods for estimating how individual muscles contribute to a movement require extensive time and experimental resources. In this study we developed an efficient method for determining how changes to lower extremity joint kinematics affect the potential of individual muscles to contribute to whole-body center-of-mass vertical (support) and anteroposterior (progression) accelerations. A 4-link 2-dimensional model was used to assess the effect of kinematic changes on muscle function. Joint kinematics were systematically varied throughout ranges observed during the momentum transfer phase of the sit-to-stand transfer. Each muscle's potential to contribute to support and progression was computed and compared to simulated potentials estimated by traditional dynamic simulation methods for young adults and individuals with knee osteoarthritis. The new method required 4-10s to compute muscle potentials per kinematic state and computed potentials were consistent with simulated potentials. The new method identified differences in muscle potentials between groups due to kinematic differences, particularly decreased anterior pelvic tilt in young adults, and revealed kinematic and muscle strengthening modifications to increase support. The methods presented provide an efficient, systematic approach to evaluate how joint kinematic adjustments alter a muscle's ability to contribute to movement and can identify potential sources of pathologic movement and rehabilitation strategies.

Effects of age and knee osteoarthritis on the modular control of walking: A pilot study.

BACKGROUND: Older adults and individuals with knee osteoarthritis (KOA) often exhibit reduced locomotor function and altered muscle activity. Identifying age- and KOA-related changes to the modular control of gait may provide insight into the neurological mechanisms underlying reduced walking performance in these populations. The purpose of this pilot study was to determine if the modular control of walking differs between younger and older adults without KOA and adults with end-stage KOA. METHODS: Kinematic, kinetic, and electromyography data were collected from ten younger (23.5 ± 3.1 years) and ten older (63.5 ± 3.4 years) adults without KOA and ten adults with KOA (64.0 ± 4.0 years) walking at their self-selected speed. Separate non-negative matrix factorizations of 500 bootstrapped samples determined the number of modules required to reconstruct each participant's electromyography. One-way Analysis of Variance tests assessed the effect of group on walking speed and the number of modules. Kendall rank correlations (τb) assessed the association between the number of modules and self-selected walking speed. RESULTS: The number of modules required in the younger adults (3.2 ± 0.4) was greater than in the individuals with KOA (2.3 ± 0.7; p = 0.002), though neither cohorts' required number of modules differed significantly from the unimpaired older adults (2.7 ± 0.5; p ≥ 0.113). A significant association between module number and walking speed was observed (τb = 0.350, p = 0.021) and individuals with KOA walked significantly slower (0.095 ± 0.21 m/s) than younger adults (1.24 ± 0.15 m/s; p = 0.005). Individuals with KOA also exhibited altered module activation patterns and composition (which muscles are associated with each module) compared to unimpaired adults. CONCLUSION: These findings suggest aging alone may not significantly alter modular control; however, the combined effects of knee osteoarthritis and aging may together impair the modular control of gait.

Effects of Optimization Technique on Simulated Muscle Activations and Forces.

Two optimization techniques, static optimization (SO) and computed muscle control (CMC), are often used in OpenSim to estimate the muscle activations and forces responsible for movement. Although differences between SO and CMC muscle function have been reported, the accuracy of each technique and the combined effect of optimization and model choice on simulated muscle function is unclear. The purpose of this study was to quantitatively compare the SO and CMC estimates of muscle activations and forces during gait with the experimental data in the Gait2392 and Full Body Running models. In OpenSim (version 3.1), muscle function during gait was estimated using SO and CMC in 6 subjects in each model and validated against experimental muscle activations and joint torques. Experimental and simulated activation agreement was sensitive to optimization technique for the soleus and tibialis anterior. Knee extension torque error was greater with CMC than SO. Muscle forces, activations, and co-contraction indices tended to be higher with CMC and more sensitive to model choice. CMC's inclusion of passive muscle forces, muscle activation-contraction dynamics, and a proportional-derivative controller to track kinematics contributes to these differences. Model and optimization technique choices should be validated using experimental activations collected simultaneously with the data used to generate the simulation.

What are the effects of simulated muscle weakness on the sit-to-stand transfer?

Populations with lower extremity muscle weakness have difficulty performing the sit-to-stand (STS) transfer. The degree of weakness that can be tolerated before compromising the ability to perform this task is unknown. Using dynamic simulations, we investigated the effects of weakness before changes in kinematics/kinetics would be required. Lower extremity muscles were weakened globally and individually and muscle forces were re-estimated as the model tracked original task kinematics/kinetics. The STS transfer was sensitive to quadriceps and plantarflexor weakness, suggesting that strengthening these muscles or changing kinematics are essential for populations who have difficulty rising from a chair independently.

Perceived Instability Is Associated With Strength and Pain, Not Frontal Knee Laxity, in Patients With Advanced Knee Osteoarthritis.

BACKGROUND: Increased varus/valgus laxity and perceived knee instability are independently associated with poor outcomes in people with knee osteoarthritis. However, the relationship between laxity and perceived instability is unclear. OBJECTIVE: To assess whether knee extensor strength, pain, and knee laxity are related to perceived knee instability in patients with advanced knee osteoarthritis. METHODS: This was a secondary analysis of a prospective observational cohort study of 35 patients (24 female; mean ± SD age, 60 ± 8 years; body mass index, 33 ± 5 kg/m2) with knee osteoarthritis awaiting total knee arthroplasty (36 knees). Within 1 month before arthroplasty, we measured isometric knee extension strength and self-reported knee pain (using the Knee injury and Osteoarthritis Outcome Score pain subscale). Patients rated their perception of knee instability as moderate to severe (n = 20) or slight to none (n = 15 patients, n = 16 knees) using the Knee Outcome Survey. We measured intraoperative varus/valgus knee laxity. RESULTS: Lower knee extension strength (P = .01) and greater pain (P<.01) were associated with the perception of moderate to severe knee instability. Laxity was not related to perceived knee instability (P = .63). CONCLUSION: Knee extension strength and pain were associated with perceived instability in people with advanced osteoarthritis. Varus/valgus laxity was not related to perceived knee instability. LEVEL OF EVIDENCE: Level 2, prognostic. J Orthop Sports Phys Ther 2019;49(7):513-517. doi:10.2519/jospt.2019.8619.

Medially-stabilized total knee arthroplasty does not alter knee laxity and balance in cadaveric knees.

Instability after total knee arthroplasty (TKA) can lead to suboptimal outcomes and revision surgery. Medially-stabilized implants aim to more closely replicate normal knee motion than other implants following TKA, but no study has investigated knee laxity (motion under applied loads) and balance (i.e., difference in varus/valgus motion under load) following medially-stabilized TKA. The primary purposes of this study were to investigate how medially-stabilized implants change knee laxity in non-arthritic, cadaveric knees, and if it produces a balanced knee after TKA. Force-displacement data were collected on 18 non-arthritic cadaveric knees before and after arthroplasty using medially-stabilized implants. Varus-valgus and anterior-posterior laxity and varus-valgus balance were compared between native and medially-stabilized knees at 0°, 20°, 60°, and 90° under three different loading conditions. Varus-valgus and anterior-posterior laxities were not different between native and medially-stabilized knees under most testing conditions (p ≥ 0.068), but differences of approximately 2° less varus-valgus laxity at 20° of flexion and 4 mm more anterior-posterior laxity at 90° were present from native laxities (p < 0.017) Medially-stabilized implant balance had ≤1.5° varus bias at all flexion angles. Future studies should confirm if the consistent laxity afforded by the medially-stabilized implant is associated with better and more predictable postoperative outcomes. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:335-349, 2019.

Forces Generated by Vastus Lateralis and Vastus Medialis Decrease with Increasing Stair Descent Speed.

Stair descent (SD) is a common, difficult task for populations who are elderly or have orthopaedic pathologies. Joint torques of young, healthy populations during SD increase at the hip and ankle with increasing speed but not at the knee, contrasting torque patterns during gait. To better understand the sources of the knee torque pattern, we used dynamic simulations to estimate knee muscle forces and how they modulate center of mass (COM) acceleration across SD speeds (slow, self-selected, and fast) in young, healthy adults. The vastus lateralis and vastus medialis forces decreased from slow to self-selected speeds as the individual lowered to the next step. Since the vasti are primary contributors to vertical support during SD, they produced lower forces at faster speeds due to the lower need for vertical COM support observed at faster speeds. In contrast, the semimembranosus and rectus femoris forces increased across successive speeds, allowing the semimembranosus to increase acceleration downward and forward and the rectus femoris to provide more vertical support and resistance to forward progression as SD speed increased. These results demonstrate the utility of dynamic simulations to extend beyond traditional inverse dynamics analyses to gain further insight into muscle mechanisms during tasks like SD.

Interpreting Musculoskeletal Models and Dynamic Simulations: Causes and Effects of Differences Between Models.

With more than 29,000 OpenSim users, several musculoskeletal models with varying levels of complexity are available to study human gait. However, how different model parameters affect estimated joint and muscle function between models is not fully understood. The purpose of this study is to determine the effects of four OpenSim models (Gait2392, Lower Limb Model 2010, Full-Body OpenSim Model, and Full Body Model 2016) on gait mechanics and estimates of muscle forces and activations. Using OpenSim 3.1 and the same experimental data for all models, six young adults were scaled in each model, gait kinematics were reproduced, and static optimization estimated muscle function. Simulated measures differed between models by up to 6.5° knee range of motion, 0.012 Nm/Nm peak knee flexion moment, 0.49 peak rectus femoris activation, and 462 N peak rectus femoris force. Differences in coordinate system definitions between models altered joint kinematics, influencing joint moments. Muscle parameter and joint moment discrepancies altered muscle activations and forces. Additional model complexity yielded greater error between experimental and simulated measures; therefore, this study suggests Gait2392 is a sufficient model for studying walking in healthy young adults. Future research is needed to determine which model(s) is best for tasks with more complex motion.

Assessing the effect of football play on knee articular cartilage using delayed gadolinium-enhanced MRI of cartilage (dGEMRIC).

The prevalence of cartilage lesions is much higher in football athletes than in the general population. Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) has been shown to quantify regional variations of glycosaminoglycan (GAG) concentrations which is an indicator of early cartilage degeneration. The goal of this study is to determine whether dGEMRIC can be used to assess the influence in cartilage GAG concentration due to college level football play. Thirteen collegiate football players with one to four years of collegiate football play experience were recruited and both knee joints were scanned using a dedicated 8-channel phased array knee coil on a 3T MRI system. The contrast concentrations within cartilage were calculated based on the T1 values from dGEMRIC scans. No substantial differences were found in the contrast concentrations between the pre- and post-season across all the cartilage compartments. One year collegiate football players presented an average contrast concentration at the pre-season of 0.116±0.011mM and post-season of 0.116±0.011mM. In players with multiple years of football play, contrast uptake was elevated to 0.141±0.012mM at the pre-season and 0.139±0.012mM at the post-season. The pre-season 0.023±0.016mM and post-season 0.025±0.016mM increase in contrast concentration within the group with multiple years of experience presented with a >20% increase in contrast uptake. This may indicate the gradual, cumulative damage of football play to the articular cartilage over years, even though the effect may not be noticeable after a season of play. Playing collegiate football for a longer period of time may lead to cartilage microstructural alterations, which may be linked to early knee cartilage degeneration.

Relationships between varus-valgus laxity of the severely osteoarthritic knee and gait, instability, clinical performance, and function.

Increased varus-valgus laxity has been reported in individuals with knee osteoarthritis (OA) compared to controls. However, the majority of previous investigations may not report truly passive joint laxity, as their tests have been performed on conscious participants who could be guarding against motion with muscle contraction during laxity evaluation. The purpose of this study was to investigate how a measure of passive knee laxity, recorded when the participant is under anesthesia, is related to varus-valgus excursion during gait, clinical measures of performance, perceived instability, and self-reported function in participants with severe knee OA. We assessed passive varus-valgus knee laxity in 29 participants (30 knees) with severe OA, as they underwent total knee arthroplasty (TKA). Participants also completed gait analysis, clinical assessment of performance (6-min walk (6 MW), stair climbing test (SCT), isometric knee strength), and self-reported measures of function (perceived instability, Knee injury, and Osteoarthritis Outcome Score (KOOS) a median of 18 days before the TKA procedure. We observed that greater passive varus-valgus laxity was associated with greater varus-valgus excursion during gait (R2 = 0.34, p = 0.002). Significant associations were also observed between greater laxity and greater isometric knee extension strength (p = 0.014), farther 6 MW distance (p = 0.033) and shorter SCT time (p = 0.046). No relationship was observed between passive varus-valgus laxity and isometric knee flexion strength, perceived instability, or any KOOS subscale. The conflicting associations between laxity, frontal excursion during gait, and functional performance suggest a complex relationship between laxity and knee cartilage health, clinical performance, and self-reported function that merits further study. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1644-1652, 2017.

Age-Related Differences in Gait Kinematics, Kinetics, and Muscle Function: A Principal Component Analysis.

Age-related increased hip extensor recruitment during gait is a proposed compensation strategy for reduced ankle power generation and may indicate a distal-to-proximal shift in muscle function with age. Extending beyond joint level analyses, identifying age-related changes at the muscle level could capture more closely the underlying mechanisms responsible for movement. The purpose of this study was to characterize and compare muscle forces and induced accelerations during gait in healthy older adults with those of young adults. Simulations of one gait cycle for ten older (73.9 ± 5.3 years) and six young (21.0 ± 2.1 years) adults walking at their self-selected speed were analyzed. Muscle force and induced acceleration waveforms, along with kinematic, kinetic, and muscle activation waveforms, were compared between age-groups using principal component analysis. Simulations of healthy older adults had greater gluteus maximus force and vertical support contribution, but smaller iliacus force, psoas force, and psoas vertical support contribution. There were no age-group differences in distal muscle force, contribution, or ankle torque magnitudes. Later peak dorsiflexion and peak ankle angular velocity in older adults may have contributed to their greater ankle power absorption during stance. These findings reveal the complex interplay between age-related changes in neuromuscular control, kinematics, and muscle function during gait.

Tibiofemoral Osteoarthritis and Varus-Valgus Laxity.

The purpose of this study was to systematically review and synthesize the literature measuring varus-valgus laxity in individuals with tibiofemoral osteoarthritis (OA). Specifically, we aimed to identify varus-valgus laxity differences between persons with OA and controls, by radiographic disease severity, by frontal plane knee alignment, and by sex. We also aimed to identify if there was a relationship between varus-valgus laxity and clinical performance and self-reported function. We systematically searched for peer-reviewed original research articles in PubMed, Scopus, and CINAHL to identify all existing literature regarding knee OA and objective measurement of varus-valgus laxity in vivo. Forty articles were identified that met the inclusion criteria and data were extracted. Varus-valgus laxity was significantly greater in individuals with OA compared with controls in a majority of studies, while no study found laxity to be significantly greater in controls. Varus-valgus laxity of the knee was reported in persons with OA and varying degrees of frontal plane alignment, disease severity, clinical performance, and self-reported function but no consensus finding could be identified. Females with knee OA appear to have more varus-valgus laxity than males. Meta-analysis was not possible due to the heterogeneity of the subject populations and differences in laxity measurement devices, applied loading, and laxity definitions. Increased varus-valgus laxity is a characteristic of knee joints with OA. Large variances exist in reported varus-valgus laxity and may be due to differences in measurement devices. Prospective studies on joint laxity are needed to identify if increased varus-valgus laxity is a causative factor in OA incidence and progression.

Muscle co-contraction during gait in individuals with articular cartilage defects in the knee.

Increased muscle co-contraction during gait is common in individuals with knee pathology, and worrisome as it is known to amplify tibiofemoral compressive forces. While knees with articular cartilage defects (ACD) are more vulnerable to compressive forces, muscle co-contraction has never been reported in this population. The purpose of this study was to evaluate the extent to which individuals with ACDs in the knee demonstrate elevated quadriceps to hamstrings muscle co-contraction on the involved limb during gait compared to the uninvolved limb and to healthy controls. We also explored the impact of participant characteristics and knee impairments on co-contraction. Twenty-nine individuals with full-thickness knee ACDs (ACD group) and 19 healthy adults (control group) participated in this study. Participants performed five gait trials at self-selected speed, during which activity of the quadriceps and hamstrings muscles were collected with surface electromyography. Three-dimensional motion capture was used to define phases of gait. Quadriceps strength and self-reported outcomes were also assessed in the same session. There were no differences in quadriceps: hamstrings co-contraction between the ACD and control groups, or between the involved and uninvolved limb for the ACD group. For both ACD and control groups, co-contraction was highest in early stance and lowest in late stance. Quadriceps strength was consistently the strongest predictor of muscle co-contraction in both the ACD and the control groups, with individuals with lower strength demonstrating greater co-contraction. Further study is needed to understand the effect of increased muscle co-contraction on joint compressive forces in the presence of varied quadriceps strength.

Tibiofemoral joint subchondral surface conformity: Individual variability with race and sex-specific trends.

BACKGROUND: Femoral and tibial subchondral surface morphology has been extensively studied to aid in anatomically correct total knee arthroplasty (TKA) implant design. Emphasis has been placed on shape variations in individual bones, and person-to-person variability in joint conformity has been overlooked. The purpose of this study is to 1) determine individual variability in key measures of tibiofemoral joint conformity, and 2) determine whether variability differs by sex or race. METHODS: Laser-scanner-generated surface models of tibiofemoral joints were obtained from 165 archival skeletons (at death: age 28.8±7.6years; 85 African-American, 80 Caucasian, 86 men, 79 women). Ratios and correlations were determined among related femoral and tibial subchondral surface areas (SA), alignment, curvatures, and linear dimensions between opposing surfaces with stratification by race and sex. RESULTS: Anterior-posterior length (R=0.80, p<0.001) and medial-lateral width (R=0.93, p<0.001) were the only linear measures that were highly correlated between the femur and tibia. Tibial and femoral surface areas were correlated among Caucasian men only (R=0.58, p<0.001; R<0.20), with a wide range of surface area ratios regardless of sex or race (SA ratio total sample: 2.32±0.39, range 1.36 to 3.62). CONCLUSIONS: There is high individual variability in tibiofemoral joint conformity at the subchondral surface, and for some measures this variability is sex-or-race specific. Key measures of joint conformity including surface area, curvature, width, and depth covary weakly or not at all, and a wide range of TKA component sizes and shapes would be required to accurately replicate native joint conformity in most people.

Muscle Forces and Their Contributions to Vertical and Horizontal Acceleration of the Center of Mass During Sit-to-Stand Transfer in Young, Healthy Adults.

Sit-to-stand transfer is a common task that is challenging for older adults and others with musculoskeletal impairments. Associated joint torques and muscle activations have been analyzed two-dimensionally, neglecting possible three-dimensional (3D) compensatory movements in those who struggle with sit-to-stand transfer. Furthermore, how muscles accelerate an individual up and off the chair remains unclear; such knowledge could inform rehabilitation strategies. We examined muscle forces, muscleinduced accelerations, and interlimb muscle force differences during sit-to-stand transfer in young, healthy adults. Dynamic simulations were created using a custom 3D musculoskeletal model; static optimization and induced acceleration analysis were used to determine muscle forces and their induced accelerations, respectively. The gluteus maximus generated the largest force (2009.07 ± 277.31 N) and was a main contributor to forward acceleration of the center of mass (COM) (0.62 ± 0.18 m/s(2)), while the quadriceps opposed it. The soleus was a main contributor to upward (2.56 ± 0.74 m/s(2)) and forward acceleration of the COM (0.62 ± 0.33 m/s(2)). Interlimb muscle force differences were observed, demonstrating lower limb symmetry cannot be assumed during this task, even in healthy adults. These findings establish a baseline from which deficits and compensatory strategies in relevant populations (eg, elderly, osteoarthritis) can be identified.