A Biomechanical Review of the Squat Exercise: Implications for Clinical Practice.

The squat is one of the most frequently prescribed exercises in the rehabilitative setting. Performance of the squat can be modified by changing parameters such as stance width, foot rotation, trunk position, tibia position, and depth. An understanding of how the various squatting techniques can influence joint loading and muscular demands is important for the proper prescription of this exercise for various clinical conditions. The purpose of this clinical commentary is to discuss how the biomechanical demands of the squat can be influenced by various modifiable parameters. General recommendations for specific clinical conditions are presented. Level of Evidence: 5.

Reliability of a method to assess corticomotor excitability of lower limb muscles using a normalized EMG motor thresholding procedure.

Given the importance of determining intervention-induced neuroplastic changes with lower extremity functional tasks, a reliable transcranial magnetic stimulation (TMS) methodology for proximal lower extremity muscles is needed. A pre-set fixed voltage value is typically used as the criterion for identifying a motor evoked potential (MEP) during the motor thresholding procedure. However, the fixed voltage value becomes problematic when the procedure is applied to proximal lower extremity muscles where active contractions are required. We sought to establish the reliability of a method measuring corticomotor excitability of gluteus maximus and vastus lateralis using normalized electromyography (EMG) as the criterion for identifying MEPs during the motor thresholding procedure. The active motor threshold for each muscle was determined using the lowest stimulator intensity required to elicit 5 MEPs that exceeded 20% maximal voluntary isometric contraction from 10 stimulations. TMS data were obtained from 10 participants on 2 separate days and compared using random-effect intra-class correlation coefficients (ICCs). Slopes from two input-output curve fitting methods as well as the maximum MEP of gluteus maximus and vastus lateralis were found to exhibit good to excellent reliability (ICCs ranging from 0.75 to 0.99). The described TMS method using EMG-normalized criteria for motor thresholding produced reliable results utilizing a relatively low number of TMS pulses.

Reactive responses of the arms increase the Margins of Stability and decrease center of mass dynamics during a slip perturbation.

Although reactive arm motions are important in recovering from a slip event, the biomechanical influences of upper extremity motions during slipping are not clear. The purpose of the current study was to determine whether reactive arm motions during slip recovery leads to increased margins of stability (MoS), and decreased center of mass (CoM) velocity and excursion. Thirty-two participants were randomized into 2 conditions: arms free and arms constrained. Participants traversed a 10-meter walkway and were exposed to an unexpected slip while wearing a protective harness. Anterior-posterior and medial-lateral MoS, as well as the CoM excursion and velocity during the slip perturbation was quantified using a three-dimensional motion capture system. In the frontal plane, individuals with their arms unconstrained demonstrated greater MoS (0.06 ± 0.03 vs -0.01 ± 0.02 m, p < 0.01), decreased CoM excursion (0.05 ± 0.02 vs 0.08 ± 0.01 m, p = 0.015), and a reduced CoM velocity (0.07 ± 0.03 vs. 0.14 ± 0.02 m/s, p < 0.01) compared to individuals with their arms constrained. In the sagittal plane, individuals with their arms unconstrained demonstrated, decreased CoM excursion (0.83 ± 0.13 vs 1.14 ± 0.20 m, p < 0.01) reduced CoM velocity (1.71 ± 0.08 vs. 1.79 ± 0.07 m/s, p = 0.02), but no differences in margins of stability (0.89 ± 0.13 vs 0.94 ± 0.10 m, p = 0.32). Our findings demonstrate that arm motions during a slip perturbation act to restore balance by minimizing displacement and velocity of the body CoM during a slip event in the frontal plane.

Femoral and acetabular features explain acetabular contact pressure sensitivity to hip internal rotation in persons with cam morphology: A finite element analysis.

BACKGROUND: Femoroacetabular impingement is characterized by premature contact between the proximal femur and acetabulum. The loss of femoral head-neck concavity associated with cam morphology leads to mechanical impingement during hip flexion and internal rotation. Other femoral and acetabular features have been linked with mechanical impingement but have not been comprehensively investigated. This study sought to determine which bony features are most influential in contributing to mechanical impingement in persons with a cam morphology. METHODS: Twenty individuals (10 female, 10 male) with a cam morphology participated. Finite element analyses incorporating subject-specific bony geometry derived from computed tomography scans were used to determine which femoral (alpha angle and femoral neck-shaft angle) and acetabular (anteversion angle, inclination angle, depth, and lateral center-edge angle) features accentuate acetabular contact pressure with increasing degrees of hip internal rotation with the hip flexed to 90°. To determine the best predictors of acetabular contact pressure sensitivity to internal rotation, all morphological variables were included in a stepwise regression with the final model subjected to a bootstrapping procedure. FINDINGS: The stepwise regression revealed that femoral neck-shaft angle, acetabular anteversion angle, acetabular inclination angle, and acetabular depth were the best combination of variables to predict contact pressure sensitivity to internal rotation, explaining 55% of the variance. Results of the bootstrap analysis revealed that a median value of 65% [37%, 89%] variance in sensitivity could be explained by these morphological variables. INTERPRETATION: Mechanical impingement and the concomitant acetabular contact pressure are modulated by multiple femoral and acetabular features in persons with a cam morphology.

Comparison of electromyographic activity of the gluteal muscles and tensor fascia lata in persons with patellofemoral pain: evaluation of selected, hip-targeted exercises using indwelling fine-wire electrodes.

INTRODUCTION: Excessive hip adduction and internal rotation are common movement impairments associated with patellofemoral pain (PFP). As such, strengthening of the hip abductors and external rotators commonly is recommended. Because tensor fascia latae (TFL) is a hip internal rotator in addition to being an abductor, it is important to select exercises that target the superior gluteus maximus (SUP-GMAX) and gluteus medius (GMED) while minimizing activation of the TFL. OBJECTIVE: To identify hip-targeted exercises resulting in greater activation of the SUP-GMAX and GMED relative to the TFL in persons with PFP. METHODS: Twelve individuals with PFP participated. Electromyographic (EMG) signals were obtained from the GMED, SUP-GMAX, and TFL using fine-wire electrodes while participants performed 11 hip-targeted exercises. Normalized EMG of GMED and SUP-GMAX was compared to that of the TFL for each exercise using repeated measures ANOVAs and descriptive statistics. RESULTS: Of the 11 hip exercises evaluated, only the clam exercise with elastic resistance resulted in significantly greater activity of both gluteal muscles (SUP-GMAX = 24.2 ± 14.4%MVIC, p = .05; GMED = 37.2 ± 19.7%MVIC, p = .008) relative to the TFL (12.5 ± 11.7%MVIC). Five exercises exhibited significantly lower activation of SUP-GMAX relative to TFL: 1) unilateral bridge: SUP-GMAX = 17.7 ± 9.8%MVIC, TFL = 34.0 ± 17.7%MVIC, p = .01; 2) bilateral bridge: SUP-GMAX = 10.0 ± 6.9%MVIC, TFL = 14.0 ± 7.5%MVIC, p = .04; 3) abduction: SUP-GMAX = 14.2 ± 11.1%MVIC, TFL = 33.0 ± 11.9%MVIC, p = .001; 4) hip hike: SUP-GMAX = 14.8 ± 12.8%MVIC, TFL = 46.8 ± 33.7%MVIC, p = .008; and 5) step-up: SUP-GMAX = 15.0 ± 5.4%MVIC, TFL = 31.7 ± 19.9 %MVIC, p = .02). No differences in gluteal activation relative to TFL were found for the remaining 6 exercises (all p > .05). CONCLUSION: The clam with elastic resistance exercise was effective at activating the SUP-GMAX and GMED greater than TFL. No other exercises achieved a similar level of muscular recruitment. When attempting to strengthen the gluteal muscles in persons with PFP, care should be taken in assuming that common hip-targeted exercises result in the desired recruitment patterns.

Is muscular strength a predictor for primary or secondary ACL injury? A scoping review of prospective studies.

OBJECTIVE: To identify strength-related risk factors of ACL injury by conducting a scoping review of the peer-reviewed literature. METHODS: PubMed and EBSCO host (CINAHL Complete, MEDLINE Complete, SPORTDiscus) were searched from inception to August 2022. Prospective studies that examined strength strength-related risk factors for ACL injury (primary and secondary) were included. PRISMA Extension for Scoping Reviews guided data charting/extraction. RESULTS: 17 studies were included (eight primary ACL injury, nine secondary ACL injury). Knee flexor strength was the most studied predictor (10 studies), followed by hip abductor strength (9 studies). Across studies, measures of muscle performance were inconsistent. Significant strength-related risk factors were reported in seven of 17 studies. Potential strength-related risk factors of primary ACL injury included measures of hip strength (abductor or external rotator) and knee strength (knee flexor/extensor ratio and knee extensor strength symmetry) for secondary ACL injury. Limited/conflicting evidence was found for all strength-related risk factors. CONCLUSION: Measures of muscle strength appear to be predictive of primary and secondary ACL injury in a subset of identified studies. The heterogeneity of study designs and lack of standardization related to strength testing make it difficult to determine the overall impact of strength in predicting ACL injury.

Persons with Patellar Tendinopathy Exhibit Greater Patellar Tendon Stress during a Single-Leg Landing Task.

PURPOSE: This study aimed to compare peak maximum principal stress in the patellar tendon between persons with and without patellar tendinopathy during a simulated single-leg landing task. A secondary purpose was to determine the biomechanical predictor(s) of peak maximum principal stress in the patellar tendon. METHODS: Using finite element (FE) modeling, patellar tendon stress profiles of 28 individuals (14 with patellar tendinopathy and 14 pain-free controls) were created at the time of the peak knee extensor moment during single-leg landing. Input parameters to the FE model included subject-specific knee joint geometry and kinematics, and quadriceps muscle forces. Independent t -tests were used to compare the peak maximum principal stress in the patellar tendon and biomechanical variables used as input variables to the FE model (knee flexion, knee rotation in the frontal and transverse planes and the peak knee extensor moment) between groups. A stepwise regression model was used to determine the biomechanical predictor(s) of peak maximum principal stress in the patellar tendon for both groups combined. RESULTS: Compared with the control group, persons with patellar tendinopathy exhibited greater peak maximum principal stress in the patellar tendon (77.4 ± 25.0 vs 60.6 ± 13.6 MPa, P = 0.04) and greater tibiofemoral joint internal rotation (4.6° ± 4.6° vs 1.1° ± 4.2°, P = 0.04). Transverse plane rotation of the tibiofemoral joint was the best predictor of peak maximum principal stress in the patellar tendon ( r = 0.51, P = 0.01). CONCLUSIONS: Persons with patellar tendinopathy exhibit greater peak patellar tendon stress compared with pain-free individuals during single-leg landing. The magnitude of peak patellar tendon stress seems to be influenced by the amount of tibiofemoral rotation in the transverse plane.

Gluteal activation during squatting reduces acetabular contact pressure in persons with femoroacetabular impingement syndrome: A patient-specific finite element analysis.

BACKGROUND: Femoroacetabular impingement syndrome is a motion-related clinical disorder resulting from abnormal hip joint morphology. Mechanical impingement, in which the aspherical femoral head (cam morphology) abuts with the acetabular rim, is created with simultaneous hip flexion, internal rotation, and adduction. Impaired function of the gluteal muscles may be contributory to femoroacetabular impingement syndrome progression. The purpose of this study was to assess the influence of gluteal muscle recruitment on acetabular contact pressure during squatting in persons with cam femoroacetabular impingement syndrome. METHODS: Eight individuals (4 males, 4 females) with a diagnosis of cam femoroacetabular impingement syndrome underwent CT imaging of the pelvis and proximal femora, and a biomechanical assessment of squatting (kinematics, kinetics, and electromyography). Two maximal depth bodyweight squat conditions were evaluated: 1) non-cued squatting; and 2) cued gluteal activation squatting. Utilizing subject-specific electromyography-driven hip and finite element modeling approaches, hip muscle activation, kinematics, bone-on-bone contact forces, and peak acetabular contact pressure were compared between squat conditions. FINDINGS: Modest increases in gluteus maximus (7% MVIC, P < 0.0001) and medius (6% MVIC, P = 0.009) activation were able to reduce hip internal rotation on average 5° (P = 0.024), and in doing so reduced acetabular contact pressure by 32% (P = 0.023). Reductions in acetabular contact pressure occurred despite no change in hip abduction and increased bone-on-bone contact forces occurring in the cued gluteal activation condition. INTERPRETATION: Our findings highlight the importance of gluteal activation in minimizing mechanical impingement and provide a foundation for interventions aimed at preventing the development and progression of femoroacetabular impingement syndrome.

The influence of isolated femur and tibia rotations on patellar tendon stress: A sensitivity analysis using finite element analysis.

The purpose of this study was to determine the influence of frontal and transverse plane rotations of the femur and tibia on peak maximum principal stress in the patellar tendon. Using finite element modeling, patellar tendon stress profiles of eight healthy individuals were obtained during a simulated squatting task (45° of knee flexion). The femur and tibia of each model were rotated 10° (in 2° increments) along their respective axes beyond that of the natural degree of rotation. This process was repeated for the transverse plane (internal and external rotation) and frontal plane (adduction and abduction). Quasi-static loading simulations were performed to quantify peak maximum principal stress in patellar tendon. Internal and external rotations of the femur and tibia that exceeded 4° beyond that of the natural rotation resulted in progressively greater patellar tendon stress (p < 0.05). Incremental femur and tibia adduction and abduction resulted in an increase in patellar tendon stress, but only at the end range of motions evaluated. These results suggest that tibiofemoral rotations in the frontal and transverse planes have the potential to influence patellar tendon stress. In particular, patellar tendon stress is highly sensitive to small degrees of tibia and/or femur motions in the transverse plane.

Does the 2D Frontal Plane Projection Angle Predict Frontal Plane Knee Moments during Stepping, Landing, and Change of Direction Tasks?

Background: Although dynamic knee valgus can be visually identified using the 2D frontal plane projection angle (FPPA), the validity of the FPPA in terms of predicting frontal plane knee kinematics has been questioned. The biomechanical utility of the FPPA may lie in its ability to predict frontal plane knee moments. Hypothesis/Purpose: The purpose of the current study was to comprehensively evaluate the ability of the FPPA to predict the frontal plane knee kinetics (peak moment, average moment, and moment at peak knee flexion) across a wide range of tasks (stepping, landing, and change of direction). Design: Crossover Study Design. Methods: Three-dimensional lower-extremity kinetics and 2D video were obtained from 39 healthy athletes (15 males and 24 females) during execution of six tasks (step down, drop jump, lateral shuffle, deceleration, triple hop, side-step-cut). Linear regression analysis was performed to determine if the 2D FPPA at peak knee flexion predicted frontal plane knee moment variables during the deceleration phase of each task (peak moment, average moment, moment at peak knee flexion). Results: The FPPA was found to significantly predict the peak frontal plane knee moment for two tasks (deceleration and side-step-cut, R2 = 12% to 25%), average frontal plane knee moment for five tasks (drop jump, shuffle, deceleration, triple hop, side-step-cut, R2 = 15% to 40%), and frontal plane knee moment at peak knee flexion for five tasks (drop jump, shuffle, deceleration, triple hop, side-step-cut, R2 = 16% to 45%). Conclusion: An increased FPPA (medial knee collapse) predicted increased knee valgus moments (or decreased knee varus moments) during landing and change of direction tasks (but not stepping). However, the predictive ability of the FPPA was weak to moderate.

Persons with patellar tendinopathy exhibit anatomical features that alter knee extensor mechanics: A comparison of persons with and without symptoms.

It has been postulated that anatomical features of the patellofemoral joint may alter knee extensor mechanics in a way that may contribute to excessive patellar tendon loading. The purpose of this study was to compare the knee extensor mechanics in persons with and without patellar tendinopathy. Twenty-eight individuals participated (14 with patellar tendinopathy and 14 pain-free controls). Sagittal magnetic resonance images of the knee were acquired at the knee flexion angle that corresponded to the knee flexion angle at the time of peak knee extensor moment during a single-leg landing task. Measurements of patellar tendon/quadriceps tendon force ratio, quadriceps moment arm, patellar tendon moment arm, and patellar height (Insall-Salvati ratio) were obtained. Independent t-tests were used to compare the variables of interest between groups. When compared to the control group, the patellar tendinopathy group exhibited significantly a greater patellar tendon/quadriceps tendon force ratio (Mean ± SD, 1.0 ± 0.1 vs 0.8 ± 0.1, p < 0.05), a larger quadriceps moment arm (Mean ± SD, 23.9 ± 2.0 mm vs 22.1 ± 2.9 mm, p < 0.05), a smaller patellar tendon moment arm (Mean ± SD, 24.2 ± 1.7 mm vs 26.3 ± 2.4 mm, p < 0.05) and a greater Insall-Salvati ratio (Mean ± SD, 1.2 ± 0.1 vs 1.1 ± 0.1, p < 0.05). These results suggest that persons with patellar tendinopathy may experience greater forces in the patellar tendon for a given level of quadriceps force.

Constraining the arms during a slip perturbation results in a higher fall frequency in young adults.

Slip and fall incidents are a major health concern. Although studies have reported the mechanical benefits of upper extremity responses during a slip to regain balance, it is not currently known if reactive arm motions aid in the recovery of a slip event. Sixty-four healthy young adults were randomized into 4 gait conditions: arms free, both arms constrained, contralateral arm to the slipping foot constrained and ipsilateral arm to the slipping foot constrained. While wearing a protective harness, participants traversed a 10-m walkway and were exposed to an unexpected slip. The group with their arms constrained exhibited a higher proportion of falls compared to the group with the arms free (62.5% vs 18.8%). In addition, individuals assigned to the contralateral arm constraint group exhibited a significantly higher proportion of falls compared to the group in which the ipsilateral arm was constrained (68.8% vs. 31.2%). Our findings suggest that arm motions aid in the recovery of balance during a slip perturbation. Motion of the arm contralateral to the slipping foot appears to be most important. Training upper extremity reactive responses training the arms may be a useful adjunct to fall prevention programs fall prevention.

Quadriceps strength symmetry predicts vertical ground reaction force symmetry during running in patients who have undergone ACL reconstruction.

OBJECTIVE: To determine whether quadriceps strength symmetry can predict peak vertical ground reaction force (vGRF) running force symmetry in patients who have undergone ACL reconstruction (ACLR). We also sought to determine a cutoff for quadriceps strength symmetry to identify patients at risk for vGRF running asymmetry. DESIGN: Retrospective cross-sectional. SETTING: Clinical facility. METHODS: Bilateral quadriceps strength and vGRF data during running were obtained from 79 patients 26-30 weeks post ACLR. Linear regression was used to determine if quadriceps strength symmetry predicted peak vGRF running force symmetry. Classification and regression tree (CART) analysis was used to determine the cutoff value for quadriceps strength symmetry to identify patients at risk for vGRF running asymmetry. RESULTS: Increased quadriceps strength symmetry predicted increased vGRF running symmetry (R2 = 0.20). CART analysis revealed that patients with quadriceps strength symmetry less than or equal to 88% were at highest risk for vGRF running asymmetry (R2 = 26%). CONCLUSION: Greater quadriceps strength symmetry is predictive of greater vGRF running force symmetry in patients who have undergone ACLR. This finding highlights the need for clinicians to consider the degree of quadriceps strength symmetry before initiating a return to running program.

Predictors of Quadriceps Strength Asymmetry after Anterior Cruciate Ligament Reconstruction: A Chi-Squared Automatic Interaction Detection Decision Tree Analysis.

INTRODUCTION: The influence of graft type on the restoration of quadriceps strength symmetry after ACL reconstruction (ACLR) has been widely studied. However, an important consideration when evaluating quadriceps symmetry is the fact that this measure can be influenced by numerous factors beyond graft type. This study sought to determine if graft type is predictive of quadriceps strength asymmetry during the first 12 months post-ACLR taking into consideration potentially influential factors (i.e., age, sex, body mass index, time post-ACLR). METHODS: We retrospectively reviewed quadriceps strength data from 434 patients (303 female patients and 131 male patients) who had previously undergone ACLR with an autograft (hamstring tendon, quadriceps tendon [QT], patellar tendon [PT]) or allograft. Chi-Squared Automatic Interaction Detection decision tree analysis was used to evaluate if graft type is predictive of quadriceps strength asymmetry during the first 12 months post-ACLR taking into consideration age, sex, body mass index, and time post-ACLR. RESULTS: The best predictor of quadriceps strength asymmetry was graft type. Specifically, three graft categories were identified: 1) allograft and hamstring tendon autograft, 2) PT autograft, and 3) QT autograft. The average quadriceps strength asymmetry for each of the three identified categories was 0.91, 0.87, and 0.81, respectively, and differed statistically from each other ( P < 0.001). The second-best predictor of quadriceps strength asymmetry was sex, albeit only in the PT and QT groups (with female patients having increased asymmetry). Female patients post-ACLR with a QT autograft were at highest risk for quadriceps strength asymmetry. CONCLUSIONS: Graft type and sex are important predictors of quadriceps strength asymmetry after ACLR. Clinicians should take these factors into consideration when designing rehabilitation protocols to restore quadriceps strength symmetry during the postoperative period.

Persons with patellofemoral pain exhibit altered hip abductor muscle recruitment while performing hip abductor exercises.

INTRODUCTION: Strengthening of the hip abductors has been advocated for persons with patellofemoral pain (PFP). It is not clear if these individuals activate the hip abductor muscles appropriately to achieve the desired therapeutic effects. OBJECTIVE: To compare activation of the hip abductor muscles between persons with and without PFP during the performance of hip abductor exercises. METHODS: Thirty-two individuals participated (12 with PFP and 20 without PFP). The average age (± standard deviation) was 29.7 ± 5.9 years for the PFP group and 28.1 ± 6.9 for the control group. Electromyographic (EMG) signals from the gluteus medius (GMED), superior gluteus maximus (SUP-GMAX), and tensor fascia lata (TFL) were obtained using fine-wire electrodes while participants performed 11 different exercises. Normalized EMG activity of each muscle was compared between groups across all exercises. RESULTS: When averaged across all exercises, persons with PFP exhibited significantly greater EMG activity of TFL (mean = 25.3% MVIC; 95% CI = 19.2, 31.3) compared to those without PFP (mean = 17.6% MVIC; 95% CI = 12.8, 22.4) and significantly lower EMG activity of SUP-GMAX (mean = 16.4% MVIC; 95% CI = 11.0, 22.0) compared to those without PFP (mean = 25.4% MVIC; 95% CI = 21.0, 29.8). Persons with PFP exhibited lower EMG activity of GMED, but only for 3 out of the 11 exercises evaluated (hip abduction, hip hike, step-up). CONCLUSION: Compared to persons without PFP, those with PFP exhibited activation differences during the performance of exercises used to target the hip abductors. Our results highlight the need for activation training prior to the initiation of strengthening exercises to achieve desired therapeutic effects.

Activation training facilitates gluteus maximus recruitment during weight-bearing strengthening exercises.

Given its tri-planar action at the hip, strengthening of gluteus maximus (GMAX) has been advocated as part of rehabilitation and injury prevention protocols for various musculoskeletal conditions. However, recruitment of GMAX during weight-bearing strengthening exercises can be challenging owing to the muscular redundancy at the hip for a given joint motion. The current study sought to determine if a 1-week activation program could result in greater GMAX recruitment during functional strengthening exercises. Pre- and post-training surface electromyography were collected from 12 healthy participants as they performed double- and single-leg squats. Between testing sessions, participants completed a GMAX activation training program consisting of isometric exercises with band resistance (twice per day for 7 days). Following the 1-week activation program, GMAX recruitment was found to increase by 57% during the double-leg squat (p = 0.005, Cohen's r = 0.73) and 53% during the single-leg squat (p = 0.006, Cohen's r = 0.70). Implementation of an initial GMAX activation program should be considered to facilitate neuromuscular adaptations that facilitate utilization of GMAX during hip strengthening exercises.

Utility of 2D Video Analysis for Assessing Frontal Plane Trunk and Pelvis Motion during Stepping, Landing, and Change in Direction Tasks: A Validity Study.

BACKGROUND: Excessive frontal plane motion of the trunk and/or pelvis has been implicated in numerous clinical conditions. To date, it is unclear whether 2D video is an appropriate surrogate for assessing frontal plane trunk and pelvis motion as a comprehensive validity study across a wide range of movements using a consistent methodology has not been performed. HYPOTHESIS/PURPOSE: The purpose of the current study was to assess the concurrent validity and agreement of frontal plane pelvis and trunk motion obtained with 2D video against the respective 3D angles during stepping, landing, and change in direction tasks. DESIGN: Crossover Study Design. METHODS: 3D kinematics and 2D frontal plane video were obtained from 39 healthy participants (15 males and 24 females) during five athletic tasks (step down, lateral shuffle, deceleration, triple hop, side-step-cut). Data were extracted at peak knee flexion. Pearson's correlation analysis was used to assess the association between the 2D and 3D frontal plane angles at the trunk and pelvis. Bland Altman plots were used to assess the level of agreement between the 2D and 3D frontal plane angles at the trunk and pelvis. RESULTS: 2D and 3D frontal plane angles for all tasks were correlated in a positive direction at the pelvis (r = 0.54 to 0.73, all p < 0.001) and trunk (r = 0.81 to 0.92, all p < 0.001). Absolute agreement in the frontal plane for all tasks and angles was below 5°. However, the 95% limits of agreement across tasks ranged from -12.8° to 21.3° for the pelvis and -11.8° to 9.4° for the trunk. CONCLUSIONS: The use of 2D video to assess frontal plane trunk and pelvis motion is appropriate during stepping, landing, and change of direction tasks, however caution is advised when high levels of agreement or accuracy is required.

The influence of hip flexion mobility and lumbar spine extensor strength on lumbar spine flexion during a squat lift.

STUDY DESIGN: Cross-sectional; Controlled laboratory study. OBJECTIVE: To examine the associations among available hip flexion motion, lumbar extensor strength and peak lumbar flexion during a squat lift task. SUMMARY OF BACKGROUND DATA: Lumbar spine flexion during lifting can result in increased strain on spinal structures. Although decreased available hip flexion motion and reduced strength of the lumbar extensor muscles has been proposed to contribute to greater lumbar flexion during lifting, direct relationships have not been explored. METHODS: Fifty healthy young adults participated (23 males and 27 females). Strength of the lumbar extensors was measured using a motor-driven dynamometer. Available hip flexion was assessed using 3D motion capture. Peak lumbar spine flexion and hip flexion were quantified during the descent phase of the squat lifting task. RESULTS: There was a significant negative association between available hip flexion and peak lumbar spine flexion during squat lifting in females (r = -0.407, p = 0.035) but not males (r = -0.341, p = 0.120). Similarly, peak lumbar spine flexion was negatively associated with lumbar extensor strength in females (r = -0.398, p = 0.040) but not males (r = -0.310, p = 0.161). During the squat lift, peak hip motion was positively associated with available hip flexion for both males and females combined (r = 0.774, p < 0.001). CONCLUSION: Females with less available hip flexion and lower lumbar extensor strength exhibit greater lumbar flexion when performing a lifting task. Clinicians should be aware of the potential contributions of such impairments when instructing patients into various lifting strategies.

Quantification of reactive arm responses to a slip perturbation.

The purpose of this study was two-fold: 1) characterize bilateral upper extremity responses during a slip event in both the sagittal and frontal planes, and 2) to examine the utility of using slip onset as the measurement reference for behavioral responses of the upper extremities using EMG latency. Sixteen healthy young adults were exposed to an unexpected slip during walking. Three-dimensional arm kinematics (excursions) and electromyographic onset latencies (bilateral deltoids) were quantified. Thirteen of the 16 participants recovered their balance following the slip perturbation. Of those who recovered, multi-planar arm responses were observed bilaterally. The arm contralateral to the slipping foot demonstrated significantly greater excursion in the frontal plane than the ipsilateral arm (p < 0.001), whereas excursions in the sagittal plane did not differ between arms (p = 0.75). Further, the frontal plane excursion of the contralateral arm was greater than sagittal plane excursion (p < 0.001). The electromyographic onset of deltoid activity was equivalent in both arms (57-76 ms), despite the differences in kinematics. Multi-plane arm motion occurs in response to a slip perturbation. Specifically, frontal plane motion of the arm contralateral to the slipping foot exhibited the greatest amount of excursion.

Classification of Runners with High versus Low Hip Adduction Based on Measures of Pelvis and Femur Morphology.

PURPOSE: To determine the most relevant pelvis and femur morphological characteristics for differentiating runners with high versus low hip adduction during running. METHODS: Fifteen female and 14 male runners underwent instrumented kinematics analysis of overground running and computed tomography scanning of pelvis and femur. The peak hip adduction angle during the stance phase of running was identified for each participant. Using the cohort average of the peak hip adduction angle as the classifying threshold, participants were categorized into high or low hip adduction groups. To determine the most relevant morphologic features for discriminating high and low hip adduction runners, a feature selection-based support vector machine classification analysis was performed. RESULTS: Of the 15 morphology variables examined, femoral head anteversion and femur length were shown to be the best discriminant variables for group classification. Together, these variables achieved a prediction accuracy of 0.93, sensitivity of 1.0, and specificity of 0.88. CONCLUSIONS: Our results highlight the importance of femur morphology in contributing to increased hip adduction during running.