Validation of gait event detection by centre of pressure during target stepping in healthy and paretic gait

BackgroundTarget-stepping paradigms are increasingly used to assess and train gait adaptability. Accurate gait-event detection (GED) is key to locating targets relative to the ongoing step cycle as well as measuring foot-placement error. In the current literature GED is either based on kinematics or centre of pressure (CoP), and both have been previously validated with young healthy individuals. However, CoP based GED has not been validated for stroke survivors who demonstrate altered CoP pattern.MethodsYoung healthy adults and individuals affected by stroke stepped to targets on a treadmill, while gait events were measured using three detection methods; verticies of CoP cyclograms, and two kinematic criteria, (1) vertical velocity and position and of the heel marker, (2) anterior velocity and position of the heel and toe marker, were used. The percentage of unmatched gait events was used to determine the success of the GED method. The difference between CoP and kinematic GED methods were tested with two one sample (two-tailed) t-tests against a reference value of zero. Differences between group and paretic and non-paretic leg were tested with a repeated measures ANOVA.ResultsThe kinematic method based on vertical velocity only detected about 80% of foot contact events on the paretic side in stroke survivors while the method on anterior velocity was more successful in both young healthy adults as stroke survivors (3% young healthy and 7% stroke survivors unmatched). Both kinematic methods detected gait events significantly earlier than CoP GED (p < 0.001) except for foot contact in stroke survivors based on the vertical velocity.ConclusionsCoP GED may be more appropriate for gait analyses of SS than kinematic methods; even when walking and varying steps.     

The influence of estimated body segment parameters on predicted joint kinetics during diplegic cerebral palsy gait

Inverse Dynamic calculations are routinely used in joint moment and power estimates during gait with anthropometric data often taken from published sources. Many biomechanical analyses have highlighted the need to obtain subject-specific anthropometric data (e.g. Mass, Centre of Mass, Moments of Inertia) yet the types of imaging techniques required to achieve this are not always available in the clinical setting. Differences in anthropometric sets have been shown to affect the reactive force and moment calculations in normal subjects but the effect on a paediatric diplegic cerebral palsy group has not been investigated. The aim of this study was to investigate the effect of using different anthropometric sets on predicted sagittal plane moments during normal and diplegic cerebral palsy gait. Three published anthropometric sets were applied to the reactive force and moment calculations of 14 Cerebral Palsy and 14 Control subjects. Statistically significant differences were found when comparing the different anthropometric sets but variability in the resulting sagittal plane moment calculations between sets was low (0.01–0.07 Nm/kg). In addition, the GDI-Kinetic, used as an outcome variable to assess whether differences were clinically meaningful, indicated no clinically meaningful difference between sets. The results suggest that the effects of using different anthropometric sets on the kinetic profiles of normal and diplegic cerebral palsy subjects are clinically insignificant.     

Children with cerebral palsy have larger Achilles tendon moment arms than typically developing children

The effectiveness of the plantarflexor muscle group to generate desired plantarflexion moments is modulated by the geometry of the Achilles tendon moment arm (ATMA). Children with cerebral palsy (CP) frequently have reduced plantarflexion function, which is commonly attributed to impaired muscle structure and function, however little attention has been paid to the potential contribution of ATMA geometry. The use of musculoskeletal modelling for the simulation of gait and understanding of gait mechanics, rely on accuracy of ATMA estimates. This study aimed to compare 3D in-vivo estimates of ATMA of adults, children with CP and typically developing (TD) children, as well as compare 3D in-vivo estimates to linearly scaled musculoskeletal model estimates. MRI scans for eight children with CP, 11 TD children and nine healthy adults were used to estimate in-vivo 3D ATMA using a validated method. A lower limb musculoskeletal model was linearly scaled to individual tibia length to provide a scaled ATMA estimate. Normalised in-vivo 3D ATMA for children with CP was 17.2% ± 2.0 tibia length, which was significantly larger than for TD children (15.2% ± 1.2, p = 0.013) and adults (12.5% ± 0.8, p < 0.001). Scaled ATMA estimates from musculoskeletal models significantly underestimated in-vivo estimates for all groups, by up to 34.7%. The results of this study show children with CP have larger normalised 3D ATMA compared to their TD counterparts, which may have implications in understanding reduced plantarflexor function and the efficacy of surgical interventions whose aim is to modify the musculoskeletal geometry of this muscle group.     

Literature Review and Comparison of Two Statistical Methods to Evaluate the Effect of Botulinum Toxin Treatment on Gait in Children with Cerebral Palsy

Aim

This study aimed at comparing two statistical approaches to analyze the effect of Botulinum Toxin A (BTX-A) treatment on gait in children with a diagnosis of spastic cerebral palsy (CP), based on three-dimensional gait analysis (3DGA) data. Through a literature review, the available expert knowledge on gait changes after BTX-A treatment in children with CP is summarized.

Methods

Part 1—Intervention studies on BTX-A treatment in children with CP between 4–18 years that used 3DGA data as an outcome measure and were written in English, were identified through a broad systematic literature search. Reported kinematic and kinetic gait features were extracted from the identified studies. Part 2—A retrospective sample of 53 children with CP (6.1 ± 2.3years, GMFCS I-III) received 3DGA before and after multilevel BTX-A injections. The effect of BTX-A on gait was interpreted by comparing the results of paired samples t-tests on the kinematic gait features that were identified from literature to the results of statistical parametric mapping analysis on the kinematic waveforms of the lower limb joints.

Results

Part 1–53 kinematic and 33 kinetic features were described in literature. Overall, there is no consensus on which features should be evaluated after BTX-A treatment as 49 features were reported only once or twice. Part 2—Post-BTX-A, both statistical approaches found increased ankle dorsiflexion throughout the gait cycle. Statistical parametric mapping analyses additionally found increased knee extension during terminal stance. In turn, feature analyses found increased outtoeing during stance after BTX-A.

Conclusion

This study confirms that BTX-A injections are a valuable treatment option to improve gait function in children with CP. However, different statistical approaches may lead to different interpretations of treatment outcome. We suggest that a clear, definite hypothesis should be stated a priori and a commensurate statistical approach should accompany this hypothesis.     

Custom sizing of lower limb exoskeleton actuators using gait dynamic modelling of children with cerebral palsy

The use of exoskeletons as an aid for people with musculoskeletal disorder is the subject to an increasing interest in the research community. These devices are expected to meet the specific needs of users, such as children with cerebral palsy (CP) who are considered a significant population in pediatric rehabilitation. Although these exoskeletons should be designed to ease the movement of people with physical shortcoming, their design is generally based on data obtained from healthy adults, which leads to oversized components that are inadequate to the targeted users. Consequently, the objective of this study is to custom-size the lower limb exoskeleton actuators based on dynamic modeling of the human body for children with CP on the basis of hip, knee, and ankle joint kinematics and dynamics of human body during gait. For this purpose, a multibody modeling of the human body of 3 typically developed children (TD) and 3 children with CP is used. The results show significant differences in gait patterns especially in knee and ankle with respectively 0.39 and ?0.33 (Nm/kg) maximum torque differences between TD children and children with CP. This study provides the recommendations to support the design of actuators to normalize the movement of children with CP.     

Multilevel Upper Body Movement Control during Gait in Children with Cerebral Palsy

Upper body movements during walking provide information about balance control and gait stability. Typically developing (TD) children normally present a progressive decrease of accelerations from the pelvis to the head, whereas children with cerebral palsy (CP) exhibit a general increase of upper body accelerations. However, the literature describing how they are transmitted from the pelvis to the head is lacking. This study proposes a multilevel motion sensor approach to characterize upper body accelerations and how they propagate from pelvis to head in children with CP, comparing with their TD peers. Two age- and gender-matched groups of 20 children performed a 10m walking test at self-selected speed while wearing three magneto-inertial sensors located at pelvis, sternum, and head levels. The root mean square value of the accelerations at each level was computed in a local anatomical frame and its variation from lower to upper levels was described using attenuation coefficients. Between-group differences were assessed performing an ANCOVA, while the mutual dependence between acceleration components and the relationship between biomechanical parameters and typical clinical scores were investigated using Regression Analysis and Spearman’s Correlation, respectively (α = 0.05). New insights were obtained on how the CP group managed the transmission of accelerations through the upper body. Despite a significant reduction of the acceleration from pelvis to sternum, children with CP do not compensate for large accelerations, which are greater than in TD children. Furthermore, those with CP showed negative sternum-to-head attenuations, in agreement with the documented rigidity of the head-trunk system observed in this population. In addition, the estimated parameters proved to correlate with the scores used in daily clinical practice. The proposed multilevel approach was fruitful in highlighting CP-TD gait differences, supported the in-field quantitative gait assessment in children with CP and might prove beneficial to designing innovative intervention protocols based on pelvis stabilization.     

Assessment of wavelet analysis of gait in children with typical development and cerebral palsy

The objective of this study was to examine the use of the continuous wavelet transform (CWT) on surface electromyographic (sEMG) signals acquired from the lower extremity muscles during gait in children with typical development (TD) and cerebral palsy (CP). This was done to explore the possibility of developing a quantitative assessment scale of motor function based on time-frequency information. An initial study was conducted on retrospective gait data from three children, matched in gender and in anthropometric variables but with differing levels of walking ability. EMG data were extracted from five lower extremity muscles to assess the degrees of differentiation. The data were processed using the CWT to derive an average scalogram, from which the instantaneous mean frequency (IMNF) was calculated. Principal component analysis was used to assess the differences between the curves. Preliminary results indicated that for select lower extremity muscles, there was a significant deviation in the IMNF curves in the child with CP as compared to the child with TD. Furthermore, as motor impairment increased, total percent explained variance to the TD curves decreased. This suggests that it might be possible to derive a physiologically based quantitative index for assessing motor function and for assessing clinical treatments in CP using the wavelet analysis.     

Trunk and hip muscle activity in early walkers with and without cerebral palsy – A frequency analysis

Poor control of postural muscles is a primary impairment in cerebral palsy (CP), yet core trunk and hip muscle activity has not been thoroughly investigated. Frequency analysis of electromyographic (EMG) signals provides insight about the intensity and pattern of muscle activation, correlates with functional measures in CP, and is sensitive to change after intervention. The objective of this study was to investigate differences in trunk and hip muscle activation frequency in children with CP compared to children with similar amounts of walking experience and typical development (TD). EMG data from 31 children (15 with CP, 16 with TD) were recorded from 16 trunk and hip muscles bilaterally. A time–frequency pattern was generated using the continuous wavelet transform and instantaneous mean frequency (IMNF) was calculated at each interval of the gait cycle. Functional principal component analysis (PCA) revealed that IMNF was significantly higher in the CP group throughout the gait cycle for all muscles. Additionally, stride-to-stride variability was higher in the CP group. This evidence demonstrated altered patterns of trunk and hip muscle activation in CP, including increased rates of motor unit firing, increased number of recruited motor units, and/or decreased synchrony of motor units. These altered muscle activation patterns likely contribute to muscle fatigue and decreased biomechanical efficiency in children with CP.     

Ground reaction forces and lower-limb joint kinetics of turning gait in typically developing children

Turning is a common locomotor task essential to daily activity; however, very little is known about the forces and moments responsible for the kinematic adaptations occurring relative to straight-line gait in typically developing children. Thus, the aims of this study were to analyse ground reaction forces (GRFs), ground reaction free vertical torque (T_Z), and the lower-limb joint kinetics of 90° outside (step) and inside (spin) limb turns. Step, spin, and straight walking trials from fifty-four typically developing children were analysed. All children were fit with the Plug-in Gait and Oxford Foot Model marker sets while walking over force plates embedded in the walkway. Net internal joint moments and power were computed via a standard inverse dynamics approach. All dependent variables were statistically analysed over the entire curves using the mean difference 95% bootstrap confidence band approach. GRFs were directed medially for step turns and laterally for spin turns during the turning phase. Directions were reversed and magnitudes decreased during the approach phase. Step turns showed reduced ankle power generation, while spin turns showed large T_Z. Both strategies required large knee and hip coronal and transverse plane moments during swing. These kinetic differences highlight adaptations required to maintain stability and reorient the body towards the new walking direction during turning. From a clinical perspective, turning gait may better reveal weaknesses and motor control deficits than straight walking in pathological populations, such as children with cerebral palsy, and could potentially be implemented in standard gait analysis sessions.     

The effects of electromyography-assisted modelling in estimating musculotendon forces during gait in children with cerebral palsy

Neuro-musculoskeletal modelling can provide insight into the aberrant muscle function during walking in those suffering cerebral palsy (CP). However, such modelling employs optimization to estimate muscle activation that may not account for disturbed motor control and muscle weakness in CP. This study evaluated different forms of neuro-musculoskeletal model personalization and optimization to estimate musculotendon forces during gait of nine children with CP (GMFCS I-II) and nine typically developing (TD) children. Data collection included 3D-kinematics, ground reaction forces, and electromyography (EMG) of eight lower limb muscles. Four different optimization methods estimated muscle activation and musculotendon forces of a scaled-generic musculoskeletal model for each child walking, i.e. (i) static optimization that minimized summed-excitation squared; (ii) static optimization with maximum isometric muscle forces scaled to body mass; (iii) an EMG-assisted approach using optimization to minimize summed-excitation squared while reducing tracking errors of experimental EMG-linear envelopes and joint moments; and (iv) EMG-assisted with musculotendon model parameters first personalized by calibration. Both static optimization approaches showed a relatively low model performance compared to EMG envelopes. EMG-assisted approaches performed much better, especially in CP, with only a minor mismatch in joint moments. Calibration did not affect model performance significantly, however it did affect musculotendon forces, especially in CP. A model more consistent with experimental measures is more likely to yield more physiologically representative results. Therefore, this study highlights the importance of calibrated EMG-assisted modelling when estimating musculotendon forces in TD children and even more so in children with CP.     

Time-frequency changes in electromyographic signals after hamstring lengthening surgery in children with cerebral palsy

Increased knee flexion during stance is a common gait deviation in the child with cerebral palsy (CP), with distal hamstring lengthening surgeries being an accepted course of treatment. Post-operatively, improvements in gait kinematics have been reported, however little change is noted in the patterns of muscle activity as portrayed by onset and offset timing in the surface electromyographic (sEMG) signals. Similar analysis based on the frequency content of the sEMG signals has seldom been applied, yet may provide additional insight into changes in muscle activity in response to surgery. The purpose of this study was to determine if changes in the time-frequency characteristics of the sEMG, extracted using wavelet analysis techniques, corresponded to improved gait kinematics observed post-surgical intervention, and whether there existed a relationship between frequency characteristics of the sEMG signals and the type of surgery required to correct gait kinematics. Data were collected from 16 children with typical development (TD) and 17 children with CP pre- and post-surgery. Muscle activity was recorded from the medial hamstring (MH) and vastus lateralis (VL) muscles, processed using the wavelet transform, and analyzed using functional principal component analyses (PCA). Results indicated that frequency differences were present pre-operatively depending if surgery was to be performed bilaterally or involved bone modification. Post-operatively, frequency characteristics of the VL more closely approximated those observed in children with TD, agreeing with the improved gait kinematics. MH characteristics, however, for the surgical groups demonstrated a deviation away for TD reflecting the altered muscle structure.     

Effects of gleno-humeral joint centre mislocation on gleno-humeral kinematics and kinetics

The gleno-humeral (GH) rotation centre is typically estimated using predictive or functional methods, however these methods may lead to location errors. This study aimed at determining a location error threshold above which statistically significant changes in the values of kinematic and kinetic GH parameters occur. The secondary aims were to quantify the effects of the direction of mislocation (X, Y or Z axis) of the GH rotation centre on GH kinematic and kinetic parameters.

Shoulder flexion and abduction movements of 11 healthy volunteers were recorded using a standard motion capture system (Vicon, Oxford Metrics Ltd, Oxford, UK), then GH kinematic and kinetic parameters were computed. The true position of the GH rotation centre was determined using a low dose x-ray scanner (EOS? imaging, France) and this position was transferred to the motion data. GH angles and moments were re-computed for each position of the GH rotation centre after errors of up to ± 20?mm were added in increments of ± 5?mm to each axis. The three-dimensional error range was 5?mm to 34.65?mm.

GH joint angle and moment values were significantly altered from 10?mm of three-dimensional error, and from 5?mm of error on individual axes. However, errors on the longitudinal and antero-posterior axes only caused very small alterations of GH joint angle and moment values respectively. Future research should develop methods of GH rotation centre estimation that produce three-dimensional location errors of less than 10?mm to reduce error propagation on GH kinematics and kinetics.     

The pediatric upper limb motion index and a temporal-spatial logistic regression: quantitative analysis of upper limb movement disorders during the Reach & Grasp Cycle

This study describes a novel pediatric upper limb motion index (PULMI) for children with cerebral palsy (CP). The PULMI is based on three-dimensional kinematics and provides quantitative information about upper limb motion during the Reach & Grasp Cycle. We also report key temporal-spatial parameters for children with spastic, dyskinetic, and ataxic CP. Participants included 30 typically-developing (TD) children (age=10.9±4.1 years) and 25 children with CP and upper limb involvement (age=12.3±3.7 years), Manual Ability Classification System (MACS) levels I-IV. The PULMI is calculated from the root-mean-square difference for eight kinematic variables between each child with CP and the average TD values, and scaled such that the TD PULMI is 100±10. The PULMI was significantly lower among children with CP compared to TD children (Wilcoxon Z=-5.06, p<.0001). PULMI scores were significantly lower among children with dyskinetic CP compared to spastic CP (Z=-2.47, p<.0135). There was a strong negative correlation between PULMI and MACS among children with CP (Spearman's rho=-.78, p<.0001). Temporal-spatial values were significantly different between CP and TD children: movement time (Z=4.06, p<.0001), index of curvature during reach (Z=3.68, p=.0002), number of movement units (Z=3.72, p=.0002), angular velocity of elbow extension during reach (Z=-3.96, p<.0001), and transport(1):reach peak velocities (Z=-2.48, p=.0129). A logistic regression of four temporal-spatial parameters, the Pediatric Upper Limb Temporal-Spatial Equation (PULTSE), correctly predicted 19/22 movement disorder subtypes (spastic versus dyskinetic CP). The PULMI, PULTSE, and key temporal-spatial parameters of the Reach & Grasp Cycle offer a quantitative approach to analyzing upper limb function in children with CP.     

Walking speed effects on the lower limb electromyographic variability of healthy children aged 7–16 years

The evaluation of surface electromyography (sEMG) is commonly performed in children with cerebral palsy (CP) and reliable interpretation necessitates knowledge of the variability in age-matched, typically developing (TD) children. Variance ratio was calculated for inter-trial sEMG linear envelope (LE) and the Instantaneous Mean Frequency (IMNF) variability in the lower limb muscle in TD children, in three different age groups during slow, comfortable speed, and fast walking. Significantly greater variability was found in the 7–9 group compared to the 13–16 years. Variability during both slow and fast walking was significantly greater compared to comfortable speed walking and was profound in the 7–9 year age group. Variability of the IMNF was significantly greater than LE in the Tibialis-Anterior, Biceps-Femoris (BF), Vastus-Lateralis (VL), and Rectus-Femoris (RF). Clinical implications are that children under 10 years are more variable than older children when walking either slower or faster than self-selected walking speed. This suggests that muscle activation patterns in gait mature at a later stage of childhood than do kinematic gait patterns. Greater precaution, therefore, is needed when comparing sEMG patterns of less than 10 years of age patient and TD children.     

A proposal for a kinetic summary measure: the Gait Kinetic Index

A new summary index for kinetic gait data is proposed (Gait Kinetic Index - GKI), BASED on six kinetic selected variables: hip, knee and ankle moments and powers on the sagittal plane. This method was applied on a control group (CG) of 18 subjects and on 57 patients with diplegic Cerebral Palsy (CP). CP showed statistical different GKI value in comparison with CG. The same is for the sub GKI with the exclusion of GKI Knee Power. The GKI seems to be a promising tool useful to measure extensively the gait pathology taking into consideration kinetic aspects of gait pattern.     

Crouch severity is a poor predictor of elevated oxygen consumption in cerebral palsy

Children with cerebral palsy (CP) expend more energy to walk compared to typically-developing peers. One of the most prevalent gait patterns among children with CP, crouch gait, is often singled out as especially exhausting. The dynamics of crouch gait increase external flexion moments and the demand on extensor muscles. This elevated demand is thought to dramatically increase energy expenditure. However, the impact of crouch severity on energy expenditure has not been investigated among children with CP. We evaluated oxygen consumption and gait kinematics for 573 children with bilateral CP. The average net nondimensional oxygen consumption during gait of the children with CP (0.18 ± 0.06) was 2.9 times that of speed-matched typically-developing peers. Crouch severity was only modestly related to oxygen consumption, with measures of knee flexion angle during gait explaining only 5–20% of the variability in oxygen consumption. While knee moment and muscle activity were moderately to strongly correlated with crouch severity (r² = 0.13–0.73), these variables were only weakly correlated with oxygen consumption (r² = 0.02–0.04). Thus, although the dynamics of crouch gait increased muscle demand, these effects did not directly result in elevated energy expenditure. In clinical gait analysis, assumptions about an individual’s energy expenditure should not be based upon kinematics or kinetics alone. Identifying patient-specific factors that contribute to increased energy expenditure may provide new pathways to improve gait for children with CP.     

The impact of increased femoral antetorsion on gait deviations in healthy adolescents

Increased femoral antetorsion leads to several gait deviations, and amongst others, an increased knee flexion was reported in mid and terminal stance. Therefore, the purpose of this retrospective study was to identify gait deviations caused by increased femoral antetorsion and to perform subgroup analyses based on sagittal knee kinematics. Patients with isolated, CT confirmed increased femoral antetorsion (n = 42) and age-matched typically developing children (TDC, n = 17) were included in this study. Patients were referred to gait analysis because of gait abnormalities going along with an increased femoral antetorsion ≥30°. Kinematic and kinetic data were recorded during 3D gait analysis and three valid gait cycles were analyzed. Principal component (PC) analysis was used to achieve data transformation. A linear mixed model was used to estimate the group effect of PC-scores of retained PCs explaining 90% of the cumulative variance. Group effects of PC-scores revealed that patients walked with more flexed hips and greater anterior pelvic tilt throughout the gait cycle. Knee flexion was increased in patients during mid and terminal stance. Increased frontal plane knee and hip joint moments were found for patients compared to TDC. Furthermore, dividing patients into two subgroups based on their sagittal knee kinematics showed that kinematic gait deviations were more pronounced in patients with higher femoral antetorsion, while deviations in joint moments were more pronounced in patients with lower femoral antetorsion. Increased femoral antetorsion showed alterations in all lower limb joints and may be not only a cosmetic problem. Therefore, 3D gait analysis should be used for clinical management and operative treatment should be considered depending on severity of gait deviations.     

An upper extremity inverse dynamics model for pediatric Lofstrand crutch-assisted gait

The objective of this study was to develop an instrumented Lofstrand crutch system, which quantifies three-dimensional (3-D) upper extremity (UE) kinematics and kinetics using an inverse dynamics model. The model describes the dynamics of the shoulders, elbows, wrists, and crutches and is compliant with the International Society of Biomechanics (ISB) recommended standards. A custom designed Lofstrand crutch system with four, six-degree-of-freedom force transducers was implemented with the inverse dynamics model to obtain triaxial UE joint reaction forces and moments. The crutch system was validated statically and dynamically for accuracy of computing joint reaction forces and moments during gait. The root mean square (RMS) error of the system ranged from 0.84 to 5.20%. The system was demonstrated in children with diplegic cerebral palsy (CP), incomplete spinal cord injury (SCI), and type I osteogenesis imperfecta (OI). The greatest joint reaction forces were observed in the posterior direction of the wrist, while shoulder flexion moments were the greatest joint reaction moments. The subject with CP showed the highest forces and the subject with SCI demonstrated the highest moments. Dynamic quantification may help to elucidate UE joint demands in regard to pain and pathology in long-term assistive device users.     

Effects of hip joint centre mislocation on gait analysis results

Methods to determine the hip joint centre (HJC) location are necessary in gait analysis. It has been demonstrated that the methods proposed in the literature involve large mislocation errors. The choice should be made according to the extent by which HJC location errors distort the estimates of angles and resultant moments at the hip and knee joints. This study aimed at quantifying how mislocation errors propagate to these gait analysis results. Angles and moments at the hip and knee joint were calculated for five able-bodied subjects during level walking. The nominal position of the HJC was determined as the position of the pivot point of a 3D movement of the thigh relative to the pelvis. Angles and moments were then re-calculated after having added to HJC co-ordinates errors in the range of +/-30 mm. Angles and moments at both hip and knee joints were affected by HJC mislocation. The hip moments showed the largest propagation error: a 30 mm HJC anterior mislocation resulted in a propagated error into flexion/extension component of about -22%. The hip abduction/adduction moment was found the second largest affected quantity: a 30 mm lateral HJC mislocation produced a propagated error of about -15%. Finally, a 30 mm posterior HJC mislocation produced a delay of the flexion-to-extension timing in the order of 25% of the stride duration. HJC estimation methods with minimum antero-posterior error should therefore be preferred.