Journal of Biomechanics

This study examined how mediolateral foot placement is controlled following mechanical perturbations that affected either whole-body linear or angular momentum. Predictive foot placement models based on center of mass state alone were compared with models that additionally included whole-body angular momentum to determine whether whole-body angular momentum contributes to foot placement control beyond linear momentum. Ten healthy adults walked on a treadmill at 2 km/h and 5 km/h while being exposed to two perturbation types: (1) a pull to the pelvis that primarily altered linear momentum (translation perturbation) and (2) simultaneous pulls to the pelvis and shoulder in opposite directions that primarily altered angular momentum (rotation perturbation). Perturbations were applied at heel strike, with a magnitude of [~]120 N and a duration of 300 ms. Whole-body kinematics were recorded using 3D motion capture and processed in OpenSim to compute linear and angular momentum. Translation perturbations caused large deviations in whole-body linear momentum with minimal changes in whole-body angular momentum, whereas rotation perturbations induced strong whole-body angular momentum deviations with smaller changes in linear momentum. Including whole-body angular momentum minimally improved foot placement predictions during early swing after rotation perturbations. These findings indicate that mediolateral foot placement is primarily governed by linear momentum dynamics.

Humans regularly walk across uneven terrain, which demands the use of reactive control strategies to maintain forward progress and stability. While reactive control during walking has been well described during straight gait, it is unclear how reactive control differs during turning gait. Because turning is asymmetrical, perturbations to the inside and outside limbs may elicit different reactive adjustments. This study investigates how unexpected underfoot perturbations alter stability measures during turning and how individuals alter their foot placement to maintain stability after such perturbations. Seven healthy adults completed walking trials around a circular track while wearing mechanized shoes that pseudo-randomly delivered underfoot perturbations to either the inside or outside limb. We calculated mediolateral margin of stability corrected for centripetal acceleration (ML MoSC), step width, and step length from kinematic data. Linear mixed effects models compared the effects of perturbation type (inversion vs eversion), perturbation limb (inside vs outside), and their interaction for each outcome measure. ML MoSC was affected by both perturbation type and perturbation limb, with larger changes observed during eversion, and outside, perturbations. Changes to step width and step time during two recovery steps after each perturbation were primarily influenced by the perturbation limb - outside perturbations elicited consistent changes during two recovery steps compared to one altered step after inside perturbations. Perturbations to the outside limb during turning disrupt gait longer than perturbations to the inside limb. This difference across perturbation limb may indicate that outside steps are more important to maintaining and recovering stability than inside ones during turning.

BackgroundJoint hyper-resistance is a common symptom in cerebral palsy (CP). It is assessed by rotating the joint of a relaxed patient. Joint rotations also occur when perturbing functional movements. Therefore, joint hyper-resistance might contribute to reactive balance impairments in CP. AimTo investigate relationships between altered muscle responses to isolated joint rotations and perturbations of standing balance in children with CP. Methods & procedures20 children with CP participated in the study. During an instrumented spasticity assessment, the ankle was rotated as fast as possible from maximal plantarflexion towards maximal dorsiflexion. Standing balance was perturbed by backward support-surface translations and toe-up support-surface rotations. Gastrocnemius, soleus, and tibialis anterior electromyography was measured. We quantified reduced reciprocal inhibition by plantarflexor-dorsiflexor co-activation and the neural response to stretch by average muscle activity. We evaluated the relation between muscle responses to ankle rotation and balance perturbations using linear mixed models. Outcomes & resultsCo-activation during isolated joint rotations and perturbations of standing balance was correlated across all levels. The neural response to stretch during isolated joint rotations and balance perturbations was not correlated. Conclusions & implicationsReduced reciprocal inhibition during isolated joint rotations might be a predictor of altered reactive balance control strategies. HighlightsO_LIImpaired reciprocal inhibition might underlie altered balance control in CP. C_LIO_LICo-activation during isolated joint rotations and balance responses is correlated. C_LIO_LIHyperreflexia is not correlated with increased response to perturbations of standing. C_LIO_LIReduced reciprocal inhibition has functional implications. C_LIO_LIIt might be valuable to clinically assess reduced reciprocal inhibition. C_LI What this paper addsIt has been hard to relate alterations in muscle coordination during functional movements to alterations in the muscles response to isolated joint rotations as applied during (clinical) assessments of hyper-reflexia. Here, we performed a more comprehensive assessment of the altered muscle response to isolated joint rotations in children with cerebral palsy (CP) by not only considering muscle activity in response to stretch but also agonist-antagonist co-activation. Muscle co-activation in response to isolated joint rotations in relaxed patients has been attributed to reduced reciprocal inhibition in the spinal cord. We found that muscle co-activation during isolated joint rotations was correlated to muscle co-activation during perturbed standing, an important functional movement. Therefore, increased muscle co-activation during standing balance control might - at least partially - result from reduced reciprocal inhibition in the spinal cord. In contrast, we found very few relations between the mean muscle activity during isolated joint rotations and perturbed standing. This might be due to the sensitivity of the response to stretch to stretch velocity, posture, and baseline muscle activity, all of which largely differed between the two conditions. Our results indicate that clinical assessment of reduced reciprocal inhibition during isolated joint rotations might provide information about balance impairments.

Markerless motion capture has emerged as a potential substitute for traditional marker-based systems, offering scalable, non-invasive acquisition of human movement. Despite increasing adoption in research and sports applications, its clinical utility for children with complex gait patterns remains an open question. To address this gap, simultaneous marker-based and markerless data were collected in 202 pediatric children (12.1 {+/-} 3.9 years). Marker-based kinematics were processed using the Shriners Childrens Gait Model (SCGM), while markerless outputs were computed using Theia3D with identical Cardan sequences. Agreement between systems was evaluated using statistical parametric mapping (SPM), root-mean-square error (RMSE), and a gait pattern classification based on the plantarflexor-knee extension index. Markerless output systematically underestimated pelvic tilt, hip rotation, and knee rotation and demonstrated reduced between-subject variance in the transverse plane. SPM revealed widespread waveform differences, although most were of negligible effect, especially in the sagittal plane. Mean sagittal-plane RMSEs were < 5{degrees} for the knee and ankle and < 8{degrees} for the pelvis and hip. Coronal-plane deviations were < 7{degrees}, whereas transverse-plane errors exceeded 10{degrees}. RMSE increased significantly with body mass index and use of a walker (p < 0.001). Agreement in sagittal-plane gait classification was moderate between systems ({kappa} = 0.60; 67% overall concordance). These results indicate that markerless motion capture is suitable for analyses emphasizing sagittal deviations but remains limited for applications requiring precise axial or frontal-plane estimation. Future work should address algorithmic underestimation of transverse motion and evaluate markerless performance across increasing severity of gait deviation.

BackgroundLower limb injuries commonly occur during sudden deceleration movements. Instructing landing with forefoot or rearfoot is hard to answer, especially considering the injury prevention and keep the performance. Therefore, the purpose of this study is to evaluate the effect of the forefoot and rearfoot landing on the biomechanical risk of lower limb injury prevention and performance during the stop-jumping task. MethodTwenty-three male health subjects were recruited for this study. During a stop-jumping task, three-dimensional kinematic and kinetic, and performance data were collected under two conditions: forefoot landing at initial ground contact and rearfoot landing at initial ground contact. Statistical parametric mapping analysis was used to compare the differences between different landing strategies. ResultSignificant differences were found in ankle internal rotation angle and ankle joint moment at foot initial contact with ground between different landing strategies. Landing with forefoot has shorter stance time compared with landing with rearfoot. In the rearfoot strike, posterior ground reaction force (GRF) and GRF inclination angle were smaller than forefoot strike in 0-14% of the stance phase. ConclusionLanding with forefoot may decrease the risk of non-contact anterior cruciate ligament injuries and have an advancement in quick reaction time, as indicated by decreased stance time, but the risk of lateral ankle sprain may increase for the stop-jump task.

Metabolic power (net energy consumed while walking per unit time) is, on average, two-to-three times greater in children with cerebral palsy (CP) than their typically developing peers, contributing to greater physical fatigue, lower levels of physical activity and greater risk of cardiovascular disease. The goal of this project was to identify the total causal effects of clinical factors that may contribute to high metabolic power demand in children with CP. We included children who 1) visited Gillette Childrens Specialty Healthcare for a quantitative gait assessment after the year 2000, 2) were formally diagnosed with CP, 3) were classified as level I-III under the Gross Motor Function Classification System and 4) were 18 years old or younger. We created a structural causal model that specified the assumed relationships of a childs gait pattern (i.e., gait deviation index, GDI) and common impairments (i.e., dynamic and selective motor control, strength, and spasticity) with metabolic power. We estimated causal effects using Bayesian additive regression trees, adjusting for factors identified by the causal model. There were 2157 children who met our criteria. We found that a childs gait pattern, as summarized by the GDI, affected metabolic power approximately twice as much as the next largest contributor. Selective motor control, dynamic motor control, and spasticity had the next largest effects. Among the factors we considered, strength had the smallest effect on metabolic power. Our results suggest that children with CP may benefit more from treatments that improve their gait pattern and motor control than treatments that improve spasticity or strength.

Understanding the vertical ground reaction force (vGRF) profile offers important insight into how humans regulate mechanical work during walking. Although the characteristic double-hump vGRF pattern is well documented, the mechanical factors underlying asymmetry in peak amplitudes and midstance trough timing remain unclear. Using a simple powered walking model and an inverted pendulum simulation with constant hip torque, we examined how step-transition work--collision and push-off--shapes the vGRF trajectory. We further compared these predictions to empirical data spanning walking speeds from 0.8-1.4 m. s-1. The simple walking model predicted symmetric vGRF profiles across speeds because collision and push-off impulses were equal, resulting in passive single-support motion. In contrast, adding hip torque within the pendular model produced stance-phase asymmetries, shifting the vGRF trough earlier when torque added energy and later when torque dissipated energy. Empirical analysis revealed that collision and push-off impulses were generally unequal except at one speed, producing asymmetric vGRF peaks. At low speeds, push-off exceeded collision; at high speeds, the reverse occurred, consistent with a need for compensatory single-support positive work. These mechanical imbalances predicted systematic shifts in trough timing toward the dominant impulse. Therefore, we propose the Vertical GRF Trough Timing Index (vGRF-TTI), combined with collision and push-off peak amplitudes, as a clinically meaningful outcome capturing the balance of step-transition work. Earlier troughs with elevated collision peaks indicate impaired push-off or constrained gait conditions, whereas later troughs with larger push-off peaks reflect compensatory or enhanced propulsion. These metrics provide sensitive, mechanism-based indicators of gait efficiency and neuromotor control.

BackgroundClinical researchers are trying to unravel the impact of different training interventions on the kinematics of human gait. However, the effects of long-term training experience on the kinematics of a healthy gait pattern remains unclear. Here we assess the effect of long-term training experience on joint angle variability during walking. MethodsHip, knee, and ankle joint angles from fourteen soccer players and sixteen controls were acquired during treadmill and overground walking. Hip-knee coupling, knee-ankle coupling and coupling angle variability (CAV) of the right leg in the sagittal plane were assessed using a vector coding technique. ResultsSoccer players showed reduced hip-knee CAV during the mid-stance and terminal-stance phases and reduced knee-ankle CAV during the pre-swing phase of gait compared to the control group. In addition, soccer players less often used an ankle coordination pattern, in which only the ankle joint but not the knee joint rotates. InterpretationThese findings show that soccer players had more stability in the ankle joint during the stance phase of the gait compared to the control group. Future studies can test whether these differences in the coordination of the ankle joint reflect the effects of long-term training on normal gait by comparing knee-ankle coupling and variability before and after exercise training interventions.

As markerless motion capture technologies develop and mature, there is an increasing demand from the biomechanics community to provide kinematic data with the same level of accuracy as current gold standard methods. The purpose of this study was to evaluate how adding synthetic data to the training dataset of a deep learning based markerless biomechanics system impacts the accuracy of kinematic measurements during two functional movements. Synchronized video from multiple camera views was captured along with marker-based data from 9 subjects who performed 3 repetitions of countermovement jumps and squats. Including synthetic data to the training reduced lower limb error on average by 65.1% and 70.1% for the countermovement jump and squat movements, respectively. These results demonstrate the promising utility of supplementing the training of a deep learning markerless motion capture system with synthetic data.

Musculoskeletal models of the shoulder are needed to understand the mechanics of overhead motions. Existing models can be scaled to represent the size of an individual person, but the kinematics are generic. We introduce a method to personalize the shoulder kinematics of a computational model of the upper body that defines the orientations of the clavicle and scapula based on glenohumeral joint angles. During five static calibration poses, we palpate and measure the orientation of the scapula. We explore the importance of representing shoulder elevation by introducing clavicle elevation as a degree of freedom that is independent of the glenohumeral angles. For ten subjects, we record the five calibration poses, ten additional static poses, and dynamic arm raises using optical motion capture. We examine the data using a dynamically-constrained inverse kinematics analysis. Personalization, independent clavicle elevation, and both in combination reduce the average upper body marker tracking error compared to the generic model in the static poses (26 mm to 17-20 mm) and in the dynamic trials (22 mm to 14-17 mm). Only personalization reduces the average scapula marker error (51 mm to 36-38 mm) and scapula axis-angle error (15{degrees} to 10{degrees}) in the static poses, and in the dynamic trials at instances that best match the static poses (53 mm to 37-40 mm, 15{degrees} to 9{degrees}). Our results show that personalizing upper body models improves kinematic tracking. We provide our experimental data, model, and methods to allow researchers to reproduce and build upon our results.

Adaptive treadmills (ATMs) that change speed based on the users gait mechanics can allow for healthy stride to stride variability observed during overground walking, while maintaining the benefits of traditional treadmill training for gait rehabilitation. To enable unilateral targeting of propulsion, we developed an adaptive split-belt treadmill (sATM) that updates the speed of each belt of the treadmill based on the users propulsion and position. The present study validated the sATM against an existing tied-belt ATM that updates the speed of both belts based on the users propulsion, step length, and position. We also determined the effect of modifying the importance of propulsion unilaterally on the sATM. To validate the sATM, fourteen young, healthy participants completed five trials at their comfortable walking speed on the tied ATM and sATM with unilateral and bilateral modifications. We hypothesized that propulsion and walking speed on the novel sATM would be equivalent to the tied ATM, and further that the sATM could be preferentially weighted to encourage higher propulsion unilaterally while maintaining walking speed. Participants maintained similar propulsion and walking speed between treadmills and belts for bilateral modifications. While a few participants increased unilateral propulsion to maintain similar walking speeds for the unilateral modification, others showed no change in propulsion with slight differences in walking speed. Our results suggest that the response to unilateral modifications was user-specific, and the sATM could be tuned individually to allow unilateral control strategies. The present study demonstrates that walking speed can be modulated on an adaptive split-belt treadmill.

Passive joint stiffness reflects the stiffness of various soft tissues across a joint. However, no previous studies have investigated the relationship between passive joint stiffness and muscle, nerve, and tendon stiffness. This study aimed to clarify whether passive ankle joint stiffness is related to stiffness in the triceps surae muscles, tibial nerve, and Achilles tendon. Thirty-eight healthy adults participated in the study. The passive ankle joint stiffness (slope of angle-passive torque curve) and shear wave velocities, which indicate soft tissue stiffness, of the triceps surae muscles and tibial nerve were measured at 5{degrees} of ankle plantarflexion and 5{degrees}, 15{degrees}, and 25{degrees} of ankle dorsiflexion. The shear wave velocity of the Achilles tendon was measured only at 5{degrees} of plantarflexion. A multiple regression model (forced-entry method) was constructed at each angle, specifying the shear wave velocities as the independent variables and passive joint stiffness as the dependent variable. At 5{degrees} of plantarflexion, no shear wave velocities were significantly related to passive joint stiffness (all p [&ge;] 0.05). At 5{degrees} and 15{degrees} of dorsiflexion, only the shear wave velocities of the tibial nerve were significantly positively related to passive joint stiffness (p = 0.024 and 0.008, respectively). At 25{degrees} of dorsiflexion, the shear wave velocities of the lateral gastrocnemius muscle and tibial nerve were significantly positively related to passive joint stiffness (p = 0.002 and 0.001, respectively). It can be concluded that both triceps surae muscles stiffness and tibial nerve stiffness are related to passive ankle joint stiffness.

BackgroundInter-joint coordination plays a key role in walking, particularly in people with cerebral palsy (CP), who experience altered movement patterns. The Continuous Relative Phase (CRP) method quantifies lower-limb coordination by assessing the phase relationships between joints. However, the reliability of CRP measurements during walking in individuals with CP remains unexplored, and may be affected by measurement variability due to marker placement errors, soft tissue artifacts, and natural movement fluctuations. Quantifying this reliability is important for appropriate clinical comparisons. This study aimed to quantify within- and between-assessor reliability of lower-limb CRP measurements in individuals with CP and their non-impaired (NI) peers. MethodsCP (n=19, age=18.4{+/-}7.3 years, GMFCS I-III) and NI (n=19, age=18.3{+/-}11.2 years) individuals completed two gait assessment sessions, each including 3D motion capture of at least 10 walking trials. Two trained assessors independently placed reflective markers and conducted gait analyses. Standard error of measurement (SEM) and minimal detectable change (MDC) were computed for knee-hip and ankle-knee coordination across gait subphases. FindingsThe SEM and MDC were lower for knee-hip than ankle-knee coordination, suggesting higher measurement reliability for proximal joint coupling. For knee-hip coordination, MDC reached 15.1{+/-}0.7{degrees}(CP) and 9.3{+/-}0.6{degrees}(NI) between assessors, and 23.8{+/-}3.0{degrees}(CP) and 9.1{+/-}1.6{degrees}(NI) within assessors. For ankle-knee coordination, MDC reached 29.0{+/-}2.6{degrees}(CP) and 25.0{+/-}3.5{degrees}(NI) between assessors, peaking at 47.3{+/-}10.9{degrees}(CP) and 28.6{+/-}1.5{degrees}(NI) in mid-swing within assessors. InterpretationThis study provides the first metrological reference for reliability of CRP-based inter-joint coordination during gait in CP. Results showed poor reliability, emphazing that such measurements must be interpreted with caution. Highlights- Knee-hip showed greater reliability than ankle-knee coordination across gait phases - Cerebral palsy individuals showed higher variability than non-impaired peers The beginning and the end of gait cycle showed the poorest reliability Pre-post comparisons should account for MDC thresholds to avoid misinterpretation

The reliability and validity of the Jumpster app (JA) was compared to the Vertec. Thirty-six participants completed 100 total trials using both tools simultaneously. Validity was assessed using correlation and tolerance analyses. Reliability was assessed using 95% predictive intervals (PI95) and tolerance limits (TL95) between the measures, comparing standard error of the measure (SEM) and coefficients of variation (CV) for each tool, and examining the intraclass correlation coefficient (ICC 2,K; upper and lower 95% CI) comparing both tools. The JA was weakly related to the Vertec (r = 0.24; p < 0.01). The tolerance analysis showed a moderately strong proportional bias of the JA (r = 0.45; p < 0.01). While all data fell within calculated PI95 {+/-}TL95, the JA SEM (14.7cm) and CV (40.30%) exceeded the Vertec SEM (3.57cm) and CV (7.22%) with an ICC of 0.55 [0.79, -0.08]. These JA is neither reliable or valid.

Generalization of movements from experienced to novel situations is a critical aspect of motor learning. It has been demonstrated that the training period when a movement is learned influences the movements generalization to untrained situations. However, little is known about how healthy aging affects these processes. For example, young adults exhibit greater generalization of movements learned on a device (e.g. split-belt treadmill or robotic arm) to movements without it (e.g, overground walking or unconstrained reaching) when participants experience small vs. large perturbations on the training device. Here, we investigated whether a similar effect would be observed in older adults. To this end, we compared the generalization of split-belt adaptation to overground walking in older (75.9{+/-}4.8 years old) and young adults (24.7{+/-}5.9 years old) when adapted gradually (i.e., small perturbations) vs. abruptly (i.e., large perturbations). We found that both age groups adapted more to the abrupt condition compared to the gradual condition, which resulted in greater adaptation effects (i.e., aftereffects) on the treadmill in the abrupt than the gradual groups. We also found that older adults generalize more than young adults, regardless of the perturbation schedule (i.e., gradual or abrupt). Our results suggest that abrupt perturbations during adaptation do not limit the generalization of movement in older adults-perhaps because they are more likely to attribute them to their own faulty movements. These results suggest that large perturbations are better than small when training older clinical populations since abrupt disturbances would lead to more adaptation and generalization of corrected movements in older people.

BackgroundPromising studies have shown that the mobility of individuals with hemiparesis due to brain lesions, such as stroke, can improve through motor adaptation protocols forcing patients to use their affected limb more. However, little is known about how to facilitate this process. Here we asked if increasing propulsion demands during split-belt walking (i.e., legs moving at different speeds) leads to more motor adaptation and more symmetric gait in survivors of a stroke, as we previously observed in subjects without neurological disorders.\n\nMethodsWe investigated the effect of propulsion forces on locomotor adaptation during and after split-belt walking in the asymmetric motor system post-stroke. To test this, 12 subjects in the chronic phase post-stroke experienced a split-belt protocol in a flat and incline session so as to contrast the effects of two different propulsion demands. Step length asymmetry and propulsion forces were used to compare the motor behavior between the two sessions because these are clinically relevant measures that are altered by split-belt walking.\n\nResultsThe incline session resulted in more symmetric step lengths during late split-belt walking and larger after-effects following split-belt walking. In both testing sessions, subjects who have had a stroke adapted to regain speed and slope-specific leg orientations similarly to young, intact adults. Importantly, leg orientations during baseline walking were predictive of those achieved during split-belt walking, which in turn predicted each individuals post-adaptation behavior.\n\nConclusionThese results indicated that survivors of a stroke can adapt their movements to meet leg-specific kinetic demands. This promising finding suggests that augmenting propulsion demands during split-belt walking could favor symmetric walking in individuals who had a stroke, possibly making split-belt interventions a more effective gait rehabilitation strategy.

The biomechanics underlying bouncing exercises are characterized by the spring-like behavior of the human leg. However, the mechanism underlying the mechanistic contribution of muscle dynamics to the adjustment of leg stiffness is unclear. This study aimed to elucidate the mechanisms governing the changes in leg stiffness during hopping at different frequencies by examining the dynamics of the muscle-tendon complex (MTC) of the medial gastrocnemius muscle (MG). We hypothesized that an increase in muscle stiffness would augment leg stiffness, thereby enabling hopping at higher frequencies. Kinematic and kinetic data were obtained using a motion capture system and force plates. Simultaneously, ultrasound images of the MG were acquired to quantify the muscle fascicle length and pennation angle. The results showed that the stiffness of the MTC increased with hop frequency and exhibited a strong correlation with the leg stiffness. In addition, with increasing frequency, the fascicle contractions shifted from isometric to concentric. To explain these results, an MTC model comprising a contractile component (CC) and series elastic component (SEC) was constructed. We observed a negative CC stiffness, which increased the MTC stiffness. Although this result appears to diverge from our initial hypothesis, the effect of negative CC stiffness on MTC stiffness can be understood, from the perspective of two springs in series, as an extension of the very high stiffness effect. This quantitative understanding of the dynamic interaction between the muscle and tendon offers a unified framework for interpreting various results of previous studies on fascicle dynamics during hopping.

Gait asymmetry is present in several pathological populations, including those with Parkinsons disease, Huntingtons disease, and stroke survivors. Previous studies suggest that commonly used discrete symmetry metrics, which compare single bilateral variables, may not be equally sensitive to underlying effects of asymmetry, and the use of a metric with low sensitivity could result in unnecessarily low statistical power. The purpose of this study was to provide a comprehensive assessment of the sensitivity of commonly used discrete symmetry metrics to better inform design of future studies. Monte Carlo simulations were used to estimate the statistical power of each symmetry metric at a range of asymmetry magnitudes, group/condition variabilities, and sample sizes. Power was estimated by repeated comparison of simulated symmetric and asymmetric data with a paired t-test, where the proportion of significant results is equivalent to the power. Simulation results confirmed that not all common discrete symmetry metrics are equally sensitive to reference effects of asymmetry. Multiple symmetry metrics exhibit equivalent sensitivities, but the most sensitive discrete symmetry metric in all cases is a bilateral difference (e.g. left - right). A ratio (e.g. left/right) has poor sensitivity when group/condition variability is not small, but a log-transformation produces increased sensitivity. Additionally, two metrics which included an absolute value in their definitions showed increased sensitivity when the absolute value was removed. Future studies should consider metric sensitivity when designing analyses to reduce the possibility of underpowered research. Summary statementStatistical power is an important factor in study design. Our results show that not all discrete symmetry metrics have similar or sufficient sensitivity to detect effects of asymmetry.

Accurate modelling of inertial properties of human lower limbs is of great interest to many tasks, from gait analysis in biomechanics to motion tracking and control in computer animation. Previous work typically simplified the human musculoskeletal structure as a chain of rigid capsules, with muscle mass lumped with body segments. Such simplifications lead to errors in the inertia matrix, and the error propagates to torque and pose estimates. In this study, we developed a data-driven model to represent the joint-space inertia of the lower-body of a human in motion. The model does not make any assumptions other than that the estimated inertia matrices must be symmetric and positive definite. We show that a joint-space inertia matrix, estimated from synchronized motion and ground force data following foot strikes, reveals inertial coupling, and that estimated inertia matrices are bilaterally symmetric and motion-type dependent. These are properties which a rigid, mass-lumped inertia matrix fails to entail. Moreover, we show that the data-driven model fits to data better than the articulated rigid body inertia model, and that when used for reconstructing lower body kinematics estimated inertia yields more accurate and stable motion.

Gait events (i.e., heel strikes and toe-offs) are essential for extracting spatiotemporal parameters and segmenting biological signals. While force platforms and optical motion capture (OMC) are ideal for identifying such events, inertial measurement units (IMUs) are cheaper and more applicable, especially for applications outside traditional lab settings. This study aimed to compare IMU- and OMC-based gait event detection to force plates. Seventeen adults walked on a walkway, stepping on two force plates while an IMU on the sacrum and retro-reflective markers on the calcaneus and 5th metatarsus captured foot kinematics. Gait events were identified using two OMC and two IMU methods (OMC1, OMC2, IMU1, IMU2). OMC1 detects gait events using vertical marker velocity shifts, OMC2 uses sagittal velocity thresholds, IMU1 applies wavelet-based differentiation and IMU2 identifies peaks in acceleration signals. Heel strikes and toe-offs were compared to force plate data, assessing root-mean-square error (RMSE), and intra-subject coefficient of variation (CoV). For heel strike events, OMC1 presented the lowest RMSE ([~]14.3ms), significantly differing from IMU1 (RMSE: 50.6ms; p<0,001) and IMU2 (RMSE: 61.1ms; p<0,001). For toe-offs, OMC1 presented the lowest RMSE ([~]17.3ms), differing from IMU1 (54.7ms; p<0,001) and IMU2 (74.8ms; p<0,001). IMU2 also showed the highest CoV ([~]23.9ms) differing from OMC1 ([~]7.1ms; p<0,001) and IMU1 (9.2ms; p<0,001). Lower accuracy and greater variability in IMU methods may stem from the approach used to detect gait events. Thus, OMC methods more accurately detect gait events than sacrum-mounted IMUs. While IMUs offer an alternative, researchers should be cautious of their accuracy and variability limitations.