Journal of Biomechanics

Slip-induced falls are a major contributor to hip fractures and injury, particularly during sideways falls where lateral trunk flexion drives impact to the hip. While past studies have focused on sagittal-plane slip mechanics, the mechanisms of inducing frontal-plane trunk excursion remain poorly understood. This study investigated which slip foot kinematic variables predict lateral trunk flexion during unexpected slips. Twenty-six healthy young adults experienced an unexpected slip while walking in a laboratory setting. Peak anteroposterior (AP) heel slip distance, velocity, and acceleration, as well as mediolateral (ML) slip distance, velocity, and acceleration, were measured using three-dimensional motion capture. A stepwise multiple linear regression identified peak AP heel slip acceleration as the sole significant predictor of lateral trunk flexion (p = .026, R2 = 0.19). One-way repeated measures ANOVA and one-tailed paired t-tests further revealed that peak heel acceleration occurred significantly earlier than both peak heel velocity (p = .012) and the onset of lateral trunk flexion (p < .001), supporting its role as an early destabilizing mechanism. In contrast, neither velocity nor slip distance predicted lateral trunk flexion magnitude. These findings suggest that peak AP heel acceleration is associated with inducing lateral trunk flexion movements that likely precede lateral falls. Reducing heel acceleration through slip-resistant flooring or footwear, or enhancing compensatory strategies such as reactive arm movements, may reduce the likelihood of losing sideways balance. This work highlights the need for balance training interventions that emphasize rapid responses to early-phase slip dynamics. HighlightsO_LIEvery 1000 cm/s2 increase in AP heel acceleration leads to 6 degrees more lateral trunk flexion C_LIO_LIHeel acceleration occurs significantly earlier than heel velocity or the onset of lateral trunk flexion C_LIO_LIFindings highlight the importance in reducing forward heel acceleration to reduce sideways loss of balance C_LI

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.

IntroductionAnterior cruciate ligament (ACL) injuries represent one of the most common and disruptive conditions in sport, with fewer than two-thirds of athletes return to competitive play. Effective return-to-play (RTP) monitoring requires multidimensional approaches that capture physical, psychological, and sport-specific components rather than reliance of isolated benchmarks. PurposeThis study aimed to longitudinally examine the RTP process of a female varsity basketball athlete following ACL reconstruction, using a framework that integrates physical performance (capacity), biomechanical sport-specific (capability), and psychological (confidence) components relative to pre-injury benchmarks. MethodsData collection included countermovement jump testing with dual force plates, on-court inertial measurement unit (IMU) monitoring of limb-loading, and psychological questionnaires, analyzed relative to pre-injury and post-surgery baselines using minimal detectable change thresholds. ResultsPre-injury monitoring indicated stable movement profiles with only minor fluctuations. Following ACL reconstruction, jump height recovered within seven weeks of RTP initiation, but notable inter-limb asymmetries persisted in force plate and IMU measures despite high confidence scores. DiscussionSymmetry improved with continued training, yet variability in on-court loading remained even after clinical clearance. These findings highlight the value of integrated, multidimensional monitoring to detect residual deficits that may be overlooked by traditional outcome-based assessments. ConclusionThis study demonstrates that integrating athlete-specific biomechanical, psychological, and sport-specific assessments relative to pre-injury baselines can support RTP decision-making to enhance individualized recovery trajectories in female athletes.

Humans must actively control their lateral balance through frontal plane work or adjusting lateral foot placement. With constant muscle efficiency and considering the energetic consequences of Center of Mass (COM) work variability, we estimated the metabolic cost of lateral balance maintenance and compared it with the Workman model. Like the Workman model, we found that lateral balance energetics were mainly associated with terrain amplitude. Increased walking speeds effect on step transition work might be offset by reduced step width. A significant rise in lateral work magnitude (+157.1%) with restricted lookahead was potentially linked to wider steps. Comparing mechanical work with the Workman model, we found significant differences in magnitudes, suggesting that the Workman model included additional costs such as force rate generation, muscle coactivation, or posture maintenance not reflected in the COM lateral work and its variability. Practitioner SummaryLateral balance maintenance during walking requires active regulation, yet it is not studied mechanically. Our study found that lateral control costs are terrain-specific and increase with restricted lookahead. With age, only variability increases. We assume associated energetics rise with both work and variability.

Impact exercises are known to increase bone mineral density (BMD) and in turn, bone strength and resistance to fracture. The biochemical pathways driving changes in BMD take months to complete, complicating our ability to understand how specific exercises influence the remodeling stimulus received by the bone. The purpose of this study was to compare several measures that have been theoretically linked to bone remodeling stimulus, including accelerations measured by Inertial Measurement Units (IMUs) at the middle of the tibia, ground reaction forces measured by force plate, joint contact forces estimated by musculoskeletal modeling, and tibia strains estimated by finite element modeling informed by high-resolution CT imaging. Twenty healthy adults (10 male: 22.1 +/- 2.2 years; 10 female: 21.3 +/- 1.3 years) participated in a biomechanical investigation of how drop height and landing style (bilateral vs. unilateral) affect the various bone remodeling stimuli. The results showed that while drop height consistently had significant direct relationships with stimulus magnitude, there was little benefit to drop heights greater than 0.4 m. In contrast, switching from a bilateral to a unilateral landing had a large positive effect. The stimuli calculated based on IMU data showed opposite trends compared to force plate and musculoskeletal modeling-based calculations, highlighting the need for caution in how IMUs are placed, and the resulting data interpreted, in the context of bone loading. A post-hoc analysis showed that a linear regression with predictor variables of kinematics, jump height, landing type (unilateral vs. bilateral) and the Ground Reaction Force FFT Integral could explain 79% of the variance in the bone remodeling stimulus that was predicted using much more sophisticated (and labor intensive) modeling. We conclude that higher level biomechanical modeling may not be necessary to understand the magnitude of a bone remodeling stimulus of an exercise.

Bipedal gait is inherently unstable, requiring a complex interplay between foot placement and centre of mass (CoM) movement to maintain balance. While various factors are known to impact walking balance, few studies have explored the specific effects of functional asymmetry on lateral stability. This study investigates how step length, step width, and CoM adaptations impact lateral gait stability in healthy young adults walking with and without a functional asymmetry induced by fully extending the left knee. The results show that step length remains unaffected by functional asymmetry regardless of speed, while step width increases under the constraint. This adjustment increases the base of support; however, the concurrent increase in lateral CoM movement reduces overall lateral stability. These findings offer valuable insights into fundamental gait adaptation and stability mechanisms, with potential implications for designing rehabilitation strategies for individuals with gait asymmetry.

ObjectivesEvaluate procedures for analyzing raw accelerometer data (inertial measurement units) to reconstruct the gait cycle using BioKinetoGraph (BKG). We examine whether footwear and walking surface influence gait (BKG) and evaluate test-retest reliability. We also examine the association between BKG and NIH 4-meter gait, and compare BKG to other neurobehavioral measures for predicting concussion symptoms. MethodsIn Study 1, a within-subjects design with 60 participants was used to examine the effects of footwear (shoes/no-shoes) and walking surface (tile floor/grass) on BKG data, and evaluate retest reliability. Study 2 employed a cross-sectional, cohort design of 1,008 participants to assess BKGs correlation with NIH 4-m gait, and prediction of Centers of Disease Control and Prevention (CDC) concussion symptoms relative to previously validated speed and balance measures. Results2x2 ANOVAs illustrate footwear and walking surface effects on BKG for the power, stride, stability, and symmetry, with variable effect sizes. Retest reliability (Pearson rs) for the no shoes/ tile surface condition ranged from .72-.91 (mean = .80, 4-day average interval). BKG correlates significantly with NIH 4-m gait. Regression analyses found BKG predicts CDC concussion symptom endorsement, and outperforms (2-3 fold) BESS and NIH 4-meter gait. ConclusionsGait assessments should be standardized for footwear and especially walking surface. When standardized (no shoes/hard surface) BKG results in strong test-retest reliability. BKG variables are strongly related to NIH 4-m gait, and are superior to standard measures of gait speed and balance when predicting concussion symptoms; offering additional information when predicting the sequalae of concussion. Summary BoxO_ST_ABSWhat is already known on the topic?C_ST_ABSO_LISensor technology to evaluate gait has established reliability and predicts a wide range of medical outcomes. However, the influence of footwear and walking surface on gait has not been studied, nor has a sensor-based gait assessment been compared to conventional measures for predicting concussion symptoms. C_LI What this study adds?O_LIGait sensor data is sensitive to footwear and walking surface, but can produce good reliability when these factors are standardized. Gait sensor scores converge with other validated measures of gait, and sensor-based measures of power, stride, symmetry, and stability can outperform established gait speed and balance measures when predicting CDC concussion symptoms. C_LI How this study might affect research, practice, or policy?O_LIUniform standards for footwear and walking surface are needed when evaluating gait in both research and practice, and sensor-based gait measures can be reliably assessed to provide insight into the behavioral sequelae of concussion, that are superior to simple gait speed. C_LI

Mediolateral gait stability can be maintained by coordinating our foot placement with respect to the center-of-mass (CoM) kinematic state. Neurological impairments can reduce the degree of foot placement control. For individuals with such impairments, interventions that could improve foot placement control could thus contribute to improved gait stability. In this study we aimed to better understand two potential interventions, by investigating their effect in neurologically intact individuals. The degree of foot placement control can be quantified based on a foot placement model, in which the CoM position and velocity during swing predict subsequent foot placement. Previously, perturbing foot placement with a force-field resulted in an enhanced degree of foot placement control as an after-effect. Moreover, muscle vibration enhanced the degree of foot placement control through sensory augmentation whilst the vibration was applied. Here, we replicated these two findings and further investigated whether Q1) sensory augmentation leads to an after-effect and Q2) whether combining sensory augmentation with force-field perturbations leads to a larger after-effect, as compared to force-field perturbations only. In addition, we evaluated several potential contributors to the degree of foot placement control, by considering foot placement errors, CoM variability and the CoM position gain ({beta}pos) of the foot placement model, next to the R2 measure as the degree of foot placement control. Sensory augmentation led to a higher degree of foot placement control as an after-effect (Q1). However, combining sensory augmentation and force-field perturbations did not lead to a larger after-effect, as compared to following force-field perturbations only (Q2). Furthermore, we showed that, the improved degree of foot placement control following force-field perturbations and during/following muscle vibration, did not reflect diminished foot placement errors. Rather, participants demonstrated a stronger active response (higher {beta}pos) as well as higher CoM variability.

Musculoskeletal geometry and muscle volumes vary widely in the population and are intricately linked to the performance of tasks ranging from walking and running to jumping and sprinting. However, our ability to understand how these parameters affect task performance has been limited due to the high computational cost of modelling the necessary complexity of the musculoskeletal system and solving the requisite multi-dimensional optimization problem. For example, sprinting and running are fundamental to many forms of sport, but past research on the relationships between musculoskeletal geometry, muscle volumes, and running performance has been limited to observational studies, which have not established cause-effect relationships, and simulation studies with simplified representations of musculoskeletal geometry. In this study, we developed a novel musculoskeletal simulator that is differentiable with respect to musculoskeletal geometry and muscle volumes. This simulator enabled us to find the optimal body segment dimensions and optimal distribution of added muscle volume for sprinting and marathon running. Our simulation results replicate experimental observations, such as increased muscle mass in sprinters, as well a mass in the lower end of the healthy BMI range and a higher leg-length-to-height ratio in marathon runners. The simulations also reveal new relationships, for example showing that hip musculature is vital to both sprinting and marathon running. We found hip flexor and extensor moment arms were maximized to optimize sprint and marathon running performance, and hip muscles the main target when we simulated strength training for sprinters. Our simulation results can help sprint and marathon runners customize strength training, and our simulator can be extended to other athletic tasks, such as jumping, or to non-athletic applications, such as designing interventions to improve mobility in older adults or individuals with movement disorders. AUTHOR SUMMARYOur study addresses the challenge of determining optimal musculoskeletal parameters for tasks like sprinting and marathon running. Existing research has been limited to observational studies and simplified simulations. To overcome these limitations, we developed a differentiable musculoskeletal simulator to optimize running performance. We replicated past findings and uncovered new insights. We confirmed the benefits of increased muscle mass for sprinters and identified key factors for marathon runners, such a mass in the lower end of the healthy BMI range and an increased leg-length-to-height ratio. Hip musculature was found to be critical for both sprinting and marathon running. Our simulation results have practical implications. They can inform customized strength training for sprinters and marathon runners. Additionally, the simulator can be extended to other athletic tasks, benefiting various sporting events. Beyond athletics, our open-source simulator has broader applications. It can determine minimal strength requirements for daily activities, guide strength training in the elderly, and estimate the effects of simulated musculoskeletal surgery.

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.

PurposeReducing strains within the tibia and fibula during running may reduce the risk of stress fractures. We examined the effect of reduced flight time during running (i.e., grounded running) on finite-element predicted bone strains within the tibia-fibula complex. MethodsNine physically active males ran on an instrumented treadmill at 2.2 m/s using a preferred and reduced flight time technique in a randomized order. Three-dimensional force and motion capture data were recorded during running and a computed tomography image was subsequently acquired for the participants left leg. An inverse-dynamics-based musculoskeletal modeling workflow was used to calculate bone-on-bone contact and muscle forces during the stance phase of running. These forces served as inputs to a participant-specific finite-element model to estimate peak bone strains and strained volume (i.e., the volume of bone experiencing strains above a specific threshold) within the tibia-fibula complex. ResultsGuided attempts to reduce flight time was successful with an 18 ms (95% CI: 12 ms, 25 ms; p<0.001) reduction in flight time. Reducing flight time was associated with significant reductions in peak tibial/fibular strains (17% lower; 95% CI: -7.1%, -25.0%; p=0.002) and strained volume (35% lower; 95% CI: -13.57%, -50.87%; p=0.007). ConclusionWe conclude that guided attempts to reduce flight time significantly reduces strains in the tibia and fibula during treadmill running at a fixed speed. These results suggest that grounded running may be a viable technique to reduce musculoskeletal loading and stress fracture risk, particularly in slow runners and those runners coming back from injury.

During daily walking, humans might contend with various perturbations from slippery surfaces in the winter to uneven sidewalks in the summer. Inertial sensors enable investigations of how humans maintain balance under these natural conditions, but conducting these outdoor studies has practical considerations that might influence study results, such as the selection of footwear under different weather conditions. Our study investigates the effects of winter and summer shoe types on gait patterns, specifically whether different shoe types induce changes in gait stability measures under the same walking environment. Twelve healthy adult participants walked indoors with winter and summer shoes while their gait kinematics were recorded using an inertial sensor-based motion capture system. Spatiotemporal measures, body kinematics, stability measures (minimum margin of stability and local divergence exponent), and stepping regressions were calculated to evaluate differences between walking in summer and winter shoes. Statistical significance was determined by paired t-tests. Varying shoe types altered spatiotemporal and kinematic measures, such as increased stride time and stance time while wearing winter shoes, but increased step width and reliance on stepping were the only stability-related changes found. Our study provides insights into the influence of footwear for inertial sensor-based gait studies in real environments, aiding the analysis and interpretation of those studies to augment our understanding of natural stability behavior.

Tears to the rotator cuff often require surgical repair. These repairs often culminate in re-tearing when sutures break through the tendon in the weeks following repair. Although numerous studies have been performed to identify suturing strategies that reduce this risk by balancing forces across sutures, none have accounted for how the viscoelastic nature of tendon influences load sharing. With the aim of providing insight into this problem, we studied how tendon viscoelasticity, tendon stiffness, and suture anchor spacing affect this balancing of forces across sutures. Results from a model of a three-row sutured re-attachment demonstrated that optimized distributions of suture stiffnesses and of the spacing of suture anchors can balance the forces across sutures to within a few percent, even when accounting for tendon viscoelasticity. Non-optimized distributions resulted in concentrated force, typically in the outermost sutures. Results underscore the importance of accounting for viscoelastic effects in the design of tendon to bone repairs.

BackgroundBalance studies usually focus on quantities describing the global body motion, such as the position of the whole-body centre of mass (CoM), its associated extrapolated centre of mass (XCoM) and the whole-body angular momentum (WBAM). Assessing such quantities using classical marker-based approach can be tedious and modify the participants behaviour. The recent development of markerless motion capture methods could bypass the issues related to the use of markers. Research questionCan we use markerless motion capture systems to study quantities that are relevant for balance studies? MethodsSixteen young healthy participants performed four different motor tasks: walking at self-selected speed, balance loss, walking on a narrow beam and countermovement jumps. Their movements were recorded simultaneously by marker-based and markerless motion capture systems. Videos were processed using a commercial markerless pose estimation software, Theia3D. The position of their CoM was computed, and the associated XCoM and WBAM were derived. Bland-Altman analysis was performed and root mean square error and coefficient of determination were computed to compare the results obtained with marker-based and markerless methods across all participants and tasks. ResultsBias remained of the magnitude of a few mm for CoM and XCoM position, and RMSE of CoM and XCoM was around 1 cm. Confidence interval for CoM and XCoM was under 2 cm except for one task in one direction. RMSE of the WBAM was less than 8% of the total amplitude in any direction, and bias was less than 1%. SignificanceResults suggest that the markerless motion capture system can be used in balance studies as the measured errors are in the range of the differences found between different models or populations in the literature. Nevertheless, one should be careful when assessing dynamic movements such as jumping, as they displayed the biggest errors. HighlightsO_LIMarkerless motion capture could bypass issues from classical marker-based approaches C_LIO_LIWe compared balance related quantities computed from both approaches C_LIO_LIMean differences were about 1cm on the position of the whole-body center of mass C_LIO_LIObtained differences are acceptable for most applications C_LI

Navigating stairs is a challenging task for many patient populations. Unfortunately, assessing lower extremity kinetics is not practical in many laboratories due in part to methodologic constraints. In this study, we designed, fabricated, and calibrated a staircase that accurately measured ground reaction forces applied to the second and fourth step. This implementation met several design criteria that included low-cost, ability to quickly move the staircase in and out of motion capture spaces, stable and safe staircase, and easily modifiable to meet the constraints of different lab layouts. We built the staircase as an outer and inner staircase assembly constructed using a modular aluminum framing system. Once positioned on our force plates that were embedded in the lab floor, we used an instrumented pole to apply known loads to a series of surface locations on the force plates and steps that were resting on top of the force plates. This calibration procedure reduced the center of pressure errors by approximately 50% for the embedded force plates and lower step (step 2) and 3-fold for the higher step (step 4). Next, we demonstrated that these steps can be integrated into a clinical gait analysis workflow. A single healthy-young adult navigated the stairs, the ground reaction forces were transformed into stair reaction forces, and these external loads were used to solve the inverse dynamics problem. This staircase provides other researchers with a new tool to assess stair navigation biomechanics. In this study, we provided the bill of materials, mechanical drawings, and calibration code necessary to modify and implement this staircase paradigm into other lab layouts.

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.

Three-dimensional gait analysis is widely used to support clinical decision-making in neuromuscular disorders, with the Conventional Gait Model (CGM) being the most commonly applied biomechanical model in clinical practice. Recent developments of the CGM, grouped under the open-source CGM2 framework, introduced methodological updates intended to improve robustness while preserving backward compatibility. However, the reliability of these successive CGM2 iterations has not been comprehensively evaluated, particularly in pathological gait populations. This study investigated within- and between-assessor reliability of lower-limb kinematics across three CGM2 versions (2.1, 2.2, and 2.3) in asymptomatic participants and individuals with cerebral palsy. Reliability was quantified using standard error of measurement and minimal detectable change across the gait cycle. Overall measurement error remained low and consistent across models and participant groups, with standard errors close to 2{degrees} and minimal detectable changes around 6{degrees}. Introducing kinematic fitting had minimal influence on reliability, while adding tracking markers on the thigh and shank produced a modest reduction in hip transverse rotation error. These findings indicate that methodological refinements implemented in CGM2 preserve the reliability of the original CGM while providing incremental improvements for clinically relevant parameters, supporting its use in both asymptomatic and pathological gait analysis and longitudinal clinical assessments

Background and ObjectivesGround reaction forces and moments (GRF&Ms), typically measured using force plates, are key inputs for musculoskeletal simulations. OpenSim currently lacks a tool to predict GRF&Ms directly from kinematics. This study presents OpenGRF, an OpenSim-based tool designed to estimate GRF&Ms and the centre of pressure (CoP) from joint kinematic data and validates its performance against force plate recordings. MethodsThe proposed methodology integrates calibrated foot-ground contact probes with an optimization framework based on computed muscle control, while CoP is computed accounting for both kinematic and dynamic contributions. For validation, a scaled FullBodyModel (37-DoF without muscles) was created for seven healthy adults performing six trials each of level walking, stair ascent, and stair descent, for a total of 126 marker-based trials. GRF&Ms predictions were compared to reference force plate data using normalized RMSE (nRMSE), Pearson correlation coefficients ({rho}), CoP error, and Statistical Parametric Mapping (SPM) ResultsResults showed high accuracy for vertical GRF (nRMSE [&le;]1.5%, {rho} [&ge;]0.94), particularly during level walking, and good accuracy for anterior-posterior GRF (nRMSE 4.4-6.1%, {rho} = 0.81-0.91). Medio-lateral GRF was less reliable, especially in stair tasks (nRMSE up to 11.2%, {rho} down to 0.48). Free moments were the most challenging quantity to predict across all tasks (nRMSE up to 28%). In contrast, ankle moments were predicted with high fidelity (nRMSE {approx}1.7%, {rho} {approx}0.98). Median CoP errors were 21-23 mm, with largest discrepancies during double support. ConclusionsOpenGRF enables physics-consistent estimation of GRF&Ms and CoP directly from kinematics, achieving the highest accuracy for vertical GRFs and predicting ankle moments that closely match those obtained from force plate measurements.

People use the mechanical interplay between stability and manoeuvrability to successfully walk. During single limb support, body states (position and velocity) that increase lateral stability will inherently resist lateral manoeuvres, decrease medial stability, and facilitate medial manoeuvres. Although not well understood, people can make behavioural decisions exploiting this relationship in anticipation of perturbations or direction changes. To characterize the behavioural component of the stability-manoeuvrability relationship, twenty-four participants performed many repetitions of a discrete stepping task involving mid-trial reactive manoeuvres (medial or lateral direction) in a Baseline (no external perturbations) and Perturbed (random mediolateral perturbations applied to their pelvis) environment. We hypothesized people would make systematic changes in lateral stability dependent on both environment (increasing lateral stability in the Perturbed environment) and anticipated manoeuvre direction (reducing lateral stability to facilitate lateral manoeuvres). Participants increased lateral stability in the Perturbed environment, coinciding with an increase in manoeuvre reaction time for laterally but not medially directed manoeuvres. Moreover, we observed lower lateral stability in both environments when people anticipated making a lateral manoeuvre when compared to medial manoeuvres. These results support the hypothesis that people behaviourally exploit the mechanical relationship between lateral stability and manoeuvrability depending on walk task goals and external environment.

In Rugby a high proportion of catastrophic cervical spine injuries occur during tackling. In the injury prevention literature, there is still an open debate on the injury mechanisms related to such injuries, with hyperflexion and buckling being under scrutiny. The aims of this study were to determine the primary cervical spine injury mechanism during head-on rugby tackling, and evaluate the effect of tackling technique on cervical spine intervertebral loading. We conducted an in silico study to examine the dynamic response of the cervical spine under loading conditions representative of accidental head-on rugby tackles by using a subject-specific musculoskeletal model of a rugby player. The computer simulations were driven by experimental in vivo data of an academy rugby player tackling a punchbag, and in vitro data of head-first impacts using a dummy head. Results showed that: i) the earlier generation of high compression and anterior shear loads with low values of flexion moments provides evidence that hyperflexion is unlikely to be the primary injury mechanism in the sub-axial cervical spine (C3-C7) during central and posterior head impact locations; ii) a higher degree of neck flexion at impact poses the cervical spine in a more hazardous position. These findings provide objective evidence to inform injury prevention strategies or rugby law changes, with the final view of improving the safety of the game of rugby.