Search

Abstract: Previous deterministic finger biomechanical models predicted that the flexor digitorum superficialis (FDS) was silent and the flexor digitorum profundus (FDP) was the only active flexor during finger flexion. Experimental studies in vivo, however, recorded activities of both flexors. In this study, in an attempt to elucidate the roles of the flexors, a probabilistic biodynamic model of the index finger was constructed to estimate the muscle–tendon forces during an experimentally measured index finger flexion movement. A Monte-Carlo simulation was performed with four model parameters, including moment arms, physiological cross sectional areas (PCSA), passive torques, and anthropometric measures as independent random variables. The muscle-tendon forces at each time point were determined using a nonlinear optimization technique. The model predicted that both FDS and FDP contributed to sustaining the movement and the FDS was not necessarily silent. The two distinct force patterns observed in vivo in experimental studies were also corroborated by the simulation. These findings, contrary to previous deterministic models’ predictions but in agreement with experimental measurements, explained the observed coactivation of FDS and FDP, and resolved the controversy regarding the roles of the flexors in finger movement dynamics. [Copyright &y& Elsevier]

Abstract: A forward dynamic model of human multi-fingered hand movement is proposed. The model represents digits 2–5 in manipulative acts as a 12-degrees-of-freedom (DOF) system, driven by torque actuators at individual joints and controlled using a parsimonious proportional–derivative (PD) scheme. The control parameters as feedback gains along with an auxiliary parameter to modulate the joint torque magnitudes and cross-coupling can be empirically identified in an iterative procedure minimizing the discrepancy between the model-prediction and measurement. The procedure is guided and computationally accelerated by pre-knowledge of relations between the parameters and kinematic responses. An empirical test based on real grasping movement data showed that the model simulated the multi-finger movements with varied inter-joint temporal coordination accurately: the grand mean of the root-mean-square-errors (RMSE) across trials performed by 28 subjects was 3.25°. Analyses of the model parameters yielded new insights into intra- and inter-person variability in multi-finger movement performance, and distinguished the less variable motor control strategy from much more variable anthropometric and physiological factors. [Copyright &y& Elsevier]

Abstract: An optimization-based model for power-grip posture prediction was proposed. The model was based on the premise that the hand prehensile configuration in a power grip best conforms to the object shape. This premise was embodied by an optimization procedure that minimized the sum of distances from the finger joints to the object surface. The model was evaluated against data from an experiment that measured the grasp postures of 28 subjects having diverse anthropometry. The intra- and inter-person variabilities in grip postures were empirically assessed and used as benchmark values for model evaluation. The evaluation showed that the root-mean-square (RMS) values of angle differences between the predicted and measured postures had a 13.7° grand mean (across all joints, subjects, and two cylindrical handles grasped), whereas the RMS values of the inter- and intra-person variabilities in measured postures had grand means of 13.0° and 4.4°, respectively. The model can be readily generalized to the prediction of postures in power-grasping objects of different shapes, and adapted for testing alternative prehensile strategies or performance criteria. [Copyright &y& Elsevier]

This paper describes the development of a novel algorithm for deriving finger segmental center of rotation (COR) locations during flexion–extension from measured surface marker motions in vivo. The algorithm employs an optimization routine minimizing the time-variance of the internal link lengths, and incorporates an empirically quantifiable relationship between the local movement of a surface marker around a joint (termed “surface marker excursion”) and the joint flexion–extension. The latter relationship constrains and simplifies the optimization routine to make it computationally tractable. To empirically investigate this relationship and test the proposed algorithm, an experiment was conducted, in which hand cylinder-grasping movements were performed by 24 subjects (12 males and 12 females). Spherical retro-reflective markers were placed at various surface landmarks on the dorsal aspect of each subject''s right (grasping) hand, and were measured during the movements by an opto-electronic system. Analysis of experimental data revealed a highly linear relationship between the “surface marker excursion” and the marker-defined flexion–extension angle: the average R2 in linear regression ranged from 0.89 to 0.97. The algorithm successfully determined the CORs of the distal interphalangeal, proximal interphalangeal, and metacarpophalangeal joints of digits 2–5 during measured motions. The derived CORs appeared plausible as examined in terms of the physical locations relative to surface marker trajectories and the congruency across different joints and individuals. [Copyright &y& Elsevier]

This study investigated whether the external marker-defined spine inter-segmental rotation is different from the internal vertebral rotation, and explored how to estimate the latter from limited surface measurement. A kinematic model was first created to elucidate analytically the relation between the external and internal rotations. A novel approach guided by the model was proposed for deriving vertebral centers of rotation (CORs) from measured planar trajectories of skin-surface markers. The approach involved a recursive procedure for establishing local (anatomical) coordinate systems, and an optimization routine that identified the maximum-likelihood circles best fitting the marker trajectories in local coordinate systems. An experiment with 10 subjects (5 males and 5 females) was conducted to test the approach along with the model. Skin-surface markers were strategically placed over individual spinous processes and other body landmarks, and recorded by an opto-electronic system while sagittally symmetric load-lifting movements were being performed. For the majority (89%) of measured motions, the COR locations for lumbar vertebrae (L2–L5) were derived successfully: solutions resulting from the optimization routine met a convergence criterion governed by the model, and were in agreement with existing data from radiographic or cadaveric studies. Empirical results confirmed the differences between the external marker-defined inter-segmental motions and corresponding internal vertebral rotations (1.1–5.8° on average, all statistically significant). The study demonstrated the necessity and viability of quantifying internal vertebral kinematics when utilizing non-invasive marker-based measurement for spine-related clinical diagnosis and biomechanical modeling. [Copyright &y& Elsevier]

Neck pain and injuries are growing healthcare burdens with women having a higher incidence rate and poorer treatment outcomes than males. A better understanding of sex differences in neck biomechanics, foundational for more targeted, effective prevention or treatment strategies, calls for more advanced subject-specific musculoskeletal modeling. Current neck musculoskeletal models are based on generic anatomy, lack subject specificity beyond anthropometric scaling, and are unable to accurately reproduce neck strengths exhibited in vivo without arbitrary muscle force scaling factors or residual torque actuators. In this work, subject-specific neck musculoskeletal models of 23 individuals (11 male, 12 female) were constructed by integrating multi-modality imaging and biomechanical measurements. Each model simulated maximal voluntary neck static exertions in three postures: neck flexion in a neutral posture, flexion in a 40° extended posture, and extension in a 40° flexed posture. Quantitative model validation showed close agreement between model-predicted muscle activation and EMG measurement. The models unveiled that (1) males have greater moment arms in one flexor muscle group and five extensor muscle groups, (2) females exhibited higher cervical spinal compression per unit exertion force in the flexed posture, and (3) the variability of compression force was much greater in females in all three exertions but most notably in the extension with a flexed "dropped head" position. These insights illuminated a plausible pathway from sex differences in neck biomechanics to sex disparities in the risk and prevalence of neck pain. [ABSTRACT FROM AUTHOR]

The study investigated the potential for obtaining more accurate spine joint reaction force (JRF) estimates from musculoskeletal models by incorporating dynamic stereo X-ray imaging (DSX)-based in vivo lumbar vertebral rotational and translational kinematics compared to generic, rhythm (RHY)-based kinematics, while also observing the influence of accompanying inputs: intervertebral segment stiffness and neutral state. A full-body OpenSim® musculoskeletal model, constructed by combining existing lower- and upper-body models, was driven based on one volunteer's (female; age 25; 60.8 kg; 176 cm) anthropometrics and kinematics from a series of upright standing and straight-legged dynamic lifting tasks. The lumbar spine portion was modified in a step-wise manner to observe effects of: (1) RHY vs. DSX lumbar kinematics; (2) No disc (bushing) stiffness (NBS); generic, linear bushing stiffness (LBS); subject-specific nonlinear bushing stiffness (NLBS); (3) Upright standing (UP) vs. Supine (SUP) neutral state; (4) Weight lifted: 4.5 kg vs. 13.6 kg. L4L5 JRF from 24 model variations based on combinations of aforementioned parameters were compared. Rhythm-based kinematics without translational components tends to over-predict JRF (31% and 39% for compression and shear, respectively) compared to DSX-based kinematics. Additionally, differences due to accompanying passive stiffness and neutral state choice combinations were even larger (>50%), indicating heightened demand on the quality of these accompanying inputs. The study not only highlights model sensitivity to choices made regarding the three primary inputs—kinematics, passive stiffness and neutral state— separately, but also how interactions between these choices can result in significant variability in joint loading estimates. [ABSTRACT FROM AUTHOR]

The lumbar facet joint (FJ) is often associated with pathogenesis in the spine, but quantification of normal FJ motion remains limited to in vitro studies or static imaging of non-functional poses. The purpose of this study was to quantify lumbar FJ kinematics in healthy individuals during functional activity with dynamic stereo radiography (DSX) imaging. Ten asymptomatic participants lifted three known weights starting from a trunk-flexed (∼75°) position to an upright position while being imaged within the DSX system. High resolution computed tomography (CT) scan-derived 3D models of their lumbar vertebrae (L2-S1) were registered to the biplane 2D radiographs using a markerless model-based tracking technique providing instantaneous 3D vertebral kinematics throughout the lifting tasks. Effects of segment level and weight lifted were assessed using mixed-effect repeated measures ANOVA. Superior-inferior (SI) translation dominated FJ translation, with L5S1 showing significantly less translation magnitudes ( Median (Md) = 3.5 mm, p < 0.0001 ) than L2L3, L3L4, and L4L5 segments (Md = 5.9 mm, 6.3 mm and 6.6 mm respectively ). Linear regression-based slopes of continuous facet translations revealed strong linearity for SI translation ( r 2  > 0.94), reasonably high linearity for sideways sliding ( Z -) ( r 2  > 0.8), but much less linearity for facet gap change ( X -) ( r 2  ∼ 0.5). Caudal segments (L4-S1), particularly L5S1, displayed greater coupling compared to cranial (L2-L4) segments, revealing distinct differences overall in FJ translation trends at L5S1. No significant effect of weight lifted on FJ translations was detected. The study presents a hitherto unavailable and highly precise baseline dataset of facet translations measured during a functional, dynamic lifting task. [ABSTRACT FROM AUTHOR]

The study aimed to map instantaneous centers of rotation (ICRs) of lumbar motion segments during a functional lifting task and examine differences across segments and variations caused by magnitude of weight lifted. Eleven healthy participants lifted loads of three different magnitudes (4.5, 9, and 13.5 kg) from a trunk-flexed (~75°) to an upright position, while being imaged by a dynamic stereo X-ray (DSX) system. Tracked lumbar vertebral (L2-S1) motion data were processed into highly accurate 6DOF intervertebral (L2L3, L3L4, L4L5, L5S1) kinematics. ICRs were computed using the finite helical axis method. Effects of segment level and load magnitude on the anterior-posterior (AP) and superior-inferior (SI) ICR migration ranges were assessed with a mixed-effects model. Further, ICRs were averaged to a single center of rotation (COR) to assess segment-specific differences in COR AP- and SI-coordinates. The AP range was found to be significantly larger for L2L3 compared to L3L4 ( p =0.02), L4L5 and L5S1 ( p <0.001). Average ICR SI location was relatively higher – near the superior endplate of the inferior vertebra – for L4L5 and L5SI compared to L2L3 and L3L4 ( p ≤0.001) – located between the mid-transverse plane and superior endplate of the inferior vertebra – but differences were not significant amongst themselves ( p >0.9). Load magnitude had a significant effect only on the SI component of ICR migration range (13.5 kg>9 kg and 4.5 kg; p =0.049 and 0.017 respectively). The reported segment-specific ICR data exemplify improved input parameters for lumbar spine biomechanical models and design of disc replacements, and base-line references for potential diagnostic applications. [ABSTRACT FROM AUTHOR]

Segmental apportionment of lumbar (L2–S1) rotation is a critical input parameter for musculoskeletal models and a candidate metric for clinical assessment of spinal health, but such data are sparse. This paper aims to quantify the time-variant and load-dependent characteristics of intervertebral contributions to L2–S1 extension during a dynamic lifting task. Eleven healthy participants lifted multiple weights (4.5, 9.1, and 13.6 kg) from a trunk-flexed to an upright position while being imaged by a dynamic stereo X-ray system at 30 frames/s. Vertebral (L2–S1) motion was tracked using a previously validated volumetric model-based tracking method that employs 3D bone models reconstructed from subject-specific CT images to obtain high-accuracy (≤0.26°, 0.2 mm) 3D vertebral kinematics. Individual intervertebral motions as percentages of the total L2–S1 extension were computed at each % increment of the motion to show the segmental apportionment. Results showed L3–L4 (25.8±2.2%) and L4–L5 (31±3.1%) together contributed a larger share (∼60% combined) compared to L2–L3 (21.7±3.7%) and L5–S1 (22.6±4.7%); L4–L5 consistently provided the largest contribution of the measured segments. Relative changes over time in L3–L4 (6±12.5%) and L4–L5 (0.5±10.2%) contribution were minimal; in contrast, L2–L3 (18±20.1%) contribution increased while L5–S1 (−33±22.9%) contribution decreased in a somewhat complementary fashion as motion progressed. No significant effect of the magnitude of load lifted on individual segmental contribution patterns was detected. The current study updated the knowledge regarding apportionment of lumbar (L2–S1) motion among individual segments, serving both as input into musculoskeletal models and as potential biomechanical markers of low back disorders. [ABSTRACT FROM AUTHOR]

Abstract: This study assessed the accuracy of surface-measured OpenSim-derived tibiofemoral kinematics in functional activities. Ten subjects with unilateral, isolated grade II PCL deficiency performed level running and stair ascent. A dynamic stereo radiography (DSX) system and a Vicon motion capture system simultaneously measured their knee or lower extremity movement. Surface marker motion data from the Vicon system were used to create subject-specific models in OpenSim and derive the tibiofemoral kinematics. The surface-measured model-derived tibiofemoral kinematics in all six degrees of freedom (DOFs) were then compared with those measured by the DSX as the benchmarks. The differences between surface- and DSX-measured tibiofemoral kinematics were found to be substantial: the overall mean (±SD) RMS differences during running were 9.1±3.2°, 2.0±1.2°, and 6.4±4.5° for the flexion–extension, abduction–adduction, and internal–external rotations, respectively, and 7.1±3.2mm, 8.8±3.7mm, and 1.9±1.2mm for anterior–posterior, proximal–distal, and medial–lateral translations, respectively. The differences were more pronounced in relatively higher speed running than in stair ascent. It was also found that surface-based measures significantly underestimated the mean as well as inter-subject variability of the differences between PCL-injured and intact knees in abduction–adduction, internal–external rotations, and anterior–posterior translation. [Copyright &y& Elsevier]

Neck muscle size and strength have been linked to lower injury risk and reduced pain. However, prior findings have been inconclusive and have failed to clarify whether there are sex differences in neck muscle size-strength relationships. Such differences may point to an underlying cause for the reported sex difference in neck pain prevalence. Thirty participants (13 males, 17 females) who underwent neck strength testing and MR imaging were analyzed. Strength was measured in three conditions that differed in posture and exertion direction. Muscle size was quantified by three metrics: anatomical cross-sectional area (ACSA), muscle volume (MV), and an estimate of physiological cross-sectional area—reconstruction-based cross-sectional area (RCSA). Inter-posture strength correlations, muscle size-strength correlations, and sex differences were analyzed with linear regression. Males were approximately 65% stronger and had significantly larger muscles. Strength varied significantly across postures, but only female strength values for different postures were significantly correlated. Observed in males only, the sternocleidomastoid (SCM) was a strong predictor of flexion strength in the neutral posture while the anterior scalene (AS) was more involved in the extended. No extensor's size was significantly linked to extension strength. A greater amount of force variation is unexplained by muscle size alone in females than in males. Males and females exhibited distinct size-strength relationships, highlighting the need for sex-specific models and analyses and the greater potential effect of non-morphometric factors on force generating capacity in females. No advantage of one muscle size metric over another in strength prediction was evidenced. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a new methodology that enables mapping of the ligament insertion sites onto bone surfaces in the knee joint by co-registration of the data acquired using digitization and computed tomography (CT). Local coordinate systems on the distal femur and proximal tibia were established by three spherical fiducial markers rigidly affixed to each bone. The fiducial marker centroid locations were identified by a least-squares sphere-fitting algorithm. An optimization correction procedure was proposed to mitigate the effect of the target registration error (TRE) on the alignment of coordinate systems for co-registration. A test with four cadaveric specimens demonstrated successful mapping of insertion sites for five ligaments. Fiducial registration error (FRE) as measured by the differences in inter-marker distances between the two modalities was smaller than 2%. The optimization procedure corrected the insertion site invisibility or partial visibility problem and improved the overall mapping quality, as indicated by substantial reduction of the mean and dispersion of distances from digitized insertion site points to the bone surfaces. [Copyright &y& Elsevier]