Dimensionality reduction is a critical step for the efficacy and efficiency of clustering analysis. Despite the multiple available methods, biomechanists have often defaulted to Principal Component Analysis (PCA). We evaluated two PCA- and one autoencoder-based dimensionality reduction methods for their data compression and reconstruction capability, assessed their effect on the output of clustering runners' based on kinematics, and discussed their implications for the biomechanical assessment of running technique. Eighty-four participants completed a 4-minute run at 12 km/h while trunk and lower-limb kinematics were collected. Data reconstruction quality was assessed for Direct PCA (PCA directly on original variables) and Fourier PCA (modelling time series as Fourier series and then applying PCA) using popular variance explained criteria; and a feedforward autoencoder (AE). Agglomerative hierarchical clustering was then applied and the agreement between the resulting partitions was assessed. Meaningful errors in the reconstructed signals were found when applying popular variance explained criteria, suggesting reconstruction error should be assessed to make a more informed decision about how many components to retain for further analysis. Direct PCA, Fourier PCA and AE yielded different clusters, warranting caution when comparing outcomes from studies that use different dimensionality reduction techniques: each method may be sensitive to different data features. Direct PCA retaining 99 % of the original variance emerged as the best compromise of data compression, reconstruction quality and cluster separability in our dataset. We encourage biomechanists to experiment with diverse dimensionality reduction methods to optimise clustering outcomes and enhance the real-world applicability of their findings.

The prediction of neointimal hyperplasia (NIH) growth, leading to vein graft failure in lower-limb peripheral arterial disease (PAD), is hindered by the multifactorial and multiscale mechanobiological mechanisms underlying the vascular remodelling process. Multiscale in silico models, linking patients' hemodynamics to NIH pathobiological mechanisms, can serve as a clinical support tool to monitor disease progression. Here, we propose a new computational pipeline for simulating NIH growth, carefully balancing model complexity/inclusion of mechanisms and readily available clinical data, and we use it to predict NIH growth for an entire vein graft. To this end, three different fittings to published in vitro data of time-averaged wall shear stress (TAWSS) vs nitric oxide (NO) production were tested for predicting long-term graft response (10-month follow-up) on a single patient. Additionally, the sensitivity of the model's predictions to different inflow boundary conditions (BCs) was assessed. The main findings indicate that: (i) a TAWSS-NO hyperbolic relationship best predicts long-term graft response; (ii) the model is insensitive to the inflow BCs if the waveform shape and the systolic acceleration time are comparable with the one acquired at the same time as the computed-tomography scan. This proof-of-concept study demonstrates the potential of using multiscale, computational techniques to predict NIH growth in lower-limb vein grafts, considering the routine clinical scenario of non-standardised data collection and sparse, incomplete datasets.

Measurement of hip external rotation strength (ERS) is important for preventive and rehabilitative purposes. ERS can be measured in 3 different positions in the isokinetic dynamometer ISOMED2000. However, it is not clear whether these measurement positions effect ERS nor if these positions are reliable in the ISOMED2000. Hence, the purpose of this study was to compare ERS in these positions, the reliability and the agreement. A cross-sectional design was conducted to compare measurement positions and a test-retest design to assess intra-rater reliability and agreement. Twenty-four healthy, physically active athletes participated in the study. Peak isometric torque was measured in the ISOMED in prone, supine, and side-lying position across two sessions on one day. Differences between positions were evaluated with the Wilcoxon-signed-rank test and cliff's delta. Reliability was assessed via intraclass correlation. Agreement was determined using the standard error of measurement (SEM), minimal detectable change (MDC), and Bland-Altman analysis (BAA). Results indicated a significant influence of measurement position on ERS (p < 0.001) with high effect sizes (>0.74). Reliability and agreement were high in all positions, but highest for the side-lying position (ICC = 0.90 [0.78, 0.96]; SEM = 0.08; MDC = 0.23; BAA_bias = 3.4 %, BAA_loA = 37 %). There were only poor to moderate correlations between measurement positions. These findings suggest that measurement position significantly affects ERS. Furthermore, the effect varies across individuals indicating that normative values cannot be used interchangeably or be adapted across positions. In diagnostic testing ERS should be measured in the same position, but preferably in the side-lying position.

We determined the effects of knee joint position on the relationship between maximal voluntary contraction (MVC) isometric plantar flexor torque and architectural properties of the plantar flexors measured at rest in healthy young adults. We obtained 3-D reconstructed muscle architecture data of the right plantar flexor muscles of nine physically active males using T1 and DTI MRI sequences with the knee in ∼5° flexion and at rest. Muscle volume, fascicle length, pennation angle, and physiological cross-sectional area were estimated for the medial and lateral gastrocnemius and the soleus muscle. MVC isometric plantar flexor torque was assessed on a dynamometer with the knee flexed and extended. MVC isometric plantar flexor torque was 59 % lower when performed with the knee flexed (93.1 ± 22.3 N∙m) vs. extended (154.4 ± 37.8 N∙m). Medial (r = 0.70, p = 0.026) and lateral gastrocnemius (r = 0.49, p = 0.048), total soleus (r = 0.79, p = 0.01), and total triceps suræ muscle volume (r = 0.77, p = 0.012) correlated with MVC isometric plantarflexion torque produced with the knee extended. However, only total soleus (r = 0.64, p = 0.028) and triceps suræ volume (r = 0.64, p = 0.031) correlated with MVC isometric plantar flexor torque produced with the knee flexed. Only the total soleus (r = 0.66, p = 0.038) and triceps suræ physiological cross-sectional area (r = 0.55, p = 0.049) correlated with MVC isometric plantar flexor torque performed with knee extended. The data suggest that knee joint position affects torque-size relationship in the gastrocnemius muscles. Additionally, it appears that the total soleus and triceps suræ muscle volumes association with MVC isometric plantar flexor torque is larger than the total physiological cross-sectional area of the triceps suræ. In conclusion, the data suggest that knee joint position affects torque-size relationship in the gastrocnemii but not in the soleus muscle.

Single-energy quantitative computed tomography (SEQCT) provides volumetric bone mineral density (vBMD) measures for bone analysis and input to image-based finite element models (FEMs). Dual-energy CT (DECT) improves vBMD by accounting for voxel-specific material variations utilizing scans at multiple x-ray energies. vBMD is also altered by reconstruction kernel that cannot be accounted for using calibration phantoms. This study compared vBMD and FEM stiffness derived from SEQCT and DECT images reconstructed with two common kernels. SEQCT and DECT images of cadaveric shoulders (n = 10) were collected using standard (STD) and boneplus (BONE) kernels. Hounsfield Units were converted to vBMD using specimen-specific calibrations. DECT STD and BONE images were generated using an established material decomposition method with 40 and 90 keV simulated monochromatic images. A proximal humerus bone section below the anatomic neck was used for vBMD analysis and FEM generation. FEMs were loaded to 1% apparent strain for stiffness measurements. Between STD and BONE kernel images, average vBMD differed 0.9 mg/cc and 4.1 mg /cc, in SEQCT and DECT images, respectively. Significant differences occurred in DECT images (p = 0.001). BONE reconstructed images produced higher vBMD measures across both SEQCT and DECT images. The difference between STD and BONE in both SEQCT- and DECT-based FEMs persisted, with larger estimated stiffness in BONE models. For six of the models DECT-based had higher stiffness than SEQCT-based models using the same kernel, although these models differed between STD and BONE kernels. Differences in stiffness between STD and BONE derived models were similar across image types (DECT: 17.5 kN/mm; SEQCT: 19.0 kN/mm). Stiffness values were significantly different within SECT kernels and between SEQCT BONE and DECT STD models. This study shows important differences in vBMD and FEM stiffness that occur due to CT-based imaging parameters alone. These results indicate that consistent imaging parameters should be used for vBMD analysis and FEM input to avoid systematic measurement errors.

Gait analysis for patients with orthopedic joint diseases is crucial to understand their functional status. Inertial measurement unit (IMU) systems, as alternatives to optical motion capture (OMC) systems, enable gait analysis outside the laboratory. However, their accuracy requires validated before widespread clinical use. Therefore, this study evaluated the validity of the Xsens IMU system for lower-limb joint angle measurements during walking in patients with hip and knee pathology and compared the error metrics among patients with hip pathology, patients with knee pathology and healthy controls. We used OMC system and Xsens IMU systems to simultaneously collect lower-limb kinematic data of 130 patients with knee pathology, 110 patients with hip pathology and 25 healthy individuals during walking at self-selected speed. Validity was assessed using root mean square error (RMSE), amplitude difference and coefficient of determination (R). The RMSEs for the patient groups ranged from 2.5° to 8.8°, with half of the joint angles showing acceptable accuracy and the rest tolerable accuracy. The joint angles measured by both systems were more consistent in the sagittal plane (R: 0.62-0.96), with lower consistency in the frontal and transverse planes (R: 0.44-0.63). Compared to the control group, both patient groups had higher RMSEs and lower R across most joint angles. Our results suggest that the Xsens IMU system offers highly comparable sagittal plane kinematic waveforms in patients with hip and knee pathology. Caution should be taken when interpreting the frontal and transverse plane kinematics and assessing patients with severe joint deformities.

Low back pain (LBP) can alters spinal kinematics. However, for adequate clinical care, a better understanding of lumbopelvic biomechanical behaviour according to the type of LBP is required. Our objectives were to identify differences in lumbopelvic rhythm (LPR) between subjects with acute low back pain (aLBP), axial spondyloarthritis (axSpA) and healthy subjects. As well as to identify correlations between LPR and sociodemographic and clinical data. In each group of 39 subjects, LPR total and by quartiles (Q) and metrological and clinical data were evaluated. No differences were found in relation to total flexion and LPR extension. However, study by Q showed less movement in aLBP compared to axSpA and healthy subjects at the Lumbar level in Q2 (p = 0.001), Pelvis in Q3 and Q4 and Trunk in Q3 (p=<0.001). In Q4 the aLBP moved the Trunk less than axSpA exclusively [-3,64°(95 % confidence interval - 6.53,-0.74), p = 0.011]. For the extension movement, the Pelvic motion of Q2 was lower for the aLBP group compared to axSpA group [-3,11°(-6.00,-0.22), p = 0.030], and Trunk motion of Q2 and Q3 (p = 0.001, p = 0.007, respectively), and Lumbar mobility of Q3 were also lower compared to axSpA and control groups (p = 0.031). Specific correlations were found for each group. aLBP with BMI, axSpA with metrology and Healthy subjects with age. Subjects with aLBP showed less lumbar, pelvic or trunk movement in Q2 and Q3 of trunk flexion and extension movements than axSpA and controls. RPL and its interrelationships with sociodemographic and clinical variables depend on the lumbar condition.

Advancing successful treatments for carpal instabilities of the wrist are hindered due, in part, to limited preclinical animal models. The purpose of this study was to evaluate the forelimb of the Yucatan minipig (YP) as a potential preclinical animal model for the human wrist by quantifying carpal biomechanics in vitro in the intact and after two ligament transection conditions. Porcine wrist biomechanics (n = 12, 5M, 7F) were determined in 28 range of motion (ROM) directions, in pronation-supination, and in volar-dorsal translation using a six-axis robotic musculoskeletal simulator. Testing was implemented in three conditions - intact, and after sequential transection of the radial intermediate ligament (RIL) and the dorsal intercarpal ligament (DIC). Mixed models were employed to examine differences in direction and conditions among male and female specimens. The intact ROM envelope was elliptical in shape and oriented toward ulnar flexion with the largest ROM about 15° from the flexion-extension axis. Transection of RIL and DIC did not alter the ROM envelope orientation, however, subtle increases in ROM were observed in extension and radial deviation following transection of both RIL and DIC. Pronation in neutral was greater than supination in all three test conditions. Volar translation increased subtly in the RIL and DIC condition. This novel study investigated the multidirectional biomechanics of the YP forelimb. ROM in the general directions of extension, radial and ulnar deviation were less than in humans, while flexion was substantially larger. These specific ligament transections had minor effects on the biomechanics of the YP forelimb.

Post-traumatic osteoarthritis (PTOA) often develops following tibial pilon fractures. Evidence suggesting PTOA development is driven by elevated articular contact stress from residual malreduction has led surgeons to strive for precise articular reduction, typically at the cost of extended operative time. Post-operative bracing using carbon fiber custom dynamic orthoses (CDOs) offers another means to decrease tibiotalar joint reaction force (JRF) and contact stress. The purpose of this cadaveric study was to measure how CDO stiffness influences ankle JRF and contact stress over the stance phase of gait. A servohydraulic load frame was used to test five cadaver ankles, with axial loading (240-330 N) and pneumatic actuation of the Achilles tendon (50-436 N) serving to quasi-statically model multiple points in the stance phase of gait. Three CDO rotational stiffness conditions were tested: (1) No CDO-0 Nm/deg, (2) low stiffness CDO-1.8 Nm/deg, and (3) moderate stiffness CDO-2.3 Nm/deg. JRF and contact stresses were measured using a piezoresistive pressure sensor inserted into the tibiotalar joint. An insole plantar pressure sensor placed between the cadaveric foot and CDO footplate measured limb/device interactions via the plantar center of pressure (COP). As limb loading progressed through stance, the plantar COP progressed from hindfoot to forefoot, as it would in normal gait. Both CDOs demonstrated decreases in JRF, reaching as high as 32% for the low CDO and 26% for the moderate CDO, with associated decreases in contact stress. This suggests that post-operative bracing could lessen PTOA risk after pilon fractures.

While levodopa is the most effective drug for symptom treatment of Parkinson's Disease (PD), its long-term use often leads to side effects such as uncontrolled involuntary movements known as levodopa-induced dyskinesia (LID). LID has been shown to increase postural sway, but the extent to which these hyperkinetic movements alter postural sway strategies has not been explored. We recruited 25 people with idiopathic PD, of which 13 exhibit clinical signs of LID, and 10 healthy older adults. Participants performed thirty-second standing trials with no added task (single-task) and with performing a cognitive dual-task, known to provoke dyskinesia. Participants with PD were tested in their practical OFF and ON states. The root-means-square (RMS) accelerations were obtained from inertial sensors attached to the lumbar, trunk, and head. Sway ratios (superior-to-inferior segment) were calculated to determine the effect of LID on postural sway strategies. Participants with LID showed greater RMS head sway, compared to those without LID and older adults. The head-to-trunk sway ratio was greater in participants with LID during the ON state or when dual-tasking. In addition, the head-to-lumbar sway ratio was greater in participants with LID in the ON state during both single- and dual-tasking. Our results reveal an altered postural control strategy in PD with LID, presenting increased sway in superior segments of the kinematic chain, leading to head instability. Unlike PD without LID and older adults, PD with LID exhibit multi-link sway in the ON state, which has important implications for measuring postural sway in the presence of dyskinesias.

Musculoskeletal disorders impact quality of life and incur substantial socio-economic costs. While in vivo and in vitro studies provide valuable insights, they are often limited by invasiveness and logistical constraints. Finite element (FE) analysis offers a non-invasive, cost-effective alternative for studying joint mechanics. This study introduces a fully automated algorithm for identifying soft-tissue attachment sites to streamline the creation of subject-specific FE knee models from magnetic resonance images. Twelve knees were selected from the Osteoarthritis Initiative database and segmented to create 3D meshes of bone and cartilage. Attachment sites were identified in three conditions: manually by two evaluators and via our automated Python-based algorithm. All knees underwent FE simulations of a 90° flexion-extension cycle, and 68 kinematic, force, contact, stress and strain outputs were extracted. The automated process was compared against manual identification to assess intra-operator variability. The attachment site locations were consistent across all three conditions, with average distances of 3.0 ± 0.5 to 3.1 ± 0.6 mm and no significant differences between conditions (p = 0.90). FE outputs were analyzed using Pearson correlation coefficients, randomized mean square error, and pairwise dynamic time warping in conjunction with ANOVA and Kruskal-Wallis. There were no statistical differences in pairwise comparisons of 67 of 68 FE output variables, demonstrating the automated method's consistency with manual identification. Our automated approach significantly reduces processing time from hours to seconds, facilitating large-scale studies and enhancing reproducibility in biomechanical research. This advancement holds promise for broader clinical and research applications, supporting the efficient development of personalized musculoskeletal models.

Forward propulsion depends on the forces generated by the triceps surae muscles and transmitted through the muscles' subtendons, which merge and twist to form the Achilles tendon (AT). As people age, the AT may undergo structural changes that could alter the subtendons' ability to transmit forces or function with some independence; prominent changes include increased tendon compliance and a proliferation of interfascicular adhesions compared to younger tendon. However, the effects of age-related changes on the subtendons are difficult to isolate in vivo. Here, we used a Hill-type musculoskeletal model of the triceps surae muscle-subtendon units to simulate the effects of age-related changes on gastrocnemius (GAS) and soleus (SOL) muscle contractile dynamics across a range of physiological force levels during fixed-end contractions. We simulated individual and dual muscle excitations with altered tendon compliance (ε = 3 %, 6 %, 9 %) and inclusion of a shared tendon. Consistent with fundamental muscle mechanics, compared to stiffer tendons, increased tendon compliance elicited more than three times the GAS and SOL fiber shortening and greater muscle excitation - effects that increased with requisite force demand. However, our model results also suggest combinatory effects of increased tendon compliance and interfascicle adhesions in the aging AT that deleteriously amplify redistribution from the GAS to the SOL which may be functionally detrimental during gait.

The anatomical connection between latissimus dorsi (LD), thoracolumbar fascia, and contralateral gluteus maximus (GM) enables myofascial force transmission (MFT) between the shoulder, trunk, and hip. This study investigates whether regular sports practice, specifically running, influences this MFT pathway. Given the potential changes in tissue stiffness from sports practice and the importance of this property for MFT, we hypothesize that runners may exhibit greater MFT between the LD and GM, resulting in altered passive properties of the lumbar and hip regions during LD contraction. This study aimed to investigate whether runners present a higher modification in lumbar stiffness and passive properties of the contralateral hip due to LD contraction than sedentary individuals. The lumbar stiffness, hip resting position, passive hip torque, and stiffness of fifty-four individuals were assessed using an indentometer and an isokinetic dynamometer, respectively, in two conditions: LD relaxed, and LD contracted. The main and interaction effects were assessed using a two-way ANOVA. The LD contraction increased lumbar stiffness (p < 0.001; η = 0.50), externally rotated the hip resting position and increased the passive hip torque and stiffness (p < 0.05; η > 0.1) in both groups. In addition, runners presented higher lumbar stiffness compared to sedentary in the LD contracted condition (p = 0.017, ES = 0.54). Although runners exhibited increased lumbar stiffness during LD contraction, the MFT from the shoulder to the hip joint occurred similarly in both groups.

Accurate estimation of joint load during a lifting/lowering task could provide a better understanding of the pathogenesis and development of musculoskeletal disorders. In particular, the values of the net force and moment at the L5-S1 joint could be an important criterion to identify the unsafe lifting/lowering tasks. In this study, the joint load at L5-S1 was estimated from the motion kinematics acquired using a multi-view markerless motion capture system without force plate. The 3D human pose estimation was first obtained on each frame using deep learning. The kinematic analysis was then performed to calculate the velocity and acceleration information of each segment. Then, the net force and moment at the L5-S1 joint were calculated using inverse dynamics with a top-down approach. This estimate was compared to a reference with a bottom-up approach. It was computed using a marker-based motion capture system combined with force plates and using personalized body segment inertial parameters derived from a 3D model of the human body shape constructed for each subject using biplanar radiographs. The average differences of the estimates for force and moment among all subjects were 14.0 ± 6.9 N and 9.0 ± 2.3 Nm, respectively. Meanwhile, the mean peak value differences of the estimates were 10.8 ± 8.9 N and 11.9 ± 9.5 Nm, respectively. This study then proposed the most rigorous comparison of mechanical loading on the lumbar spine using computer vision. Further work is needed to perform such an estimation under realistic industrial conditions.

Muscle fatigue, the transient decrease in muscle power, leads to low levels of physical activity and an inability to perform activities of daily living. Altered muscle coordination in response to fatigue may contribute to impaired physical performance. We sought to determine whether lower extremity muscle coordination during gait changes differently depending on susceptibility to fatigue (i.e., fatigability). Thirty-one older adults completed muscle power testing before and after a 30-min walk, with the change in power used to categorize participants as more or less fatigable. We used non-negative matrix factorization to identify muscle modules from electromyography (EMG) from the 2nd minute as our measure of baseline muscle coordination. Changes in muscle coordination were determined by computing the variance in the 30th minute's EMG accounted for by the baseline modules across all muscles (tVAF) and in individual muscles (mVAF). We compared tVAF between the 2nd and 30th minutes of the walk in individuals who were more and less fatigable. We used mVAF to explore the contribution of changes in individual muscle activity to tVAF. There was a decrease in tVAF overall in response to the walk (p < 0.001; 92.3 ± 1.6 % vs. 89.0 ± 4.3 %) but this did not differ between groups (interaction p = 0.66). There were significant associations between mVAF and tVAF for knee extensor, knee flexor, and ankle dorsiflexor muscles. Our results suggest that muscle coordination changes over the course of a walk in older adults but that this change does not differ between more and less fatigable older adults.

Understanding muscle response to exercise is critical for optimizing training strategies. This study investigated the effects of dorsiflexion and plantar flexion exercises on T2* values in the tibialis anterior (TA) and soleus (SOL) muscles and explored their relationship with muscle cross-sectional area (MCA), strength, and perceived exertion. Forty participants were divided into two exercise protocols: 30 performed dorsiflexion, 16 performed plantar flexion, and 6 completed both. T2* values were measured pre-and post-exercise using a 1.5 T MRI scanner. MCA and muscle strength were assessed via MRI and a dynamometer, while perceived exertion was measured using the Borg scale. Results showed that TA T2* values significantly increased after dorsiflexion (9.04 ± 4.21 ms), peaking 600 s post-exercise, whereas SOL T2* changes during plantar flexion were minimal (1.29 ± 1.05 ms). A significant correlation (r = 0.41, p = 0.026) was observed between T2* changes and Borg scale scores during dorsiflexion, but not with muscle strength (r = 0.08) or MCA (r = 0.35). No significant correlations were found for the SOL during plantar flexion. General linear model analysis showed a significant main effect of dorsiflexion on T2* values (p < 0.0001) and perceived exertion within the dorsiflexion protocol (p = 0.044). These findings suggest that dorsiflexion induces greater metabolic disturbances in the TA compared to plantar flexion. The results emphasize the importance of exercise-specific approaches for assessing muscle function and highlight the role of perceived exertion in evaluating muscle response.

This study investigated the impact of joint positioning on ultrasound shear wave elastography measurements in the Achilles and patellar tendons. Twenty-eight healthy adults underwent SWE assessment of shear wave speed (SWS) and coefficient of variation in SWS (CV-SWS) at three ankle positions (neutral, 10° plantar flexion, and 20° dorsiflexion) and two knee positions (90° flexion and full extension), at two academic sites. Participant positioning for ankle testing differed between sites (prone vs long-sitting)-while knee testing used consistent positioning. At the ankle, both joint and participant positioning significantly affected SWS. In the prone position, SWS was lower in neutral compared to dorsiflexed position (3.07 ± 1.13  m/s vs. 3.95 ± 1.03  m/s, p = 0.013). In long-sitting, SWS was lower in neutral compared to plantarflexed position (2.85 ± 0.53  m/s vs. 4.86 ± 1.92  m/s, p = 0.016); and SWS was higher in the plantarflexed position when participants were in long-sitting compared to prone (4.86 ± 1.92  m/s vs. 3.25 ± 1.13  m/s, p = 0.016). Participant positioning affected CV-SWS, with higher variability observed in prone compared to long-sitting in plantarflexed (29.3 ± 15.5 % vs 12.4 ± 9.12 %, p = 0.005) and neutral ankle angles (p = 0.03). At the knee, joint position significantly influenced SWS, with higher values in flexed versus extended positions (6.48 ± 3.1  m/s vs. 4.60 ± 2.3  m/s, p = 0.007). Extending the knee reduced CV-SWS compared to flexed position (14.5 ± 11.2 vs 19.2 ± 13.4, p = 0.044). In conclusion, joint position significantly affected SWS measurements in both the Achilles and patellar tendons, while participant positioning influenced measurement variability. Thus, standardizing joint and participant positioning is important to enhance the reliability of SWE assessments of tendon elasticity.

The shear viscoelastic behavior of eye's supporting orbital fat is unstudied in humans, yet is important during and after rapid movement. This investigation quantified viscoelastic characteristics of human orbital fat in constitutive form suitable for numerical simulation. Fresh human orbital fat was harvested postmortem from 6 male and 7 female donors of average age 78 ± 13 years. Fat samples were trimmed to disks of 20 ± 3.0 (standard deviation) mm average diameter and 2.1 ± 0.2 mm thickness. In 8 samples each, the following four testing protocols were performed: strain sweep from 0.0015 to 50 % at 1 Hz; viscometry at 0.1 s shear rate; stress relaxation at physiological temperature; and frequency sweep from 0.159 to 15.9 Hz at 0.5 % strain to validate the Prony series parameters fitting stress relaxation behavior. Orbital fat exhibited viscoelastic behavior under dynamic shear with a 0.5 % linear viscoelastic strain limit. Storage modulus G averaged 737 ± 310 Pa, and loss modulus G averaged 197 ± 76 Pa. Values were similar for strain and frequency sweep testing. At rupture, shear stress averaged 617 ± 366 Pa and rupture strain averaged 200 ± 70 %. The long-term relaxation modulus averaged 646 ± 264 Pa at 100 s. Frequency sweep testing validated the parameters of the Prony series fitted to the experimental stress relaxation data. Human orbital fat is linearly viscoelastic within a range typical of biological materials, and exhibits similar viscoelastic behavior for strain and frequency sweep testing. Stress relaxation data for human orbital fat has been parameterized for constitutive models that can be implemented in finite element analysis.

The comfort of a chair should be assessed based on the alignment between the chair's features and the user's behavioral characteristics. Considering modern habits and types of seating, prolonged work often leads to frequent shifts from correct posture (Torso Upright with Parallel Legs) to poor posture (Torso Flexion with Crossed Legs). Thus, exploring how to select an appropriate chair type to accommodate these frequent transitions between TUPLs and TFCLs becomes a topic worthy of thorough investigation. This study collected kinematic and ground reaction force data from 16 healthy males performing the TUPLs-to-TFCLs transition on two types of chairs, Stationary and Swivel, and then processed the data using a whole-body musculoskeletal model. The results revealed the following: (1) TFCLs significantly increased the lumbar L4-L5 flexion angle, lateral bending angle, flexion moment, axial rotation moment, pressure, and shear force. Lower back muscle activation increased significantly during TFCLs, whereas abdominal muscle activation decreased (P < 0.05). (2) During the TUPLs-to-TFCLs transition, the SW chair presented a significantly greater lumbar L4-L5 joint lateral bending angle, lateral bending torque, axial rotation torque, and shear force than the ST chair, with lower activation of the lower back and abdominal muscles (P < 0.05). This study revealed that although the comfort derived from reduced muscle activation during TFCLs exists, it actually increases the shear force on the lumbar intervertebral disc. Furthermore, individuals with the habit of TFCL should exercise caution when using SW chairs to reduce lumbar load and potential health risks.

Facilitating forward movement while maintaining dynamic stability during transitions like sit-to-walk (STW) requires coordination from many muscles. Age-related muscle, sensory, and neural decline can introduce compensatory biomechanics when completing STW, such as adjusting initial foot position or rising with arm support. Many previous STW studies restrict arm movement and prescribe symmetric foot positions, therefore the purpose of this study was to quantify lower limb muscle excitations and joint moments in STW transitions from four initial foot positions [symmetric, posterior offset, wide, narrow] and two arm placements [hands on knees, arms folded] in 15 younger and 15 older adults. Peak knee and ankle joint extension moments, as well as peak electromyography of five bilateral lower-limb muscles were analyzed. In all conditions, older adults had larger knee extension moments, whereas younger adults had larger ankle plantarflexion moments. Older adults generated larger peak excitation from the knee extensor muscles during rising compared to younger adults, consistent with the higher knee extension moments. Older adults had greater peak dorsiflexor and plantarflexor muscle excitation while rising compared to younger adults. Posterior offset and wide foot positions required the largest peak ankle plantarflexion and knee extension moments and plantarflexor muscle excitation. Arm-supported rising decreased peak knee extensor muscle excitation. In addition, there were interaction effects between age and initial foot position/arm placement for multiple quantities, indicating that the effects of foot and arm placement vary with age. These results inform assessments of movement performance and guidelines for rising given individual lower limb capability.