Introduction: The accuracy of musculoskeletal models and simulations as methods for predicting muscle functional outputs is always improving. However, even the most complex models contain various assumptions and simplifications in how muscle force generation is simulated. One common example is the application of a generalised (\"generic\") force-velocity relationship, derived from a limited data set to each muscle within a model, uniformly across all muscles irrespective of whether those muscles have \"fast\" or \"slow\" contractile properties. Methods: Using a previously built and validated musculoskeletal model and simulation of trotting in the mouse hindlimb, this work examines the predicted functional impact of applying muscle-specific force-velocity properties to typically fast (extensor digitorum longus; EDL) and slow-contracting (soleus; SOL) muscles. Results: Using \"real\" data led to EDL producing more positive work and acting significantly more spring-like, and soleus producing more negative work and acting more brake-like in function compared to muscles modelled using \"generic\" force-velocity data. Extrapolating these force-velocity properties to other muscles considered \"fast\" or \"slow\" also substantially impacted their predicted function. Importantly, this also further impacted EDL and SOL function beyond that seen when changing only their properties alone, to a point where they show an improved match to ex vivo experimental data. Discussion: These data suggest that further improvements to how musculoskeletal models and simulations predict muscle function should include the use of different values defining their force-velocity relationship depending on their fibre-type composition.

Obesity is a known risk factor for development of osteoarthritis (OA). Numerical tools like finite-element (FE) models combined with degenerative algorithms have been developed to understand the interplay between OA and obesity. In this study, we aimed to predict knee cartilage degeneration in a cohort of obese adults to investigate the importance of patient-specific information on degeneration predictions. We used a validated FE modeling approach and three different age-dependent functions (step-wise, exponential, and linear) to simulate cartilage degradation under overloading in the knee joint. Gait motion analysis and magnetic resonance imaging data from 115 obese individuals with knee OA were used for musculoskeletal and FE modeling. Cartilage degeneration predictions were contrasted with Kellgren-Lawrence (KL) and Boston-Leeds Osteoarthritis Knee Score (BLOKS) grades. The findings show that overall, the similarities between numerical predictions and clinical measures were better for the medial (average area under the curve (AUC) = 0.62) compared to the lateral compartment (average AUC = 0.52) of the knee. Classification results for KL grades, full patient-specific models and patient-specific geometry with generic gait data showed higher AUC values (AUC = 0.71 and AUC = 0.68, respectively) compared to generic geometry and patient-specific gait (AUC = 0.48). For BLOKS grades, AUC values for both full patient-specific models and for patient-specific geometry with generic gait locomotion were higher (AUC  = 0.66 and AUC = 0.64, respectively) compared to when the generic geometry and patient-specific gait were used (AUC = 0.53). In summary, our study highlights the importance of considering individual information in knee OA prediction. Nevertheless, our findings suggest that personalized gait play a smaller role in the OA prediction and classification capacity than personalized joint geometry.

The kinetic demands of the spine can be assessed using a top-down (TD) or bottom-up (BU) approach, which start calculations from the either the hands or from the feet, respectively. Biomechanists have traditionally favored a BU approach, though existing modeling approaches encourage a TD approach. Regardless of the approach the demands should be similar, provided the external forces and linked segment parameters are equivalently measured and modeled. Demonstrating a level of agreement between the two approaches can help evaluate a model. Further, having both approaches can be advantageous when data is inaccurate or unavailable for one. The purpose of this study was to compare the internal moments and forces at multiple lumbar and thoracic intervertebral joint (IVJ) levels during lifting tasks from an established OpenSim thoracolumbar spine model that applies a TD approach and a similar model modified to adopt a BU approach. Kinematics and external forces were recorded from twelve participants during sagittal and lateral lifts of different lifting speeds and crate masses. For both approaches IVJ kinetics were estimated using a standard OpenSim modeling pipeline. The BU and TD approach IVJ joint moments generally agreed both temporally (R2 = .94 ± .17) and in magnitude (RMSE=6.2 ± 3.5 Nm) of the primary planes of movement. There were however some temporal fit exceptions for off axes moments with low magnitudes (i.e., < 10 Nm). Bland-Altman plots also indicated acceptable agreement for IVJ peak forces (BU-TD difference of 12 ± 111 and 8 ± 31 N in compression and resultant shear, respectfully). These results support the application of the BU approach and the assigned linked segment parameters of the model. The new BU model is available on the SimTK site (https://simtk.org/projects/spine_ribcage).

During forward flexion, spine motion varies due to age and sex differences. Previous studies showed that lumbar/pelvis range of flexion (RoF) and lumbo-pelvic ratio (L/P) are age/sex dependent. How variation of these parameters affects lumbar loading in a normal population requires further assessment. We aimed to estimate lumbar loads during dynamic flexion-return cycle and the differences in peak loads (compression) and corresponding trunk inclinations due to variation in lumbar/pelvis RoF and L/P. Based on in vivo L/P (0.11-3.44), temporal phases of flexion (early, middle, and later), the lumbar (45-55°) and hip (60-79°) RoF; full flexion-return cycles of six seconds were reconstructed for three age groups (20-35, 36-50 and 50+ yrs.) in both sexes. Six inverse dynamic analyses were performed with a 50th percentile model, and differences in peak loads and corresponding trunk inclinations were calculated. Peak loads at L4-L5 were 179 N higher in younger males versus females, but 228 N and 210 N lower in middle-aged and older males, respectively, compared to females. Females exhibited higher trunk inclinations (6°-20°) than males across all age groups. Age related differences in L4-L5 peak loads and corresponding trunk inclinations were found up to 415 N and 19° in males and 152 N and 13° in females. With aging, peak loads were reduced in males but were found non-monotonic in females, whereas trunk inclinations at peak loads were reduced in both sexes from young to middle/old age groups. In conclusion, lumbar loading and corresponding trunk inclinations varied notably due to age/sex differences. Such data may help distinguishing normal or pathological condition of the lumbar spine.

While shoulder injuries resulting from the bench press exercise are commonly reported, no biomechanical evidence for lowering injury risk is currently available. Therefore, the aim of the present study was to compare musculoskeletal shoulder loads and potential injury risk during several bench press variations. Ten experienced strength athletes performed 21 technical variations of the barbell bench press, including variations in grip width of 1,1.5 and 2 bi-acromial widths (BAW), shoulder abduction angles of 45°, 70° and 90°, and scapula poses including neutral, retracted, and released conditions. Motions and forces were recorded by an opto-electronic measurement system and an instrumented barbell. An OpenSim musculoskeletal shoulder model was employed to estimate joint reaction forces in the glenohumeral and acromioclavicular joints. Time-series of joint reaction forces were compared between techniques by statistical non-parametric mapping. Results showed that narrower grip widths of < 1.5 BAW decreased acromioclavicular compression (p < 0.05), which may decrease the risk for distal clavicular osteolysis. Moreover, scapula retraction, as well as a grip width of < 1.5 BAW (p < 0.05), decreased glenohumeral posterior shear force components and rotator cuff activity and may decrease the risk for glenohumeral instability and rotator cuff injuries. Furthermore, results showed that mediolaterally exerted barbell force components varied considerably between athletes and largely affected shoulder reaction forces. It can be concluded that the grip width, scapula pose and mediolateral exerted barbell forces during the bench press influence musculoskeletal shoulder loads and the potential injury risk. Results of this study can contribute to safer bench press training guidelines.

Reactive lower limb muscle function during walking plays a key role in balance recovery following tripping, and ultimately fall prevention. The objective of this study was to evaluate muscle and joint function in the recovery limb during balance recovery after trip-based perturbations during walking. Twenty-four healthy participants underwent gait analysis while walking at slow, moderate and fast speeds over level, uphill and downhill inclines. Trip perturbations were performed randomly during stance, and lower limb kinematics, kinetics, and muscle contribution to the acceleration of the whole-body centre of mass (COM) were computed pre- and post-perturbation in the recovery limb. Ground slope and walking speed had a significant effect on lower limb joint angles, net joint moments and muscle contributions to support and propulsion during trip recovery (p < 0.05). Specifically, increasing walking speed during trip recovery significantly reduced hip extension in the recovery limb and increased knee flexion, particularly when walking uphill and at higher walking speeds (p < 0.05). Gluteus maximus played a critical role in providing support and forward propulsion of the body during trip recovery across all gait speeds and ground inclinations. This study provides a mechanistic link between muscle action, joint motion and COM acceleration during trip recovery, and underscores the potential of increased walking speed and ground inclination to increase fall risk, particularly in individuals prone to falling. The findings of this study may provide guidelines for targeted exercise therapy such as muscle strengthening for fall prevention.

BACKGROUND: The musculoskeletal models have been improved to estimate accurate knee compression force (KCF) and have been used to reveal the causal relationship between KCF and muscle weakness. Previous studies have explored how muscle weakness influences the KCF during gait; however, the influence of muscle weakness is possibly larger during activities that require deeper knee flexion (e.g., stair ambulation) than other activities (e.g., gait) because of the small knee contact area of articular surfaces.
OBJECTIVE: To explore how muscle weakness influences the KCF during stair ambulation.
METHODS: Ten young adults performed stair ascent and descent tasks at a comfortable speed. Based on a previous study, we created muscle weakness models of rectus femoris (RF), vastus muscles (VAS), gluteus medius (Gmed), and gluteus maximus (Gmax), and the medial and lateral KCF (KCFmed and KCFlat) during stair ambulation were calculated.
RESULTS: Similar to the gait, the Gmed weakness increased KCFmed and decreased KCFlat during stair ascent and descent. Whereas, unlike the gait, the Gmax weakness increased KCFmed during stair ascent and the VAS weakness decreased KCFmed and KCFlat during stair ascent and descent. Moreover, the percentage changes in KCF were similar (or large) during stair ambulation compared with those during gait.
CONCLUSIONS: Considering the KCF alterations caused by each muscle weakness, the weaknesses in Gmax and Gmed might lead to cartilage loss and pain in the knee, and the VAS weakness might lead to low stability of the knee. The symptom during stair ambulation might help precisely identify the muscle requiring rehabilitation.

Walking is one of the main activities of daily life and gait analysis can provide crucial data for the computation of biomechanics in many fields. In multiple applications, having reference data that include a variety of gait conditions could be useful for assessing walking performance. However, limited extensive reference data are available as many conditions cannot be easily tested experimentally. For this reason, a musculoskeletal model in OpenSim coupled with gait data (at seven different velocities) was used to simulate seven carried loads and all the combinations between the two parameters. The effects on lower limb biomechanics were measured with torque, power, and mechanical work. The results demonstrated that biomechanics was influenced by both speed and load. Our results expand the previous literature: in the majority of previous work, only a subset of the presented conditions was investigated. Moreover, our simulation approach provides comprehensive data that could be useful for applications in many areas, such as rehabilitation, orthopedics, medical care, and sports.

[This corrects the article DOI: 10.3389/fbioe.2024.1285845.].

Femoral stem fractures (FST) are often accompanied by muscle injuries, however, what muscle injuries affect fracture healing and to what extent is unknown. The purpose of this study was to analyze the extent to which different muscles affect FST healing through a combined musculoskeletal dynamics and finite element approach. Modeling the lower extremity musculoskeletal system for 12 different muscle comprehensives. Muscle and joint reaction forces on the femur were calculated and these data were used as boundary conditions input to the FSTs model to predict the degree of muscle influence on fracture healing. Finally, we will investigate the extent to which muscle influences FST healing during knee flexion. Muscle and joint forces are highly dependent on joint motion and have a significant biomechanical influence on interfragmentary strain (IFS) healing. The psoas major (PM), gastrocnemius lateralis (GL) and gastrocnemius medialis (GM) muscles play a major role in standing, with GM > PM > GL, whereas the gluteus medius posterior (GMP), vastus intermedius (VI), vastus medialis (VM), vastus lateralis superior (VLS), and adductor magnus distalis (AMD) muscles play a major role in knee flexion, with VLS > VM > VI > AMD > GMP. Mechanical stimulus-controlled healing can be facilitated when the knee joint is flexed less than 20°. Different muscles exert varying degrees of influence on the healing of fractures. Therefore, comprehending the impact of particular muscles on fracture site tissue FST healing can aid orthopedic surgeons in formulating improved surgical and rehabilitation strategies.