Relationships between trunk muscle activation and thoraco-lumbar kinematics in non-specific chronic low back pain subgroups during a forward bending task.

BACKGROUND: Trunk muscle activity and thoraco-lumbar kinematics can discriminate between non-specific chronic low back pain (NSCLBP) subgroups and healthy controls. However, research commonly focuses on lumbar kinematics, with limited understanding of relationships between kinematics and muscle activity across clinical subgroups. Similarly, the thoracic spine, whilst intuitively associated with NSCLBP, has received less attention and potential relationships between spinal regions and muscle activity requires exploration. RESEARCH QUESTION: Is there a relationship between trunk muscle activation and regional thoracic and lumbar kinematics in NSCLBP subgroups during a forward bending task? METHODS: Observational, case-control study. Fifty subgrouped NSCLBP motor control impairment participants (27 Flexion Pattern (FP-MCI), 23 Active Extension Pattern (AEP-MCI)) and 28 pain-free controls were evaluated using 3D motion analysis (Vicon™) and surface electromyography during a forward bending and return to upright task. Mean sagittal angles for the upper-thoracic (UTx), lower-thoracic (LTx), upper-lumbar (ULx) and lower-lumbar (LLx) regions were compared with normalised (% sub-maximal voluntary contraction) mean amplitude electromyography of bilateral transversus abdominis/internal oblique, external oblique, superficial lumbar multifidus and erector spinae (longissimus thoracis) muscles between groups. Pearson correlations were computed to assess relationships (significance p < 0.01). RESULTS: AEP-MCI individuals demonstrated statistically significant relationships between superficial lumbar multifidus and ULx and LLx kinematics (-.812 to.659). FP-MCI individuals exhibited statistically significant relationships between erector spinae and superficial lumbar multifidus and LLx and LTx kinematics (-.686 to.664) in both task phases, and between external oblique and LTx during forward bending) (-.459 to.572). Correlations were moderate to strong for all significant relationships (-.812 to .664). SIGNIFICANCE: Relationships between muscle activity and regional spinal kinematics varied between NSCLBP subgroups, suggesting that those with flexion- or extension-related LBP adopt different motor control strategies when performing a bending task. As effectively mechanical biomarkers, these findings may inform treatment by improving understanding of varied motor strategies in subgroups.

Estimating Joint Kinematics and Muscles Forces During Robotic Rehabilitation to Detect and Counteract Reduced Ankle Mobility.

The paper presents a solution to detect active ankle joint movement while a patient undergoes therapy with a robotic lower limb rehabilitation device that neither restricts nor actively supports ankle dorsi- or plantarflexion. The presented method requires the addition of only two accelerometer sensors to the system as well as a musculoskeletal model of the lower limb. Using forward kinematics and inverse dynamics, it enables knee and ankle joint kinematic tracking in the sagittal plane and muscle force estimation. This is an extension of a previous work in which only hip joint tracking was possible and, thus, muscle force estimation was limited. The correlation results of the current validation study with 12 healthy subjects show high correlation (R=0.88±0.09) between the kinematics estimated with the proposed method and those calculated from a gold standard motion capture setup for all three joints (hip, knee, and ankle). The correlation results of the estimated m. tibialis anterior muscle force against electromyography measurements (R = 0.62±0.27) are promising and a first application to a patient data set shows potential for future clinical application.

Predictive Control of Peak Achilles Tendon Force in a Simulated System of the Human Ankle Joint with a Parallel Artificial Actuator During Hopping.

Latest advances in wearable exoskeletons for the human lower extremity predominantly focus on minimising metabolic cost of walking. However, there currently is no robotic exoskeleton that gains control on the mechanics of biological tissues such as biological muscles or series-elastic tendons. Achieving robotic control of biological tissue mechanics would enable prevention of musculoskeletal injuries or the personalization of rehabilitation treatments following injury with levels of precisions not attained before. In this paper, we introduce a new framework that uses nonlinear model predictive control (NMPC) for the closed-loop control of peak tendon force in a simulated system of the human ankle joint with parallel exoskeletal actuation. We propose a computationally efficient NMPC's inner model consisting of explicit, closed-form equations of muscle-tendon dynamics along with those of the ankle joint with parallel actuation. The proposed formulation is tested and verified on movement data collected during dynamic ankle dorsiflexion/plantarflexion rotations executed on a dynamometer as well as during walking and running on a treadmill. The framework designed using the NMPC controller showed a promising performance in keeping the Achilles tendon force under a predefined threshold. Results indicated that our proposed model was generalizable to different muscles and gaits and suitable for real-time applications due to its low computational time.

Novel Design and Implementation of a Neuromuscular Controller on a Hip Exoskeleton for Partial Gait Assistance.

Exoskeletons intended for partial assistance of walking should be able to follow the gait pattern of their users, via online adaptive control strategies rather than imposing predefined kinetic or kinematic profiles. NeuroMuscular Controllers (NMCs) are adaptive strategies inspired by the neuromuscular modeling methods that seek to mimic and replicate the behavior of the human nervous system and skeletal muscles during gait. This study presents a novel design of a NMC, applied for the first time to partial assistance hip exoskeletons. Rather than the two-phase (stance/swing) division used in previous designs for the modulation of reflexes, a 5-state finite state machines is designed for gait phase synchronisation. The common virtual muscle model is also modified by assuming a stiff tendon, allowing for a more analytical computation approach for the muscle state resolution. As a first validation, the performance of the controller was tested with 9 healthy subjects walking at different speeds and slopes on a treadmill. The generated torque profiles show similarity to biological torques and optimal assistance profiles in the literature. Power output profiles of the exoskeleton indicate good synchronization with the users' intended movements, reflected in predominantly positive work by the assistance. The results also highlight the adaptability of the controller to different users and walking conditions, without the need for extensive parameter tuning.

Joint power, joint work and lower limb muscle activity for transitions between level walking and stair ambulation at three inclinations.

To enhance human mobility, training interventions and assistive lower limb wearable robotic designs must draw insights from movement tasks from daily life. This study aimed to analyze joint peak power, limb and joint work, and muscle activity of the lower limb during a series of stair ambulation conditions. We recruited 12 subjects (25.4±4.5 yrs, 180.1±4.6 cm, 74.6±7.9 kg) and studied steady gait and gait transitions between level walking, stair ascent and stair descent for three staircase inclinations (low 19°, normal 30.4°, high 39.6°). Our analysis revealed that joint peak power, limb and joint work, and muscle activity increased significantly compared to level walking and with increasing stair inclination for most of the conditions analyzed. Transition strides had no increased requirements compared to the maxima found for steady level walking and steady stair ambulation. Stair ascent required increased lower limb joint positive peak power and work, while stair descent required increased lower limb joint negative peak power and work compared to level walking. The most challenging condition was high stair inclination, which required approximately thirteen times the total lower limb joint positive and negative net work during ascent and descent, respectively. These findings suggest that training interventions and lower limb wearable robotic designs must consider the major increases in lower limb joint and muscle effort during stair ambulation, with specific attention to the demands of ascent and descent, to effectively improve human mobility.

Numerical investigation of intra-abdominal pressure and spinal load-sharing upon the application of an abdominal belt.

Chronic low back pain patients may experience spinal instability. Abdominal belts (ABs) have been shown to improve spine stability, trunk stiffness, and resiliency to spinal perturbations. However, research on the contributing mechanisms is inconclusive. ABs may increase intra-abdominal pressure (IAP) and reduce paraspinal soft tissue contribution to spine stability without increasing spinal compressive loads. A finite element model (FEM) of the spine inclusive of the T1-S1 vertebrae, intervertebral discs (IVDs), ribcage, pelvis, soft tissues, and abdominal cavity, without active muscle forces was developed. An identical FEM with an AB was developed. Both FEMs underwent trunk flexion. Following validation, the models' intervertebral rotation (IVR), IAP, IVD pressure, and tensile stress in the multifidus (MF), erector spinae (ES), and thoracolumbar fascia (TLF) were compared. The inclusion of an AB resulted in a 3.8 kPa IAP increase, but a decreased average soft tissue tensile stress of 0.28 kPa. The TLF withstood the majority of tension being transferred across the paraspinal soft tissues (>70 %). The average IVR in the AB model decreased by 10 %, with the lumbar spine experiencing the largest reduction. The lumbar IVDs of the AB model likewise showed a 31 % reduction in average IVD pressure. Using an AB improved trunk bending stiffness, primarily in the lumbar spine. Wearing an AB had minimal effect on reducing tensile stress in theES. The skewed stress distribution towards the TLF suggests its large contribution to spine stability and the potential advantage in unloading the structure when wearing an AB, measured herein at8 %.

Muscle dynamics analysis by clustered categories during jogging in patients with anterior cruciate ligament deficiency.

BACKGROUND: Patients with anterior cruciate ligament (ACL) deficiency (ACLD) tend to have altered lower extremity dynamics. Little is known about the changes in dynamic function and activation during jogging in patients with ACLD. METHODS: Twenty patients with an injured ACL before ACL reconstruction (ACLD group) and nine healthy male volunteers (control group) were recruited. Each volunteer repeated the jogging experiment five times. Based on the experimental data measured, a musculoskeletal multibody dynamics model was employed to simulate the tibiofemoral joint dynamics during jogging. Eighteen muscles were used for analysis. The obtained dynamics data were used for clustering and curve difference analysis. RESULTS: The 18 muscles studied were divided into 3 categories. All the quadriceps, the soleus, the gastrocnemius, and the popliteus were classified as label 1. All the hamstrings were classified as label 2, and the sartorius muscles were classified as label 3. Among them, the classification of the short head of the biceps femoris was significantly different between the two groups (P < 0.001). The force curves of all 18 muscles and the between-group differences were studied according to clustered categories. Most muscle force in label 1 was approaching zero in the terminal stance phase, which was significantly lower than that in the control group (P < 0.05). The muscle force in label 2 had areas with significant differences in the stance phase. Muscle force in label 3 was significantly lower than that in the control group in the pre-swing phase. CONCLUSIONS: This study showed that there are various changes of muscle function and activation in patients with ACLD. Through clustering and curve analysis, the joint reactions and changes of different muscle forces in the gait cycle between the ACLD and control groups could be further clarified.

The effects of joint hypermobility on pain and functional biomechanics in adolescents with juvenile fibromyalgia: secondary baseline analysis from a pilot randomized controlled trial.

BACKGROUND: Joint hypermobility is a common clinical finding amongst hereditary connective tissue disorders that is observed in pediatric rheumatological settings, and often associated with chronic pain. Joint hypermobility may also contribute to deficits in physical functioning and physical activity, but previous findings have been inconsistent. It is possible that physical activity impairment in joint hypermobility may be due to chronic aberrant movement patterns subsequent to increased joint laxity. METHOD: As part of a larger randomized pilot trial of juvenile onset fibromyalgia (JFM), a secondary analysis was conducted to explore whether adolescents with JFM and joint hypermobility differed from non-joint hypermobility peers in terms of pain, daily functioning, and biomechanics (i.e., kinetics and kinematics) during a moderately vigorous functional task. RESULTS: From the larger sample of adolescents with JFM (N = 36), 13 adolescents (36.1%) met criteria for joint hypermobility and 23 did not have joint hypermobility. Those with joint hypermobility exhibited poorer overall functioning (Md = 20, Q1,Q3 [5.8, 7.6] vs. Md = 29, Q1,Q3 [5.1, 7.6]) but there were no differences in pain (Md = 6.9, Q1,Q3 [22, 33], vs. Md = 6.45, Q1,Q3 [15, 29.5]). Inspection of time-series plots suggests those with joint hypermobility exhibited decreased hip flexion and frontal plane hip moment (e.g., resistance to dynamic valgus) during the landing phase (early stance) and greater hip and knee transverse plane moments during the propulsion phase (late stance) of the drop vertical jump task (DVJ). No other differences in lower extremity biomechanics were observed between study groups. CONCLUSIONS: In this exploratory study, there were small but notable differences in biomechanics between patients with JFM who also had joint hypermobility versus those without joint hypermobility during a landing and jumping task (e.g., DVJ). These differences may indicate decreased joint stiffness during landing, associated with increased joint laxity and decreased joint stability, which may put them at greater risk for injury. Further study with a larger sample size is warranted to examine whether these biomechanical differences in patients with JFM and joint hypermobility affect their response to typical physical therapy or exercise recommendations.

The Effects of Exoskeleton Assistance at the Ankle on Sensory Integration During Standing Balance.

Exoskeleton devices can reduce metabolic cost, increase walking speed, and augment load-carrying capacity. However, little is known about the effects of powered assistance on the sensory information required to achieve these tasks. To learn how to use an assistive device, humans must integrate novel sensory information into their internal model. This process may be disrupted by challenges to the sensory systems used for posture. We investigated the exoskeleton-induced changes to balance performance and sensory integration during quiet standing. We asked 11 unimpaired adults to perform a virtual reality-based test of sensory integration in balance (VRSIB) on two days while wearing the exoskeleton either unpowered, using proportional myoelectric control, or with regular shoes. We measured postural biomechanics, muscle activity, equilibrium scores, postural control strategy, and sensory ratios. Results showed improvement in balance performance when wearing the exoskeleton on firm ground. The opposite occurred when standing on an unstable platform with eyes closed or when the visual information was non-veridical. The balance performance was equivalent when the exoskeleton was powered versus unpowered in all conditions except when both the support surface and the visual information were altered. We argue that in stable ground conditions, the passive stiffness of the device dominates the postural task. In contrast, when the ground becomes unstable the passive stiffness negatively affects balance performance. Furthermore, when the visual input to the user is non-veridical, exoskeleton assistance can magnify erroneous muscle inputs and negatively impact the user's postural control.

Effects of poling camber angle on the biomechanics of cross-country sit-skiing.

Cross-country sit-skiers use double poling (DP) technique to drive the slide. The aim of this study is to analyze how poling camber angle affect the capacity of power output and biomechanical parameters of the DP process. Twenty-four non-disabled college students (24.67 ± 1.46 years old) were recruited to perform three successive 30-s maximal effort tests with different poling camber angles of 0°, 15°, 24° and 30° using a sit-skiing ergometer. The biomechanical parameters, output power and muscle activation of the subjects were analyzed. The results showed that DP output power increased with the increase of poling camber angle at 15° (597.78 ± 150.31 J), 24° (610.94 ± 158.96 J, P = 0.011) and 30° (629.10 ± 168.78 J, P < 0.001) compared with 0° (590.65 ± 148.95 J). However, effective output power decreased with the increase of camber angle. Poling with camber angle of 24° had the shortest cycle time 1.53 ± 0.17 s, compared with other abduction angle (0°, 1.57 ± 0.19 s, 15°, 1.55 ± 0.16 s, and 30°, 1.56 ± 0.19 s). Compared with 0° (1.02 ± 0.14 m), the cycle distance significantly increased at poling camber angles of 24° (1.07 ± 0.12 m, P = 0.029) and 30° (1.11 ± 0.13 m, P < 0.001). With the increase of poling camber angle, the shoulder and elbow joint range of motions and joint moments were significantly increased. This study found that poling with shoulder abducted increased the output power but decreased the efficiency. By analyzing the poling angle and poling force, we find that the optimal poling camber angle may depend on the terrain or the skiing speed. These results may guide the competition techniques and tactics in the matches, and may further influence the strength-training programs of cross-country sit-skiing athletes.

Speed-specific optimal contractile conditions of the human soleus muscle from slow to maximum running speed.

The soleus is the main muscle for propulsion during human running but its operating behavior across the spectrum of physiological running speeds is currently unknown. This study experimentally investigated the soleus muscle activation patterns and contractile conditions for force generation, power production and efficient work production (i.e. force-length potential, force-velocity potential, power-velocity potential and enthalpy efficiency) at seven running speeds (3.0 m s-1 to individual maximum). During submaximal running (3.0-6.0 m s-1), the soleus fascicles shortened close to optimal length and at a velocity close to the efficiency maximum, two contractile conditions for economical work production. At higher running speeds (7.0 m s-1 to maximum), the soleus muscle fascicles still operated near optimum length, yet the fascicle shortening velocity increased and shifted towards the optimum for mechanical power production with a simultaneous increase in muscle activation, providing evidence for three cumulative mechanisms to enhance mechanical power production. Using the experimentally determined force-length-velocity potentials and muscle activation as inputs in a Hill-type muscle model, a reduction in maximum soleus muscle force at speeds ≥7.0 m s-1 and a continuous increase in maximum mechanical power with speed were predicted. The reduction in soleus maximum force was associated with a reduced force-velocity potential. The increase in maximum power was explained by an enhancement of muscle activation and contractile conditions until 7.0 m s-1, but mainly by increased muscle activation at high to maximal running speed.

Clinically-accessible and laboratory-derived predictors of biomechanical response to standalone and supported lateral wedge insoles in people with knee osteoarthritis.

BACKGROUND: Lateral wedge insoles (both standalone and those incorporating individualized arch support) have been frequently studied for the effects on knee joint loading and pain in people with knee osteoarthritis. It has been shown that many people who use these insoles do not obtain the intended biomechanical effect, and thus may not experience a clinical benefit. The ability to identify biomechanical responders to lateral wedge insoles before research or clinical intervention is an important objective for efficient resource use and optimizing patient outcomes. The purpose of our exploratory, hypothesis-generating study was to provide an initial assessment of variables that are associated with the biomechanical response to lateral wedge insoles in people with knee osteoarthritis. METHODS: We collected a number of demographic (age, sex, body mass index, foot posture), clinical (knee pain, foot pain, radiographic disease severity), and walking-related (speed, knee alignment, frontal plane subtalar movement, and foot rotation) outcomes from 53 individuals with painful, radiographically-confirmed knee osteoarthritis. The walking-related outcomes were obtained using equipment both from the research laboratory and the clinical setting. We used logistic regression to generate predictive models to determine candidate variables associated with a reduction in the knee adduction moment during walking - a surrogate for tibiofemoral load distribution, and a known biomechanical risk factor for osteoarthritis progression - with the use of standalone and arch-supported lateral wedge insoles. Three different response thresholds (2%, 6%, and 10% reductions in the knee adduction moment) were used. RESULTS: In general, biomechanical responders were those who walked faster, were female, had less varus alignment, and had less severe radiographic severity. Findings were similar between the standalone and arch-supported lateral wedge insoles, as well as between models using the laboratory-derived or clinically-available measures of walking performance. CONCLUSIONS: Our hypothesis-generating study provides valuable information that will inform future research into the efficient and effective use of lateral wedge insoles in the conservative management of knee osteoarthritis.

The Structural Layers of the Porcine Iris Exhibit Inherently Different Biomechanical Properties.

Purpose: The purpose of this study was to isolate the structural components of the ex vivo porcine iris tissue and to determine their biomechanical properties. Methods: The porcine stroma and dilator tissues were separated, and their dimensions were assessed using optical coherence tomography (OCT). The stroma underwent flow test (n = 32) to evaluate for permeability using Darcy's Law (ΔP = 2000 Pa, A = 0.0391 mm2), and both tissues underwent stress relaxation experiments (ε = 0.5 with initial ramp of δε = 0.1) to evaluate for their viscoelastic behaviours (n = 28). Viscoelasticity was characterized by the parameters ß (half width of the Gaussian distribution), τm (mean relaxation time constant), E0 (instantaneous modulus), and E∞ (equilibrium modulus). Results: For the stroma, the hydraulic permeability was 9.49 ± 3.05 × 10-6 mm2/Pa · s, and the viscoelastic parameters were ß = 2.50 ± 1.40, and τm = 7.43 ± 4.96 s, with the 2 moduli calculated to be E0 = 14.14 ± 6.44 kPa and E∞ = 6.08 ± 2.74 kPa. For the dilator tissue, the viscoelastic parameters were ß = 2.06 ± 1.33 and τm = 1.28 ± 1.27 seconds, with the 2 moduli calculated to be E0 = 9.16 ± 3.03 kPa and E∞ = 5.54 ± 1.98 kPa. Conclusions: We have established a new protocol to evaluate the biomechanical properties of the structural layers of the iris. Overall, the stroma was permeable and exhibited smaller moduli than those of the dilator muscle. An improved characterization of iris biomechanics may form the basis to further our understanding of angle closure glaucoma.

Magnitude and variability of individual elbow load in repetitive baseball pitching.

In baseball pitchers the elbow is exposed to high and repetitive loads (i.e. external valgus torque), caused by pitching a high number of balls in a practice session or game. This can result in overuse injuries like the ulnar collateral ligament (UCL) injury. To understand injury mechanisms, the effect of repetitive pitching on the elbow load magnitude and variability was investigated. In addition, we explored whether repetitive pitching affects elbow muscle activation during pitching. Fifteen pitchers threw each 60 to 110 balls. The external valgus torque and electromyography of three elbow muscles were quantified during each pitch. Linear mixed model analyses were performed to investigate the effect of repetitive pitching. On a group level, the linear mixed models showed no significant associations of repetitive pitching with valgus torque magnitude and variability and elbow muscle activity. Significant differences exist between pitchers in their individual trajectories in elbow valgus torque and muscle activity with repetitive pitching. This shows the importance of individuality in relation to repetitive pitching. In order to achieve effective elbow injury prevention in baseball pitching, individual characteristics of changes in elbow load and muscle activity in relation to the development of UCL injuries should be investigated.

Characterization of the ankle dynamic joint stiffness as a function of gait speed for overground and treadmill walking.

The ankle dynamic joint stiffness (DJS), defined as the slope of the joint angle-moment plot, measures the resistance of the ankle joint to movement when the foot is in contact with the ground. DJS helps to stabilize the ankle joint, and its characterization helps to identify gait pathology and assist foot prosthesis design. This study analyzes the available gait dynamics data to obtain ankle DJS parameters for population groups according to age, gender, and gait speed for overground and treadmill walking. This study classified the groups into five walking speeds normalized using the Froude number. Herein, 12 ankle DJS parameters were determined. These include four linear segments: controlled plantar flexion (CP), early response phase (ERP), large response phase (LRP), and descending phase (DP), their corresponding turning points, the net mechanical work, the absorbed work, and the loop direction. Ankle dynamics data for 92 individuals was collected from two gait data repositories. The analysis reveals a notable disparity in stiffness values between overground and treadmill gait. Specifically, the CP stiffness is significantly higher for overground gait. In contrast, the DP stiffness displays an opposing pattern, with higher values observed during treadmill walking. A negative stiffness for LRP was found at fast speeds for all groups. The sorted data, analysis tools, and findings of this study are meant to help practitioners design prosthetic and rehabilitation devices based on age, gender, and walking environment at different gait speeds.

Development of a novel FE model for investigation of interactions of multi-motion segments of the lumbar spine.

The spinal anatomy is composed of a series of motion segments (MSs). Although finite element (FE) analysis has been extensively used to investigate the spinal biomechanics with various simplifications of the spinal structures, it is still a challenge to investigate the interactions of different MSs. Anatomical studies have shown that there are major spine ligaments connecting not only single-MS (i.e., two consecutive vertebrae) but also spanning multi-vertebral bones or multi-MSs. However, the effects of the multi-MS spanning ligaments on the spine biomechanics have not been investigated previously. This study developed an FE model of the lumbar spine by simulating the anterior longitudinal ligaments (ALLs) in two portions, one connecting a single-MS and the other spanning two MSs, with varying physiological cross-sectional area (PCSA) ratios of the two portions. The spine biomechanics during extension motion were investigated. The results showed that on average, the constraining forces by the two-MS spanning elements were ∼18% of those of the single-MS ALL elements when the PCSA ratio was 50%, but the two-MS ALL elements also applied compressive forces on the anterior surfaces of the vertebrae. Decreases in intradiscal pressure were also calculated when the two-MS spanning ALL elements were included in the spine model. The multi-MS spanning ligaments were shown to synergistically function with the single-MS elements in spine biomechanics, especially in the interactions of different MSs. The novel lumbar FE model could therefore provide a useful analysis tool for investigation of physiological functions of the spine.

Dose-response effect of incremental lateral-wedge hardness on the lower limb Biomechanics during typical badminton footwork.

Badminton footwork has been characterised with jump-landing, cross step, side side and lunges, which requires movement agility to facilitate on-court performance. A novel badminton shoe design with systematic increase of lateral wedge hardness (Asker C value of 55, 60, 65, and 70) was developed and investigated in this study, aiming to analyse the dose-response effect of incremental wedge hardness on typical badminton footwork. Stance time and joint stiffness were employed to investigate the footwork performance, and the factorial Statistical non-Parametric Mapping and Principal Component Analysis (PCA) were used to quantify the biomechanical responses over the stance. As reported, shorter contact times (decreased by 8.9%-13.5%) and increased joint stiffness (in side step) of foot-ankle complex were found, suggesting improved footwork stability and agility from increased hardness. Time-varying differences were noted during the initial landing and driving-off phase of cross and side steps and drive-off returning of lunges, suggesting facilitated footwork performance. The reconstructed modes of variations from PCA further deciphered the biomechanical response to the wedge dosage, especially during drive-off, to understand the improved footwork agility and stability.

Elastic ankle exoskeletons influence soleus fascicle dynamics during unexpected perturbations.

Spring-based passive ankle exoskeletons have been designed to emulate the energy conservation and power amplification roles of biological muscle-tendon units during locomotion. Yet, it remains unknown if similar assistive devices can serve the other elastomechanical role of biological muscle-tendon units - power attenuation. Here we explored the effect of bilateral passive ankle exoskeletons on neuromuscular control and muscle fascicle dynamics in the ankle plantarflexors during rapid, unexpected vertical perturbations. We recorded muscle activation and soleus fascicle length changes during hopping with and without exoskeleton assistance (0 and 76 Nm rad-1) on elevated platforms (20 cm), which were removed at an unknown time. Our results demonstrate that exoskeleton assistance leads to a reduction in soleus muscle activation, increases in fascicle length change and decreases in muscle forces during perturbed hopping. These changes have competing effects on the mechanics and energetics of lower limb muscles, likely limiting the capacity for series elastic tissues to absorb energy. As we strive towards the design of wearable assistive devices for everyday locomotion, information regarding real-time muscle-tendon behavior may enable tunable assistance that adapts to both the user and the environment.

Influence of different calibration methods on surface electromyography-informed musculoskeletal models with few input signals.

BACKGROUND: Although model personalization is critical when assessing individuals with morphological or neurological abnormalities, or even non-disabled subjects, its translation into routine clinical settings is hampered by the cumbersomeness of experimental data acquisition and lack of resources, which are linked to high costs and long processing pipelines. Quantifying the impact of neglecting subject-specific information in simulations that aim to estimate muscle forces with surface electromyography informed modeling approaches, can address their potential in relevant clinical questions. The present study investigates how different methods to fine-tune subject-specific neuromuscular parameters, reducing the number of electromyography input data, could affect the estimation of the unmeasured excitations and the musculotendon forces. METHODS: Three-dimensional motion analysis was performed on 8 non-disabled adult subjects and 13 electromyographic signals captured. Four neuromusculoskeletal models were created for 8 participants: a reference model driven by a large set of sEMG signals; two models informed by four electromyographic signals but calibrated in different fashions; a model based on static optimization. FINDINGS: The electromyography-informed models better predicted experimental excitations, including the unmeasured ones. The model based on static optimization obtained less reliable predictions of the experimental data. When comparing the different reduced models, no major differences were observed, suggesting that the less complex model may suffice for predicting muscle forces with a small set of input in clinical gait analysis tasks. INTERPRETATION: Quantitative model performance evaluation in different conditions provides an objective indication of which method yields the most accurate prediction when a small set of electromyographic recordings is available.

Determinants of 1500-m Front-Crawl Swimming Performance in Triathletes: Influence of Physiological and Biomechanical Variables.

PURPOSE: To analyze the associations between physiological and biomechanical variables with the FINA (International Swimming Federation) points (ie, swimming performance) obtained in 1500-m front-crawl swimming to determine whether these variables can be used to explain triathletes' FINA points. METHODS: Fourteen world-class, international and national triathletes (10 male: 23.24 [3.70] y and 4 female: 23.36 [3.76] y) performed a 1500-m front-crawl swimming test in a short-course pool. Heart rate (HR), oxygen uptake (V˙O2), and blood lactate concentrations were obtained before and after the test. HR was also measured during the effort. Highest V˙O2 value (V˙O2peak) was estimated by extrapolation. Clean swimming speed, turn performance, stroke rate, stroke length, and stroke index (SI) were obtained by video analysis. RESULTS: Average 1500-m performance times were 1088 (45) seconds and 1144 (31) seconds for males and females, respectively. HR after the effort, V˙O2peak, aerobic contributions, total energy expenditure, energy cost, and turn performance presented moderate negative associations with swimming performance (r ≈ .5). In contrast, respiratory exchange ratio, anaerobic alactic contribution, clean swimming speed, stroke length, and SI were positively related, with clean swimming speed and SI having a strong large association (r ≈ .7). A multiple stepwise regression model determined that 71% of the variance in FINA points was explained by SI and total energy expenditure, being predictors in 1500-m front-crawl swimming. CONCLUSIONS: Swimming performance in triathletes was determined by the athletes' energy demands and biomechanical variables. Thus, coaches should develop specific technique skills to improve triathletes' swimming efficiency.