Force-velocity Characteristics Research Articles

Voluntary activation of human knee extensors during isotonic shortening contractions.

The interpolated twitch technique (ITT) is often used to assess voluntary activation during isometric contractions; however, this may have limited relevance to dynamic contractions. Although the ITT has been applied to relatively slow isokinetic contractions (< 150°/s), it has received limited consideration during unconstrained velocity (i.e., isotonic) contractions, despite their relevance to natural movements. Here, we explored the ITT during isotonic knee extension contractions using a modified dynamometer. Young males (n = 6) and females (n = 4) performed isometric and isotonic knee extension contractions of sub-maximal and maximal intensities with doublet (150 Hz) muscle belly stimulations to assess voluntary activation. Following each voluntary isotonic contraction (velocity range ~ 35°/s to ~ 275°/s), resting potentiated doublets were evaluated during passive joint rotation at the same angular velocity achieved during voluntary efforts, to account for force-velocity characteristics. Correlations between voluntary activation and the proportion of maximal torque or power were evaluated for isometric and isotonic contractions, respectively. Isometric voluntary activation was strongly correlated with increasing torque output (r = 0.96, p < 0.001). Doublet torque during passive joint rotation displayed a hyperbolic relationship with increasing angular velocity (r = 0.98, p < 0.001). Isotonic voluntary activation was strongly correlated with increasing power output (r = 0.89, p < 0.001). During maximal effort contractions, no differences were observed in voluntary activation between isometric and isotonic conditions (89.4% vs. 89.2%, p = 0.904). The ITT is a valid approach to evaluate voluntary activation during an isotonic contraction using a modified dynamometer. Participants were able to achieve a similar high level of voluntary activation during isometric and isotonic contractions.

Conserved mammalian muscle mechanics during eccentric contractions.

Skeletal muscle has a broad range of biomechanical functions, including power generation and energy absorption. These roles are underpinned by the force-velocity relationship, which comprises two distinct components: a concentric and an eccentric force-velocity relationship. The concentric component has been extensively studied across a wide range of muscles with different muscle properties. However, to date, little progress has been made in accurately characterising the eccentric force-velocity relationship in mammalian muscle with varying muscle properties. Consequently, mathematical models of this muscle behaviour are based on a poorly understood phenomenon. Here, we present a comprehensive assessment of the concentric force-velocity and eccentric force-velocity relationships of four mammalian muscles (soleus, extensor digitorum longus, diaphragm and digastric) with varying biomechanical functions, spanning three orders of magnitude in body mass (mouse, rat and rabbits). The force-velocity relationship was characterised using a hyperbolic-linear equation for the concentric component a hyperbolic equation for the eccentric component, at the same time as measuring the rate of force development in the two phases of force development in relation to eccentric lengthening velocity. We demonstrate that, despite differences in the curvature and plateau height of the eccentric force-velocity relationship, the rates of relative force development were consistent for the two phases of the force-time response during isovelocity lengthening ramps, in relation to lengthening velocity, in the four muscles studied. Our data support the hypothesis that this relationship depends on cross-bridge and titin activation. Hill-type musculoskeletal models of the eccentric force-velocity relationship for mammalian muscles should incorporate this biphasic force response. KEY POINTS: The capacity of skeletal muscle to generate mechanical work and absorb energy is underpinned by the force-velocity relationship. Despite identification of the lengthening (eccentric) force-velocity relationship over 80years ago, no comprehensive study has been undertaken to characterise this relationship in skeletal muscle. We show that the biphasic force response seen during active muscle lengthening is conserved over three orders of magnitude of mammalian skeletal muscle mass. Using mice with a small deletion in titin, we show that part of this biphasic force profile in response to muscle lengthening is reliant on normal titin activation. The rate of force development during muscle stretch may be a more reliable way to describe the forces experienced during eccentric muscle contractions compared to the traditional hyperbolic curve fitting, and functions as a novel predictor of force-velocity characteristics that may be used to better inform hill-type musculoskeletal models and assess pathophysiological remodelling.

Force-Velocity Relationship in Cycling and Arm Cranking: A Comparison of Men and Women.

This study was aimed at comparing the force-velocity relationship during cycling and arm cranking in males and females. Thirty-two male and twenty-two female healthy volunteers performed a force-velocity test on a cycle ergometer and a cranking ergometer in a randomly selected order. The theoretical values of the maximum force at zero speed (F0) and the maximum velocity at zero braking force (v0) for the lower and upper limbs were determined, and the maximum anaerobic power (Pmax) was calculated from the individual force-velocity relationship. The Pmax and F0 of the upper limbs related to the lower limbs correspond to 78.2 ± 14.3% and 80.1 ± 17.3% in men and 65.5 ± 12.5% and 74.5 ± 6.5% in women, respectively. The theoretical maximum velocity v0 of the upper limbs to the lower limbs attained 129.1 ± 29.0% in men and 127.4 ± 26.4% in women. The results of the study can serve as reference data for the force-velocity characteristics of the upper and lower limbs of male and female athletes. The results can be used both in training and rehabilitation programs, where the starting point is the objectification of possible strength deficits in various areas of the force-velocity characteristic spectrum of the muscles of the upper and lower limbs.

Development of a new dynamic hysteresis model for magnetorheological dampers with annular-radial gaps considering fluid inertia and compressibility

Modeling the inherent non-linear dynamic hysteresis behavior of magnetorheological (MR) fluid dampers is a fundamental necessity for their practical implementation. Quasi-static models, have been developed under very low frequency and small amplitude excitation conditions, are only beneficial in the early stages of MR dampers design and are sufficient for manufacturing requirements. These models, however, have failed to predict the non-linear hysteresis behavior of MR dampers (i.e., force-displacement and force-velocity characteristics) under low to moderate ranges of frequency and displacement amplitudes. The main contribution of the present work is to formulate a novel physic-based dynamic model to predict the non-linear behavior of MR dampers featuring an annular-radial bypass valve under different applied currents, and low to moderate input excitations. The proposed model incorporates unsteady fluid behavior and the fluid compressibility effect on the hysteretic response without relying on experimental data to evaluate the model parameters. To derive the model, the velocity profile and the pressure drop have been derived by solving the fluid momentum equation via the Laplace transform technique and Cauchy residue theory. The continuity equation has been employed for considering the compressibility contribution to hysteretic response based on the variation of bulk moduli. The predicted results for the force-displacement and force-velocity behavior of the MR damper were subsequently compared with the experimental results. Results suggest the effectiveness of the proposed model for predicting the dynamic hysteresis characteristics of the annular-radial MR fluid dampers under the relatively sufficient variation of mechanical and magnetic loading conditions considered.

Force-velocity profiling of elite wheelchair rugby players by manipulating rolling resistance over multiple wheelchair sprints.

This study investigated the effect of increased rolling resistance on wheelchair sprint performance and the concomitant force-velocity characteristics. Thirteen wheelchair rugby (WCR) athletes completed five 15 s wheelchair sprints in their own rugby wheelchair on an instrumented dual-roller wheelchair ergometer. The first sprint was performed against a close to overground resistance and in each of the following sprints, the resistance increased with 80% of that resistance. A repeated-measures ANOVA examined differences between sprints. Subsequently, linear regression analyses examined the individual force-velocity relations and then, individual parabolic power output curves were modeled. Increased rolling resistance led to significantly lower velocities (-36%), higher propulsion forces (+150%) and higher power outputs (+83%). These differences were accompanied by a lower push frequency, higher push time, yet a constant recovery time and contact angle. The modeled linear regressions (R2 = 0.71 ± 0.10) between force and velocity differed a lot in slope and intercept among individual athletes. The peak of the power output parabola (i.e., the optimal velocity) occurred on average at 3.1 ± 0.6 ms-1 . These individual force-velocity profiles can be used for training recommendations or technological changes to better exploit power generation capabilities of the WCR athletes' musculoskeletal system.

The effect of a combined sprint training intervention on sprint force-velocity characteristics in junior Australian football players.

Sprint performance in junior Australian football (AF) players has been shown to be a differentiating quality in ability level therefore developing sprint characteristics via sprint-specific training methods is an important aspect of their physical development. Assisted sprint training is one training method used to enhance sprint performance yet limited information exists on its effect on sprint force-velocity characteristics. Therefore, the main aim of this study was to determine the influence of a combined sprint training intervention using assisted and maximal sprint training methods on mechanical characteristics and sprint performance in junior Australian football players. Upon completing familiarization and pre-testing, twenty-two male junior Australian football (AF) players (age 14.4±0.3 years, body mass 58.5 ±10.0 kg, and height 1.74 ±0.08 m) were divided into a combined sprint training (CST) group (n = 14), and a maximal sprint training (MST) group (n = 8) based on initial sprint performance over 20-meters. Sprint performance was assessed during maximal 20-meter sprint efforts via a radar gun (36 Hz), with velocity-time data used to derive force-velocity characteristics and split times. All subjects then completed a 7-week in-season training intervention consisting of maximal sprinting (MST & CST groups) and assisted sprinting (CST only), along with their usual football specific exercises. Moderate to large pre-post within group effects (-0.65 ≤ ES ≥ 0.82. p≤0.01) in the CST group for relative theoretical maximal force (F0) and power (Pmax) were reflected in improved sprint performance from 0-20 m, thereby creating a more force-oriented F-v profile. The MST group displayed statistically significant pre-post differences in sprint performance between 10-20 m only (ES = 0.18, p=0.04). Moderate to high relative reliability was achieved across all sprint variables (ICC = 0.65-0.91), except for the force-velocity slope (SFV) and decrement in ratio of forces (DRF) which reported poor reliability (ICC = 0.41-0.44), while the CST group exceeded the pre-post minimal detectable change (MDC) in most sprint variables suggesting a 'true change' in performance across the intervention. It is concluded that implementing a short-term, combined sprint training intervention consisting of assisted and maximal sprint training methods may enhance sprint mechanical characteristics and sprint performance to 20-meters in junior AF players.

Force-Velocity Profiling in Club-Based Field Hockey Players: Analyzing the Relationships between Mechanical Characteristics, Sex, and Positional Demands

The purpose of this study was to investigate differences between sex and positional demands in club-based field hockey players by analyzing vertical force-velocity characteristics. Thirty-three club-based field hockey athletes (16 males - age: 24.8 ± 7.3yrs, body mass: 76.8 ± 8.2kg, height: 1.79 ± 0.05m; 17 females - age: 22.3 ± 4.2yrs, body mass: 65.2 ± 7.6kg, height: 1.66 ± 0.05m) were classified into two key positional groups (attacker or defender) based on dominant field position during gameplay. Force-velocity (F-v) profiles were established by performing countermovement jumps (CMJ) using a three-point loading protocol ranging from body mass (i.e., zero external mass, 0%) to loads corresponding to 25% and 50% of their own body mass. Across all loads, between-trial reliability of F-v and CMJ variables was determined by intraclass correlation coefficients (ICCs) and coefficient of variation (CV) and deemed to be acceptable (ICC: 0.87-0.95, CV% 2.8-8.2). Analysis by sex identified male athletes had significantly greater differences in all F-v variables (12.81-40.58%, p ≤ 0.001, ES = 1.10-3.19), a more enhanced F-v profile (i.e., greater theoretical maximal force, velocity, and power values), plus overall stronger correlations between relative maximal power (PMAX) and jump height (r = 0.67, p ≤ 0.06) when compared to female athletes (-0.71≤ r ≥ 0.60, p = 0.08). Male attackers demonstrated a more 'velocity-oriented' F-v profile compared to defenders due to significant mean differences in theoretical maximal velocity (v0) (6.64%, p ≤ 0.05, ES: 1.11), however differences in absolute and relative theoretical force (F0) (15.43%, p ≤ 0.01, ES = 1.39) led to female attackers displaying a more 'force-oriented' profile in comparison to defenders. The observed mechanical differences identify the underpinning characteristics of position specific expression of PMAX should be reflected in training programmes. Therefore, our findings suggest F-v profiling is acceptable to differentiate between sex and positional demands in club-based field hockey players. Furthermore, it is recommended field hockey players explore a range of loads and exercises across the F-v continuum through on-field and gym-based field hockey strength and conditioning practices to account for sex and positional mechanical differences.

Design and experimental characterization of a bypass magnetorheological damper featuring variable stiffness and damping

Magnetorheological (MR) fluid dampers (MRFDs) with variable stiffness and variable damping capability (VSVD-MRFDs) have demonstrated excellent vibration mitigation performance. However, there are limited studies on the development of bypass VSVD-MRFDs which offer both higher dynamic range and output force, apart from simple maintenance and straightforward assembly. In this study, a novel large-capacity VSVD-MRFD with an annular-radial bypass MR valve, as opposed to the typical practice of implementing the valve within the traveling piston in the hydraulic cylinder of the MRFD, is proposed. The main contribution of the present work includes: (a) providing the conceptional design and experimental dynamic characterization of the proposed VSVD-MRFD; (b) investigating the feasibility of the proposed damper for realizing the VSVD characteristics under wide ranges of loading conditions. A test rig was, thus, designed to perform experimental characterization of the proposed VSVD-MRFD under wide ranges of mechanical loading and magnetic field conditions. A qualitative analysis including force-displacement, and force-velocity characteristics, together with a quantitative analysis including dynamic range, equivalent viscous and stiffness coefficients, were conducted as a function of loading frequency, displacement amplitude, and applied current. Results showed a maximum dynamic range and maximum output force of 4.5 and 7.8 kN, respectively. Also, the maximum relative increase in the equivalent viscous and stiffness coefficients were obtained, respectively, as 425% and 488%, when the applied current is increased from zero to 2 A. The results confirm the potential of the proposed VSVD-MRFD for applications in off-road suspension systems. The externally designed bypass MR valve permits a straightforward design modification for realizing wide scalability of damping force in different applications (e.g., off-road vehicle suspension systems).

Relationship between Maximum Force-Velocity Exertion and Swimming Performances among Four Strokes over Medium and Short Distances: The Stronger on Dry Land, the Faster in Water?

Evaluating force-velocity characteristics on dry-land is of the utmost importance in swimming, because higher levels of these bio-motor abilities positively affect in-water performance. However, the wide range of possible technical specializations presents an opportunity for a more categorized approach that has yet to be seized. Therefore, the aim of this study was to identify feasible differences in maximum force-velocity exertion based on swimmers' stroke and distance specialization. To this scope, 96 young male swimmers competing at the regional level were divided into 12 groups, one for each stroke (butterfly, backstroke, breaststroke, and front crawl) and distance (50 m, 100 m, and 200 m). They performed two single pull-up tests, 5-min before and after competing in a federal swimming race. We assessed force (N) and velocity (m/s) exertion via linear encoder. There were no significant differences between pre-post maximum force-velocity exertions, despite the decreasing trend. Force-parameters highly correlated with each other and with the swimming performance time. Moreover, both force (t = -3.60, p < 0.001) and velocity (t = -3.90, p < 0.001) were significant predictors of swimming race time. Sprinters (both 50 m and 100 m) of all strokes could exert significantly higher force-velocity compared to 200 m swimmers (e.g., 0.96 ± 0.06 m/s performed by sprinters vs. 0.66 ± 0.03 m/s performed by 200 m swimmers). Moreover, breaststroke sprinters presented significantly lower force-velocity compared to sprinters specialized in the other strokes (e.g., 1047.83 ± 61.33 N performed by breaststroke sprinters vs. 1263.62 ± 161.23 N performed by butterfly sprinters). This study could provide the foundation for future research regarding the role of stroke and distance specializations in modeling swimmers' force-velocity abilities, thus influencing paramount elements for specific training and improvement towards competitions.

An Anthropomorphic Robotic Finger With Innate Human-Finger-Like Biomechanical Advantages Part II: Flexible Tendon Sheath and Grasping Demonstration

The human hand has a fantastic ability to interact with various objects in the dynamic unstructured environment of our daily activities. We believe that this outstanding performance benefits a lot from the unique biological features of the hand musculoskeletal system. In Part I of this article, a bio-inspired anthropomorphic robotic finger was developed, based on which two human-finger-like biomechanical advantages were elaborately investigated, including the anisotropic variable stiffness associated with the ligamentous joints and the enlarged feasible force space associated with the reticular extensor mechanisms. In Part II, the fingertip force-velocity characteristics resulting from the flexible tendon sheath are studied. It indicates that the fingertip force–velocity workspace can be greatly augmented owing to the self-adaptive morphing of the flexible tendon sheaths, showing the average improvement of 41.2% theoretically and 117.5% experimentally compared with the results of 2 mm, 4 mm, and 6 mm size rigid tendon sheaths. Grasping tests and comparisons are then conducted with four three-fingered robotic hands (one with the robotic finger proposed in Part I, one with hinge joints, one with linear extensors, and one with rigid tendon sheaths) and the human hands of six subjects to handle various objects on flat, rough, and soft surfaces. The results show that the novel bio-inspired design in this research could improve the grasping success rates of the robotic hand. Compared with the grasping test results from the robotic hand with the bio-inspired robotic finger proposed in Part I, the overall grasping performance of a robotic hand with hinge joints, linear extensors, and rigid tendon sheaths decreases by 10%, 6%, and 17%, respectively. The results have also shown that with the embedded biomechanical advantages, even without complex control and sensory systems, the robotic fingers can achieve very comparable performance to human fingers in the grasping demonstrations presented, indicating average 94% of the success rate achieved by the human fingers. Successfully demonstrating 14 of 16 grasp types in the Cutkoskey taxonomy further shows the human-finger-like grasping capability of the proposed robotic fingers.

Hybrid modeling of the nonlinear behaviors for magnetorheological energy absorber

Precise forecast of strong nonlinear behaviors of magnetorheological energy absorber (MREA) plays a significant role in its semi-active control and engineering applications. This study proposes a hybrid model based on the support vector regression (SVR) and adaptive differential evolution (i.e., JADE) algorithm for MREA, called the JADE-SVR model. The JADE algorithm is employed to find the optimal hyperparameters of SVR to improve prediction accuracy and generalization ability. The JADE-SVR model is developed using the training data randomly partitioned from an impact test database, and then applied to predict the nonlinear behaviors of MREA in the test set, including force–time, force–displacement and force–velocity characteristics. It is demonstrated that the JADE-SVR model is capable of forecasting strong nonlinear behaviors of MREA under various impact conditions. Moreover, the JADE-SVR model is compared with other SVR-based models to validate its prediction accuracy and convergence rate further. In addition to four statistical indicators, a novel desirability function is used to assess the overall performance of these models. The results show that the JADE-SVR model has the optimal values for all the performance indices, and the largest overall desirability of 0.6274. Therefore, it can be concluded that the JADE-SVR model is the best one for forecasting the dynamic behaviors of MREA under high-impact loadings.

Effects of diaphragmatic control on multiparametric analysis of the sniff nasal inspiratory pressure test and inspiratory muscle activity in healthy subjects.

BackgroundWe investigated the influence of diaphragmatic activation control (diaphC) on the relaxation rate, contractile properties and electrical activity of the inspiratory muscles of healthy subjects. Assessments were performed non-invasively using the sniff inspiratory pressure test (SNIP) and surface electromyography, respectively.MethodsTwenty-two subjects (10 men and 12 women) performed 10 sniff maneuvers in two different days: with and without diaphC instructions. For the SNIP test with diaphC, the subjects were instructed to perform intense activation of the diaphragm. The tests with the best SNIP values were used for analysis.ResultsThe maneuver with diaphC when compared to the maneuver without diaphC exhibited significant lower values for: SNIP (p <0.01), maximum relaxation rate (MRR) (p <0.01), maximum rate of pressure development (MRPD) (p <0.01), contraction times (CT) (p = 0.02) and electrical activity of the sternocleidomastoid (SCM) (p <0.01), scalene (SCL) (p = 0.01) and intercostal (CI) (p = 0.03) muscles. In addition, the decay constant (tau, τ) and relaxation time (½ RT) did not present any changes.ConclusionThe diaphragmatic control performed during the SNIP test influences the inspiratory pressure and the contractile properties of inspiratory muscles. This occurs due to changes in the pattern of muscle recruitment, which change force velocity characteristics of the test. Thus, instruction on diaphC should be encouraged for better performance of the SNIP test and for evaluation targeting the diaphragm muscle activity.

Eccentric Force-Velocity Characteristics during a Novel Squat Protocol in Trained Rugby Union Athletes-Pilot Study.

Eccentric strength characteristics have been shown to be important factors in physical performance. Many eccentric tests have been performed in isolation or with supramaximal loading. The purpose of this study was to investigate within- and between- session reliability of an incremental eccentric back squat protocol. Force plates and a linear position transducer captured force-time-displacement data across six loading conditions, separated by at least seven days. The reliability of eccentric specific measurements was assessed using coefficient of variation (CV), change in mean, and intraclass correlation coefficient (ICC). Eccentric peak force demonstrated good ICC (≥0.82) and TE (≤7.3%) for each load. Variables based on mean data were generally less reliable (e.g., mean rate of force development, mean force, mean velocity). This novel protocol meets acceptable levels of reliability for different eccentric-specific measurements although the extent to which these variables affect dynamic performance requires further research.

Force-velocity characteristics of lower extremity muscles in male high-altitude climbers

IntroductionThe study aimed to assess the force-velocity parameters of knee muscles in male high-altitude climbers and to compare the obtained results with the control group.MethodsOverall, 31 male subjects participated in the tests. The study group comprised 12 world-renowned Polish high-altitude climbers. The control group consisted of 19 professional soldiers on active duty. The groups did not differ significantly in the average age, body weight, or height. The force-velocity parameters of knee muscles were assessed under isokinetic conditions. The velocities of 60°/s and 180°/s were used.ResultsThe values of peak torque, total work, average power, and the agonist/antagonist ratio were higher and the acceleration and deceleration times were shorter in the study group in comparison with the control group. In particular, the differences in the parameters describing the knee flexors of both limbs proved to be statistically significant. The &lt;i&gt;p&lt;/i&gt;-value of the &lt;i&gt;t&lt;/i&gt;-test for the dominant limb knee flexors at the velocity of 60°/s was 0.0134 for peak torque, 0.0198 for total work, and 0.0019 for mean power. At the velocity of 180°/s, the &lt;i&gt;p&lt;/i&gt; values equalled 0.0001, &lt; 0.0001, and 0.0002, respectively. The effect size of each test was greater than 0.92.ConclusionsSignificant differences in force-velocity parameters of knee muscles were observed between the group of high-mountain climbers and the control group. The increase in the agonist/antagonist ratio and the decrease in the acceleration and deceleration times recorded in the group of high-altitude climbers are indicative of a change in the postural and dynamic mechanisms.

Driving Safety Improved with Control of Magnetorheological Dampers in Vehicle Suspension

The article is dedicated to the control of magnetorheological dampers (MR) included in a semi-active suspension of an all-terrain vehicle moving along a rough road profile. The simulation results of a half-car model and selected feedback vibration control algorithms are presented and analysed with respect to the improvement of driving safety features, such as road holding and vehicle handling. Constant control currents correspond to the passive suspension of different damping parameters. Independent Skyhook control of suspension parts represents the robust and widely used semi-active algorithm. Furthermore, its extension allows for the control of vehicle body heave and pitch vibration modes. Tests of the algorithms are carried out for a vehicle model that is synthesised with particular emphasis on mapping different phenomena occurring in a moving vehicle. The coupling of the vehicle to the road and environment is described by non-linear tire-road friction, rolling resistance, and aerodynamic drag. The pitching behaviour of the vehicle body, as well as the deflection of the suspension, is described by a suspension sub-model that exhibits four degrees of freedom. Further, three degrees of freedom of the complete model describe longitudinal movement of the vehicle and angular motion of its wheels. The MR damper model that is based on hyperbolic tangent function is favoured for describing the key phenomena of the MR damper behaviour, including non-linear shape and force saturation that are represented by force-velocity characteristics. The applied simulation environment is used for the evaluation of different semi-active control algorithms supported by an inverse MR damper model. The vehicle model is subjected to vibration excitation that is induced by road irregularities and road manoeuvres, such as accelerating and braking. The implemented control algorithms and different configurations of passive suspension are compared while using driving-safety-related quality indices.

New method to evaluate the rate of convergence for two-dimensional system involving friction damper

The performances of the two-dimensional system involving friction damper acquire highly sophisticated understanding both in steady-state and transient-state since the inherent strong nonlinearity associated with force-velocity characteristics, which highlightings the necessity to investigate the rate of convergence during transient-state. In this paper, we propose a numerical model of a two-dimensional system comprising friction damper and calculate the responses during transient-state with highly accurate slip/stick transition times and positions. We introduce the distances between two adjacent sticking points belong to two consequent orbits to describe the evolvements of responses during transient-state. We reveal that the distances are close to be logarithmically proportional to the cycle numbers during transient-state and oscillate around some sufficiently tiny value during steady-state. We establish a criterion of the nearness of approach to the steady-state by observing the critical cycle number i0 determined by the slope rate and intercept and employ it to evaluate the rate of convergence during transient-state. We consider several representative cases and utilize the new method to see how the rate of convergence is affected by constant normal load and present an approximation for the optimized constant normal load for the system with fastest rate of convergence.

Force-velocity characteristics of isolated myocardium preparations from rats exposed to subchronic intoxication with lead and cadmium acting separately or in combination

This investigation continues our study of the effects of Pb-Cd poisoning on the heart, extending the enquiry from isometric to auxotonic contractions, thereby examining the effect on the ability of myocardial tissues to perform mechanical work. Different shifts were revealed in myocardial force-velocity relations following subchronic exposure of rats to lead acetate and cadmium chloride acting separately, in combination, or in combination with a bioprotective complex (BPC). The experiments were conducted on isolated preparations of trabecules and papillary muscles of the right ventricle in physiological loading conditions and on isolated heart muscle contractile proteins examined by the in vitro motility assay. The results of the latter correlate with the shifts in the ratio of cardiac myosin isoforms. The amount of work performed by the myocardium was calculated on the basis of the tension-shortening loop area and was found to be similar in the preparations from all experimental groups. This fact presumably reflects adaptive capacity of the myocardial function even when contractility is damaged due to the metallic intoxication of a moderate severity.Some characteristics of rat myocardium altered by the impact of lead-cadmium intoxication became fully or partly normalized if intoxication developed against background administration of a bioprotective complex (BPC). Together with previously reported results obtained in the isometric mode of contractility, all these results strengthen the scientific foundations of risk assessment and risk management projects in the occupational and environmental conditions characterized by human exposure to lead and/or cadmium.

Lower extremity muscle strength across the adult lifespan in multiple sclerosis: Implications for walking and stair climbing capacity.

Persons with multiple sclerosis (pwMS) accumulate impairments in muscle strength and limitations in physical function as the disease progresses. The same symptoms are seen in the biological process of aging (years of age>60years). Nonetheless, no previous studies have examined the combined effects of aging and multiple sclerosis (MS) in regards to lower extremity muscle strength concomitant with physical function (e.g. walking capacity). The aim of this cross-sectional study was to examine potential differences in pwMS vs. healthy controls (HC) across the adult lifespan in these outcomes. A total of n=53 pwMS and n=48 age-matched HC were enrolled, and divided into groups of young (≤ 44years), middle-aged (45-59years), and old (≥ 60years). Assessment of knee extensor (KE) and plantar flexor (PF) muscle force-velocity characteristics (isometric and dynamic concentric force at contraction velocities of 0-300deg.s-1) alongside assessment of timed 25ft walk tests (T25FWT), two minute walk tests (2MWT), and 9-step stair ascend (9SSA) were carried out. Overall, substantial impairments in KE and PF isometric (ranging from -18 to -23%) and dynamic muscle strength (ranging from -26 to -38%) alongside limitations in T25FWT, 2MWT, and 9SSA (ranging from -24 to -33%) were observed in pwMS vs. HC (p<0.05). With advanced age (young → middle-aged → old), more pronounced impairments in PF (but not KE) isometric and dynamic muscle strength, alongside limitations in T25FWT, 2MWT, and 9SSA, were observed in pwMS vs. HC (p<0.05). In pwMS, associations were observed between physical function and isometric KE+PF muscle strength (r2=0.21-0.45) as well as dynamic KE+PF muscle strength (r2=0.36-0.61) (p<0.05). Substantial impairments of KE and PF muscle strength occur in pwMS across the adult lifespan accompanied by/associated with limitations in walking and stair climbing capacity, with PF muscle strength being preferentially affected in pwMS vs. HC.

Modelling and analysis of a magneto-rheological damper featuring non-magnetized flow paths in the piston

The damping characteristics of a magneto-rheological damper featuring non-magnetized flow paths in the piston are analysed using the Eyring constitutive model considering both viscous and minor hydraulic losses. The force–displacement and force–velocity characteristics of the magneto-rheological damper with non-magnetized flow paths were experimentally evaluated under different excitations and magnetic field intensity. Experimental results revealed relatively largerpre-yield-like region, attributed to flows through the non-magnetized paths, which increased with an increase in the coil current. A mathematical model of the damper was subsequently formulated using the Eyring constitutive model considering pressure drop across the piston and viscous effect in addition to the current-dependent friction. The Eyring model parameters were identified as a function of the magnetic field intensity and thus the coil current. For this purpose, a finite element model was formulated to identify a relation between the coil current and the magnetic field intensity. The validity of the proposed model is demonstrated by comparing the model-predicted force–velocity characteristics with the measured data under different applied currents. The model results are also compared with those obtained from the widely reported modified Bouc–Wen model and the Bingham constitutive model. The comparisons showed that the Eyring constitutive model can yield more accurate predictions of the damping properties compared to the Bingham model but similar to those from the modified Bouc–Wen model, while the Bouc–Wen model involves identifications of considerably greater number of parameters. The proposed model provided more accurate prediction of the damping force in the pre-yield region compared to the other models.

Diaphragm: The Relationship between Blood Supply Regulation and Characteristics of the Contractile Function

The diaphragm is a unique skeletal muscle; it is active throughout the lifetime and, therefore, differs from locomotor muscles in the properties of muscle fibers and the mechanisms of blood supply control. The aim of this review was to survey the structural characteristics of diaphragm muscle tissue, which provide its integral contractile properties; to compare the activity of vascular tone control mechanisms in the diaphragm and locomotor muscles; and to explore their relationship with the regulation of contractile function. The diaphragm differs from the majority of skeletal muscles by a high content of both slow fatigue-resistant muscle fibers of type I and fast fibers of type IIb, which provides endurance and high force-velocity characteristics of the diaphragm. The muscle fibers in the diaphragm are smaller, and the density of capillarization is much higher than in locomotor muscles. Arteries and arterioles that regulate blood supply to the diaphragm capillary bed combine the properties of arteries from muscles composed mainly of oxidative or mainly of glycolytic fibers. Such variety provides blood flow in the diaphragm adequate to its functional load with various patterns of activity. The mechanisms of vasoregulation in the diaphragm can qualitatively differ in the proximal and distal parts of the vascular bed. The functional properties of the proximal arteries can in part be explained by their proximity to the aorta and their small length. The contractile characteristics and blood supply of the diaphragm in various conditions should be considered when conducting respiratory muscle training in sports and rehabilitation medicine.