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Abstract Background Blood flow restriction (BFR) resistance training has demonstrated efficacy in promoting strength gains beneficial for rehabilitation. Yet, the distinct functional advantages of BFR strength training using high-load and low-load protocols remain unclear. This study explored the behavioral and neurophysiological mechanisms that explain the differing effects after volume-matched high-load and low-load BFR training. Methods Twenty-eight healthy participants were randomly assigned to the high-load blood flow restriction (BFR-HL, n = 14) and low-load blood flow restriction (BFR-LL, n = 14) groups. They underwent 3 weeks of BFR training for isometric wrist extension at intensities of 25% or 75% of maximal voluntary contraction (MVC) with matched training volume. Pre- and post-tests included MVC and trapezoidal force-tracking tests (0–75%–0% MVC) with multi-channel surface electromyography (EMG) from the extensor digitorum. Results The BFR-HL group exhibited a greater strength gain than that of the BFR-LL group after training (BFR_HL: 26.96 ± 16.33% vs. BFR_LL: 11.16 ± 15.34%)(p = 0.020). However, only the BFR-LL group showed improvement in force steadiness for tracking performance in the post-test (p = 0.004), indicated by a smaller normalized change in force fluctuations compared to the BFR-HL group (p = 0.048). After training, the BFR-HL group activated motor units (MUs) with higher recruitment thresholds (p

Background: Blood flow restriction (BFR) resistance training has demonstrated efficacy in promoting strength gains beneficial for rehabilitation. Yet, the distinct functional advantages of BFR strength training using high-load and low-load protocols remain unclear. This study explored the behavioral and neurophysiological mechanisms that explain the differing effects after volume-matched high-load and low-load BFR training. Methods: Twenty-eight healthy participants were randomly assigned to the high-load blood flow restriction (BFR-HL, n = 14) and low-load blood flow restriction (BFR-LL, n = 14) groups. They underwent 3 weeks of BFR training for isometric wrist extension at intensities of 25% or 75% of maximal voluntary contraction (MVC) with matched training volume. Pre- and post-tests included MVC and trapezoidal force-tracking tests (0–75%–0% MVC) with multi-channel surface electromyography (EMG) from the extensor digitorum. Results: The BFR-HL group exhibited a greater strength gain than that of the BFR-LL group after training (BFR_HL: 26.96 ± 16.33% vs. BFR_LL: 11.16 ± 15.34%)(p = 0.020). However, only the BFR-LL group showed improvement in force steadiness for tracking performance in the post-test (p = 0.004), indicated by a smaller normalized change in force fluctuations compared to the BFR-HL group (p = 0.048). After training, the BFR-HL group activated motor units (MUs) with higher recruitment thresholds (p < 0.001) and longer inter-spike intervals (p = 0.002), contrary to the BFR-LL group, who activated MUs with lower recruitment thresholds (p < 0.001) and shorter inter-spike intervals (p < 0.001) during force-tracking. The discharge variability (p < 0.003) and common drive index (p < 0.002) of MUs were consistently reduced with training for the two groups. Conclusions: BFR-HL training led to greater strength gains, while BFR-LL training better improved force precision control due to activation of MUs with lower recruitment thresholds and higher discharge rates. [ABSTRACT FROM AUTHOR]

Steel-reinforced concrete spans in road bridges have been widely used since the late 1950s, in the configuration of large-span bridges built across significant water barriers. To date, the issue of the need to reconstruct such span structures, including those designed and built according to the project “43282 km” developed by TsNIIPSK, is becoming increasingly relevant. The authors analyze the stages of the production of works of a particular object, developed for the implementation of the entire complex of works on its reconstruction using the method of force regulation. The presented order of work was successfully implemented during construction of the bridge over Kabarga River in Primorsky Krai. This made it possible to preserve the existing structure of the span (main beams and braces), replacing the worn-out reinforced concrete slab with a new - metal orthotropic one, while ensuring that the conditions of strength and stability of the flexural-torsional shape of solid-walled beams are met, in the process of dismantling the existing roadway slab and constructing a new one. Considering that steel-reinforced concrete bridges are built across large water barriers and have a very significant cost due to their large length, reconstruction using existing supports can significantly reduce the cost of construction, so, the possibility of upgrading the existing steel-reinforced concrete span structure is, undoubtedly, relevant. Based on the structural and technological measures presented by the authors, it is possible to carry out and effectively implement work on the reconstruction of the existing steel-reinforced concrete bridges that do not fully meet modern requirements for load capacity and throughput.

Abstract Background The biomechanical background of the transitory force decrease following a sudden reduction in the stimulation frequency under selected experimental conditions was studied on fast resistant motor units (MUs) of rat medial gastrocnemius in order to better understand the mechanisms of changes in force transmission. Methods Firstly, MUs were stimulated with three-phase trains of stimuli (low–high–low frequency pattern) to identify patterns when the strongest force decrease (3–36.5%) following the middle high frequency stimulation was observed. Then, in the second part of experiments, the MUs which presented the largest force decrease in the last low-frequency phase were alternatively tested under one of five conditions to analyse the influence of biomechanical factors of the force decrease: (1) determine the influence of muscle stretch on amplitude of the force decrease, (2) determine the numbers of interpulse intervals necessary to evoke the studied phenomenon, (3) study the influence of coactivation of other MUs on the studied force decrease, (4) test the presence of the transitory force decrease at progressive changes in stimulation frequency, (5) and perform mathematical analysis of changes in twitch-shape responses to individual stimuli within a tetanus phase with the studied force decrease. Results Results indicated that (1) the force decrease was highest when the muscle passive stretch was optimal for the MU twitch (100 mN); (2) the middle high-frequency burst of stimuli composed of at least several pulses was able to evoke the force decrease; (3) the force decrease was eliminated by a coactivation of 10% or more MUs in the examined muscle; (4) the transitory force decrease occured also at the progressive decrease in stimulation frequency; and (5) a mathematical decomposition of contractions with the transitory force decrease into twitch-shape responses to individual stimuli revealed that the force decrease in question results from the decrease of twitch forces and a shortening in contraction time whereas further force restitution is related to the prolongation of relaxation. Conclusions High sensitivity to biomechanical conditioning indicates that the transitory force decrease is dependent on disturbances in the force transmission predominantly by collagen surrounding active muscle fibres.

Purpose: A period of extra-efficient force production ("boost") followed by a decline in force ("sag") is often observed at the onset of unfused tetanic contractions. We tested the hypothesis that in human muscle boost and sag are diminished in repeated contractions separated by short rest periods and are re-established or enhanced following long rest periods. Methods: Two sets of 3 unfused tetanic contractions were evoked in the right quadriceps muscle group of 29 participants via percutaneous stimulation of the femoral nerve. Contractions consisted of 20 pulses evoked at inter-pulse intervals of 1.25 × twitch time to peak torque. Contractions were evoked 5 s apart and sets were evoked 5 min apart. Results: The ratio of the angular impulse of pulses 1–10 to the angular impulse of pulses 11–20 was used as the boost indicator. By this metric, boost was higher (P < 0.05) in the first relative to the second and third contractions within a set, but did not differ between sets (Set 1: 1.31 ± 0.15, 1.18 ± 0.12, 1.14 ± 0.12 vs Set 2: 1.34 ± 0.17, 1.17 ± 0.13, 1.14 ± 0.13). Sag (the percent decline in torque within each contraction) was also higher (P < 0.05) in the first relative to the second and third contractions within a set, but did not differ between sets (Set 1: 40.8 ± 7.5%, 35.4 ± 6.8%, 33.2 ± 7.8% vs Set 2: 42.1 ± 8.0%, 35.5 ± 6.8%, 33.9 ± 7.2%). Participants' sex and resistance training background did not influence boost or sag. Conclusion: Boost and sag are sensitive to contractile history in whole human quadriceps. Optimizing boost may have application in strength and power sports. [ABSTRACT FROM AUTHOR]

Periodontal mechanoreceptors (PMRs) are refined neural receptors present in abundance at the root apex and have a pivotal role in oral fine motor control. This case-control study aimed to evaluate the oral fine motor control of teeth treated with endodontic microsurgery (EMS) in comparison with the control teeth using a standardized behavioral biting task. Fourteen eligible participants performed 5 trials of an oral fine motor control task that involved holding and splitting half of a peanut positioned on a force transducer with their EMS treated tooth and its contralateral control incisor tooth (28 teeth in total). The outcome variables were the mean food holding force, intra- and intertrial variability of the holding force, food splitting force, splitting duration, and the frequency of the stepwise splitting phase. The data were analyzed with parametric and nonparametric tests. The results showed no statistically significant differences in the holding force, inter- and intratrial variability of the holding force, splitting force, or splitting duration between the teeth treated with EMS and the control (P >.05). However, there was a significantly higher frequency of stepwise ramp increase during the splitting phase with EMS treated teeth compared with the control (48% and 37%, respectively; P <.05). EMS treated teeth showed similar force regulation and oral fine motor control as the contralateral control. The findings of this study suggest that EMS treatment does not perturb the sensory information of PMRs and maintains the force regulation and oral fine motor control of the teeth. [ABSTRACT FROM AUTHOR]

New Findings: What is the central question of this study?How do contraction‐induced reductions in twitch duration, without changes in twitch force, affect summation of twitch pairs into higher force contractions in skeletal muscle?What is the main finding and its importance?Abbreviating twitch duration with a brief contraction resulted in enhanced summation of fully fused twitch pairs, but impaired summation in partially fused twitch pairs even after accounting for the differences in relaxation of the first twitch. An inherent mechanism which enhances relaxation without sacrificing force generation in forceful contractions would benefit cyclic muscle activities, such as locomotion. During electrically evoked contractions of skeletal muscle, the interplay between twitch duration and the time between electrical stimuli (inter‐pulse interval, IPI) determines how effectively twitch forces summate into high force contractions. A brief muscle contraction can impair summation by abbreviating twitch duration, though it is not clear if these impairments occur at all physiologically relevant IPI. This study was designed to test how a brief contraction affects summation of nominally isometric twitch pairs with IPIs lasting 10–5000 ms. Left adductor pollicis muscles of human participants (n = 9) were electrically activated using stimulus pairs applied both before (Pre) and after (Post) a 10 Hz, 1.0 s contraction. Force–time records were mathematically separated into Pulse 1 (single twitch) and Pulse 2 (summated twitch) components. The ratio of Pulse 2 peak force to Pulse 1 peak force was used as our measure of summation effectiveness. Consistent with the observed decline of Pulse 1 duration at Post relative to Pre (4.7 ± 0.6%; P < 0.001; duration was defined as the time from stimulation to the time required for active force to decline by 50%), summation effectiveness was higher at Pre than at Post at IPIs of 100–333 ms. Summation effectiveness was not different between Pre and Post at IPIs of 50–83 ms or 500–5000 ms. Intriguingly, summation effectiveness was higher at Post than at Pre at IPIs of 10–25 ms. In summary, a brief contraction has complex effects on the relationship between inter‐pulse interval and summation effectiveness. Future experiments are needed to reveal the mechanisms behind this novel observation. [ABSTRACT FROM AUTHOR]

In the past decade, micro- and nanomachines (MNMs) have made outstanding achievements in the fields of targeted drug delivery, tumor therapy, microsurgery, biological detection, and environmental monitoring and remediation. Researchers have made significant efforts to accelerate the rapid development of MNMs capable of moving through fluids by means of different energy sources (chemical reactions, ultrasound, light, electricity, magnetism, heat, or their combinations). However, the motion of MNMs is primarily investigated in confined two-dimensional (2D) horizontal setups. Furthermore, three-dimensional (3D) motion control remains challenging, especially for vertical movement and control, significantly limiting its potential applications in cargo transportation, environmental remediation, and biotherapy. Hence, an urgent need is to develop MNMs that can overcome self-gravity and controllably move in 3D spaces. This review delves into the latest progress made in MNMs with 3D motion capabilities under different manipulation approaches, discusses the underlying motion mechanisms, explores potential design concepts inspired by nature for controllable 3D motion in MNMs, and presents the available 3D observation and tracking systems., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Effects of a sudden decrease in the stimulation frequency for motor unit force were studied in rat medial gastrocnemius. For 161 functionally isolated single motor units of three types (S, FR, FF), unfused tetanic contractions were evoked by three-phase trains of stimuli (low-high-low frequency). The course of the tetanus at the onset of the third phase of the force recording was analyzed in tetani with variable fusion degree. For 78 units within the third phase of tetanus, a transitory force decrease to a level lower than in the first phase (identical frequency), was observed. This phenomenon was more frequent for fast fatigue resistant (65.9%) than for fast fatigable and slow motor units (27.1% and 35.5%, respectively). Moreover, the force decrease was strongest for fast resistant motor units (up to 36.5%) and when contractions evoked at variable frequencies of stimulation were compared, the highest amplitudes of the studied force decrease were noted for middle-fused tetani (0.50-0.90). A new phenomenon of transitory force decrease in tetanic contractions of motor units with a decrease in stimulation frequency was found. Most probably, the phenomenon is dependent on disturbances in the force transmission by collagen surrounding active muscles fibers. [ABSTRACT FROM AUTHOR]

The muscle force is the sum of forces of multiple motor units (MUs), which have different contractile properties. During movements, MUs develop unfused tetani, which result from summation of twitch-shape responses to individual stimuli, which are variable in amplitude and duration. The aim of the study was to develop a realistic muscle model that would integrate previously developed models of MU contractions and an algorithm for the prediction of tetanic forces. The proposed model of rat medial gastrocnemius muscle is based on physiological data: excitability and firing frequencies of motoneurons, contractile properties, and the number and proportion of MUs in the muscle. The MU twitches were modeled by a six-parameter analytical function. The excitability of motoneurons was modeled according to a distribution of their rheobase currents measured experimentally. Processes of muscle force regulation were modeled according to a common drive hypothesis. The excitation signal to motoneurons was modeled by two form types: triangular and trapezoid. The discharge frequencies of MUs, calculated individually for each MU, corresponded to those recorded for rhythmic firing of motoneurons. The force of the muscle was calculated as the sum of all recruited MUs. Participation of the three types of MUs in the developed muscle force was presented at different levels of the excitation signal to motoneurons. The model appears highly realistic and open for input data from various skeletal muscles with different compositions of MU types. The results were compared with three other models with different distribution of the input parameters. NEW & NOTEWORTHY The proposed mathematical model of rat medial gastrocnemius muscle is highly realistic because it is based strictly on experimentally determined motor unit contractile parameters and motoneuron properties. It contains the actual number and proportion of motor units and takes into consideration their different contributions to the whole muscle force, depending on the level of the excitation signal. The model is open for input data from other muscles, and additional physiological parameters can also be included. [ABSTRACT FROM AUTHOR]

The aim of this study was to determine the role of titin in preventing the development of sarcomere length nonuniformities following activation and after active and passive stretch by determining the effect of partial titin degradation on sarcomere length nonuniformities and force in passive and active myofibrils. Selective partial titin degradation was performed using a low dose of trypsin. Myofibrils were set at a sarcomere length of 2.4 µm and then passively stretched to sarcomere lengths of 3.4 and 4.4 µm. In the active condition, myofibrils were set at a sarcomere length of 2.8 µm, activated, and actively stretched by 1 µm/sarcomere. The extent of sarcomere length nonuniformities was calculated for each sarcomere as the absolute difference between sarcomere length and the mean sarcomere length of the myofibril. Our main finding is that partial titin degradation does not increase sarcomere length nonuniformities after passive stretch and activation compared with when titin is intact but increases the extent of sarcomere length nonuniformities after active stretch. Furthermore, when titin was partially degraded, active and passive stresses were substantially reduced. These results suggest that titin plays a crucial role in actively stretched myofibrils and is likely involved in active and passive force production. [ABSTRACT FROM AUTHOR]

A system for regulating the parameters of electrostimulated drawing (the temperature in the deformation zone and the drawing force) is considered. This system produces a control signal for the unit generating powerful current pulses. The basic operating principle is periodic discharge of a precharged capacitor to a low-resistance load. For regulation of the pulse amplitude and increase in system power, the uncontrollable dc source in the charger is replaced by two irreversible thyristor converters, which are in series and operate in the same direction. That produces a controllable voltage at the power capacitors. To optimize capacitor charging, a two-loop subordinate control system is employed: the external loop regulates the voltage, while the internal loop regulates the current that charges the capacitors. The high speed of the transient processes in electrostimulated drawing—in particular, the rapid temperature rise in the deformation zone on account of the large current pulse (up to 10 kA) and the high pulse frequency (up to 400 Hz)—means that manual control is practically impossible. To boost the reliability and quality of electrostimulated drawing using a powerful current-pulse generator, an automatic control system for electrostimulated drawing is developed. It includes a single-loop system for regulating the drawing force and also delayed temperature feedback in the deformation zone. The dependence of the drawing force and temperature on the frequency of the current pulses is established by means of laboratory research and calculations using both new and familiar methods. A model of the proposed control system in MATLAB-Simulink software permits analysis of the operating conditions in electrostimulated drawing under automated control. The model is consistent with the actual parameters obtained in research on the electroplastic effect. The proposed model permits improvement in the characteristics and operating conditions of electrostimulated drawing. The formalized structure of the system, the proposed model of the system in MATLAB-Simulink software, and the oscillograms of the transient processes are considered. The single-loop automatic control system for the drawing force, with flexible temperature feedback in the deformation zone, permits optimization of the operating conditions and improvement in the reliability of electrostimulated drawing. The proposed system is recommended for use in studying electrostimulated deformation and also for the control of wire drawing in production conditions [ABSTRACT FROM AUTHOR]

This paper is concerned with the compliant force regulation for a robot manipulator interacting with a deformable environment described as a potential field. We extend the conventional result of the asymptotic force regulation based on the energy shaping method for a port-Hamiltonian system. Additionally, we add an extra compensator and equip a modified integrator dynamics to the proposed controller in order to introduce sufficient degrees of freedom. Those degrees of freedom enable dealing with a wider class of disturbances, which are possibly time-varying and appear in the dynamics of all the variables of the closed-loop system. Moreover, first, we prove that the proposed controller achieves asymptotic force regulation at the desired position without disturbances. Second, we prove that the closed-loop system with the proposed controller becomes input-to-state stable with respect to any bounded disturbances and guarantees that both the solution of the system and resultant interaction force remain bounded. [ABSTRACT FROM AUTHOR]

Integrins mediate cell adhesion to extracellular matrix and transduce signals bidirectionally across the membrane. Integrin α V β 3 has been shown to play an essential role in tumor metastasis, angiogenesis, hemostasis and phagocytosis. Integrins can take several conformations, including the bent and extended conformations of the ectodomain, which regulate integrin functions. Using a biomembrane force probe, we characterized the bending and unbending conformational changes of single α V β 3 integrins on living cell surfaces in real-time. We measured the probabilities of conformational changes, rates and speeds of conformational transitions, and the dynamic equilibrium between the two conformations, which were regulated by tensile force, dependent on the ligand, and altered by point mutations. These findings provide insights into how α V β 3 acts as a molecular machine and how its physiological function and molecular structure are coupled at the single‐molecule level. [ABSTRACT FROM AUTHOR]

The effects of hypothermia and hyperthermia on mammalian skeletal muscle function have previously been reported. However, their effects on the contractile properties of different motor unit (MU) types were not described. This study aimed to explore the effect of temperature on contractile properties of MUs in rat medial gastrocnemius kept at 25 °C (hypothermia), 37 °C (normothermia), and 41 °C (hyperthermia). Hypothermia prolonged the twitch time parameters of all MU types, shifting the steep part of the force-frequency curve towards lower frequencies and increasing its steepness. In addition, it reduced the rate of force development but not the twitch and tetanus forces of slow-twitch (S) MUs. Moreover, it reduced the tetanic force of fast-twitch fatigable (FF) MUs and increased the twitch force of fast-twitch fatigue-resistant (FR) MUs. In contrast, hyperthermia had opposite effects on twitch time properties and the force-frequency relationship. The twitch-to-tetanus ratio decreased for FF and FR MUs, and the steep part of the force-frequency curve shifted towards higher frequencies and decreased in steepness. Our findings indicate that FF MUs are the most sensitive and S MUs are the least sensitive to temperature. Furthermore, force control processes involving changes in motoneuronal firing frequency were radically modified for fast MUs, especially FF MUs. [ABSTRACT FROM AUTHOR]

Inspired by the strong nonlinearity of origami cartons, a novel origami-inspired constant-force mechanism (OriCFM) is systematically studied for obtaining a stable quasi-static constant force output. The proposed OriCFM is evolved by adding equivalent springs at horizontal and oblique creases of the rigid origami mechanism. Based on the geometry relationship, the mechanical model of the origami carton is established and its zero-stiffness is obtained. The influence of structural parameters and spring stiffness on the constant force characteristic of the OriCFM is analyzed in detail. By configuring different numbers of horizontal springs, various output characteristics could be observed in the prototype experiment. The case of 4 oblique springs and 2 horizontal springs exhibited partial constant-force output, and the constant-force range accounted for 48.54% of the total experimental stroke. Comparisons of theoretical results with experimental data were carried out to demonstrate the feasibility and effectiveness of the proposed origami-inspired mechanism. The outcomes of this work could expand the applications of origami mechanisms and promote the development of low-stiffness systems. • A novel origami-inspired constant-force mechanism (OriCFM) is proposed. • The OriCFM can achieve stable force output in a large constant-force range. • With influential and tunable parameters, the OriCFM has strong design flexibility. • The quasi-static output of OriCFM is verified by numerical and experimental studies. [ABSTRACT FROM AUTHOR]

Cell-matrix adhesion complexes are multi-protein structures linking the extracellular matrix (ECM) to the cytoskeleton. They are essential to both cell motility and function by bidirectionally sensing and transmitting mechanical and biochemical stimulations. Several types of cell-matrix adhesions have been identified and they share many key molecular components, such as integrins and actin-integrin linkers. Mechanochemical coupling between ECM molecules and the actin cytoskeleton has been observed from the single cell to the single molecule level and from immune cells to neuronal cells. However, the mechanisms underlying force regulation of integrin-mediated mechanotransduction still need to be elucidated. In this review article, we focus on integrin-mediated adhesions and discuss force regulation of cell-matrix adhesions and key adaptor molecules, three different force-dependent behaviors, and molecular mechanisms for mechanochemical coupling in force regulation. [ABSTRACT FROM AUTHOR]

In this brief, we address the robust force regulation problem of mechanical systems in physical interaction with compliant environments. The control method that we present is entirely derived under the energy shaping framework. Note that for compliant interactions, standard energy shaping methods (i.e., potential shaping controls using static-state feedback actions) cannot guarantee asymptotic stability since they are not robust to unmodeled forces. To cope with this issue, in this brief, we integrate force sensory feedback with a robust energy shaping design. This methodology allows us to incorporate integral force controls while preserving in closed loop the port-Hamiltonian structure, something that is not possible with traditional force regulators. We discuss the practical implementation of our method and provide simple numerical algorithms to compute in real time some of its control terms. To validate our approach, we report an experimental study with an open architecture robot manipulator. [ABSTRACT FROM PUBLISHER]

ABSTRACT Introduction: We quantified submaximal torque regulation during low to moderate intensity isometric hip flexion contractions in individuals with stroke and the associations with leg function. Methods: Ten participants with chronic stroke and 10 controls performed isometric hip flexion contractions at 5%, 10%, 15%, 20%, and 40% of maximal voluntary contraction (MVC) in paretic, nonparetic, and control legs. Results: Participants with stroke had larger torque fluctuations (coefficient of variation, CV), for both the paretic and nonparetic legs, than controls ( P < 0.05) with the largest CV at 5% MVC in the paretic leg ( P < 0.05). The paretic CV correlated with walking speed (r2 = 0.54) and Berg Balance Score (r2 = 0.40). At 5% MVC, there were larger torque fluctuations in the contralateral leg during paretic contractions compared with the control leg. Conclusions: Impaired low-force regulation of paretic leg hip flexion can be functionally relevant and related to control versus strength deficits poststroke. Muscle Nerve 49: 225-232, 2014 [ABSTRACT FROM AUTHOR]

Abstract: In this study bimanual grip–force coordination was quantified using a novel “Gripper” system that records grip forces produced while holding a lower and upper unit, in combination with the lift force necessary to separate these units. Children with unilateral cerebral palsy (CP) (aged 5–14 years, n =12) were compared to age matched typically developing (TD) children (n =23). Compared to TD, the CP-group is much slower and takes 50% more time to generate grip and lift forces with more fixating force before lifting the upper unit. In addition the coordination between forces in both hands is reduced. The CP-group increases the lift force in the upper hand 2.5 times more than the holding force when pulling the two units apart, while this is only 1.5 times in TD. Moreover, the correlation between forces generated in both hands in the CP-group is lower. The lack of fine tuning of the forces, measured by the linearity error is increased, especially when the magnet load keeping the unit together is low. The results indicate an impaired pull–hold synergy between upper and lower hand and the lift force. Bimanual tasks evaluating bimanual grip and lift forces in children with CP and can give us new insights in the underlying force control mechanisms of the spastic hand. [Copyright &y& Elsevier]

Abstract: Objective: This study determined the variability and regularity of force production in impaired upper extremities of chronic stroke survivors. Two hypotheses included: (1) stroke will increase the variability and regularity of force output in comparison to age-matched controls and (2) degree of motor impairments will be positively correlated with the variability and regularity of force output. Methods: Nine chronic stroke and nine age-matched controls performed unimanual isometric wrist and finger extension movements for 20s to three different target force levels. Force output was indexed by magnitude, accuracy, variability, and regularity. Stroke motor impairment levels were determined by Fugl-Meyer upper extremity assessment. Results: The stroke group demonstrated increased variability and regularity of the force output. Further, motor impairments scaled with increase in variability and regularity of force output. Conclusions: The variability and regularity of force differentiated isometric contractions performed by chronic stroke survivors from age-matched controls. Moreover, in clinical settings an objective assessment of force control on variability and regularity appears to be most meaningful at 25% of MVC. Significance: Increased variability contributes to reduced steadiness in force output. Increased regularity characterizes the adaptability losses in motor capabilities following stroke. This knowledge may facilitate planning and evaluating rehabilitation protocols. [ABSTRACT FROM AUTHOR]

Background. Rehabilitation of the upper extremity in children with hemiplegic cerebral palsy has not been compared to the same intensity of therapy combined with injected botulinum toxin (BTX). Objective. To measure the short-term (2 weeks) and long-term (6 and 9 months) effects of a standardized functional training program versus without the addition of chemodenervation of forearm and hand muscles. Methods. Twenty children with spastic hemiplegia, aged 4 to 16 years, were matched for baseline characteristics and then randomized to standardized functional physical and occupational therapies for 6 months (PT/OT group) or to the same therapies plus multimuscle BTX-A (BTX+ group). Main outcome measures were isometric generated force, overshoot and undershoot (force production error), active and passive range of motion by goniometry (ROM), stretch restricted angle (SRA) of joints, Ashworth scores at the elbow and wrist, and the Melbourne assessment of unilateral upper limb function. All measures were performed at baseline, 2 weeks after BTX-A, 6 months (end of therapy), and then 3 months after termination of the therapy. Results. Clinical measures (muscle tone, active ROM of wrist and elbow) showed improvement in both groups. However, no significant differences emerged between groups on functional measures. Generated force decreased directly after the BTX-A injection but increased during the therapy period. The PT/OT group, however, showed a significantly higher increase in force and accuracy with therapy compared with the BTX+ therapy group. Conclusions. Functional rehabilitation therapies for the upper extremity increase manual isometric flexor force at the wrist and ROM, but BTX injections cause weakness and do not lead to better outcomes than therapy alone. [ABSTRACT FROM AUTHOR]

Abstract: In this study the hypothesis was tested that children with spastic hemiplegia rely more on externally guided visual feedback when trying to keep force constant with their affected hand (AH) as compared to their non-affected hand (NAH) and as compared to controls. An isometric force task in which a cursor had to be moved to a visually specified target that disappeared half way the task, was performed by 19 children with cerebral palsy (CP), spastic hemiplegia, aged between 5 and 16 years and an aged matched control group. It was found that the absolute deterioration of performance after withdrawal of target visualization did differ between AH, NAH and controls. The absolute error was smaller and the variability was larger in the hemiplegic hand. However, the normalized force error and co-efficient of variation increased similarly between groups. Furthermore, power spectrum density analysis of the force signal showed that both hands in both groups had a similar loss in the energy in the 2–3Hz range when target visualization was removed. These results suggest that CP children are equally able to produce stable force without visually monitoring their performance than children without CP, provided they are allowed to operate within their own force range. [Copyright &y& Elsevier]

Locomotion is essential for vertebrate survival. Forces required for movement are generated by skeletal muscle. Skeletal muscle shortening and/or force generation occur via parallel sliding of two protein filaments: actin and myosin. This is driven by the cycling of cross-bridges, whose unitary nanometer length change and picoNewton force output are fueled by conversion of chemical energy, stored in the form of adenosine triphosphate, into a change in myosin protein configuration. The range of force and length changes of a muscle is determined by factors such as muscle cross section, fiber angle, tendon attachment, and lever geometry, but also by the metabolic pathways available for adenosine triphosphate synthesis and by enzymes involved in cross-bridge cycling. In addition, muscle mechanical activity is affected by the extent of actin and myosin filament overlap. Force output can be graded by selective recruitment of motor units and/or by variation of force output from individual units. The cost of locomotion is subject to species differences and is affected by the environment and form of movement, with an energy efficiency of up to 0.4. Overall, design principles of vertebrate skeletal muscle may serve as an interesting reference point for novel actuator technologies. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of contact transition from free motion to constrained motion for robots. Stability of transition from free motion to constrained motion is essential for successful operation of a robot performing general tasks such as surface following and surface finishing. Uncertainty in the location of the constraint can cause the robot to impact the constraint surface with a nonzero velocity, which may lead to bouncing of the robot end-effector on the surface. A new stable discontinuous transition controller is proposed to deal with contact transition problem. This discontinuous transition control algorithm is used when switching from free motion to constrained motion. Control algorithm for a complete robot task is developed. Extensive experiments with the proposed control strategy were conducted with different levels of constraint uncertainty and impact velocities. Experimental results show much improved transition performance and force regulation with the proposed controller. Details of the experimental platform and typical experimental results are given [ABSTRACT FROM PUBLISHER]

Fatigue is the decrease in active force that happens after repeated muscle stimulation, and post tetanic twitch potentiation (PTP) is the increase in twitch force observed after repeated muscle stimulation. This study investigated the effects of length on the interaction between fatigue and PTP, as these two forms of force regulation are length-dependent and may coexist. A total number of 14 subjects were tested in 3 days, in which fatigue and PTP were induced in the knee extensor muscles in three different knee angles (30°, 60° and 90°; full extension=0°). PTP was evaluated in rested and fatigued muscles with twitch contractions elicited before and after 10 s maximal voluntary contraction (MVC), and fatigue was evaluated with nine 50 Hz electrically elicited contractions (5 s duration, 5 s interval between contractions). Fatigue was length-dependent, with force values that were (mean ± SEM) 59 ± 5, 56 ± 3 and 38 ± 1% of maximal force at 90°, 60° and 30°, respectively. PTP was also length-dependent. Rested muscles showed PTP of 39 ± 4, 47 ± 2 and 68 ± 5% at 90°, 60° and 30°, respectively. Fatigued muscles showed PTP of 44 ± 3, 55 ± 6 and 68 ± 5%, at 90°, 60° and 30°, respectively. This study shows that fatigue and PTP may represent independent mechanisms, as they regulate force in opposite directions and are both enhanced in short muscle lengths. [ABSTRACT FROM AUTHOR]

With the goal of filling the gap between theory and industrial applications, an implicit hybrid control scheme is proposed in this paper, designed to fit as much as possible the conventional industrial robot control architecture. The dynamic effects due to joint compliance, which are a major source of performance degradation in industrial robots, are fully taken into account. The scheme is based on a task description particularly suited for a direct integration in conventional robot programming tools and aims at exerting the force control action without affecting the trajectory tracking. Only the differential kinematic model (Jacobian) of the robot is needed in the design of the force control law, while the force control loop is charged with rejecting dynamic disturbances due to motion. A thorough experimental validation of the strategy, both in terms of force regulation and trajectory tracking capabilities, is discussed, based on experiments performed on an industrial robot, endowed with a six-axis wrist force/torque sensor and with a laser distance sensor. [ABSTRACT FROM PUBLISHER]

Unfused tetanic contractions evoked in fast motor units exhibit extra-efficient force production at the onset of contraction, an effect called "boost". Boost is diminished in subsequent contractions if there is a short rest period between contractions, but can be re-established with a longer period of rest. We tested the hypothesis that contractile activity and rest could enhance boost-related metrics. Two sets of 3 unfused tetani were evoked 3 min apart in fast fatigable (FF) and fast fatigue-resistant (FR) motor units of the rat medial gastrocnemius. The greatest changes occurred in the first unfused tetanic contractions. Relative to the first contraction in the first set, the first contraction in the second set exhibited higher peak force during boost in a subset of motor units (76% of FF and 48% of FR). Enhanced force during boost was influenced by interaction of slowing of twitch contraction time (up to 20% and 25%, for FF and FR motor units, respectively), half-relaxation time (up to 37% and 49% for FF and FR motor units, respectively), and potentiation of the first twitch (up to 13% and 5% for FF and FR motor units, respectively). Examination of twitches evoked between sets suggested opportunity for greater enhancement of boost with shorter intervening rest periods. The phenomenon of enhanced boost following motor unit activity may interest sports scientists. [ABSTRACT FROM AUTHOR]

Wild African elephants are voracious eaters, consuming 180 g of food per minute. One of their methods for eating at this speed is to sweep food into a pile and then pick it up. In this combined experimental and theoretical study, we elucidate the elephant's unique method of picking up a pile of food by compressing it with its trunk. To grab the smallest food items, the elephant forms a joint in its trunk, creating a pillar up to 11 cm tall that it uses to push down on food. Using a force sensor, we show the elephant applies greater force to smaller food pieces, in a manner that is required to solidify the particles into a lump solid, as calculated by Weibullian statistics. Elephants increase the height of the pillar with the force required, achieving up to 28% of the applied force using the self-weight of the pillar alone. This work shows that elephants are capable of modulating the force they apply to granular materials, taking advantage of their transition from fluid to solid. In the future, heavy robotic manipulators may also form joints to compress and lift objects together., (© 2018 The Author(s).)