Force Decrement Research Articles

Nanoscale-roughness influence on pull-off adhesion force in liquid and air.

The pull-off adhesion force was measured by atomic force microscopy in sphere-plate geometry in water where a capillarylike behavior develops due to nanobubbles and was compared to the corresponding capillary adhesion in air. The sphere and the plate were coated with gold, and the pull-off adhesion force was measured as a function of the evolving surface roughness of the plate, and the retraction velocity of the interacting surfaces. In absolute magnitude, the pull-off force in air is larger than that in liquid by an order of magnitude or more, but in both cases, the pull-off force follows a monotonic decrease with increasing roughness. However, the relative decrement of the adhesion force in water was approximately 300%, and significantly higher than that in air for the same change of the rms roughness in the range ∼7-14 nm. Finally, the adhesion force in water shows a relatively complex dependence on the retraction velocity of the interacting surfaces as the roughness increases due to possible deformation of the nanobubbles and the bridges they form between the surfaces.

Using force or EMG envelope as feedback signal for motor control system

PurposeThis work studied muscle neuro-mechanics during symmetrical up-going ramp (UGR) and down-going ramp (DGR). Aim: to evaluate during the modulation of muscular action the outcome of force feedback (FF) or neural feedback (NF) on the behavior of the trailing signals - i.e. the EMG envelope (eEMG) for FF or force signal for NF. MethodSubjects: 20. Investigated muscles: dorsal interosseous (FDI) and tibialis anterior (TA). Detected signals: force and EMG. Visual feedback: force (FF), eEMG (NF). Effort triangles: ramps duration 7.5 s, vertex at 50 and 100 % of the maximal voluntary action. Eventually, each subject performed FF50%, FF100%, NF50% and NF100% per each muscle. In each condition the areas beneath the force and eEMG signals were computed to calculate the ratios between the DGR and UGR values during the different tasks (force area DGR / force area UGR; eEMG area DGR / eEMG area UGR). Electro-mechanical coupling efficiency (EMCE) was estimated through the eEMG area / force area ratio for both UGR and DGR in each condition. Resultsa) FF. FDI: eEMG area ratio was 0.84 ± 0.15 and 0.73 ± 0.17 for FF50% and FF100%, respectively. TA: eEMG area ratio was 0.88 ± 0.11 and 0.91 ± 0.17 for FF50% and FF100%, respectively. b) NF: FDI: force area ratio was 1.18 ± 0.13 and 1.17 ± 0.13 for NF50% and NF100%, respectively. TA: force area ratio was 1.17 ± 0.21 and 1.07 ± 0.19 for NF50% and NF100%, respectively. c) DGR EMCE was greater than UGR EMCE in all four tasks. ConclusionThe influence of UGR on deployed EMCE in the following force decrement phase underpins the changes of trailing signals area during DGR. This underlines the necessity of a careful evaluation of the features of FF or NF for experimental studies or rehabilitation purposes involving the motor control system.

Dissimilar laser beam welding of AISI 2507 super duplex stainless to AISI 317L austenitic stainless steel

In this research, AISI 2507 super duplex/AISI 317L austenitic stainless steels were joined by laser beam welding technique. The effects of heat input and post weld heat treatment (PWHT) on surface, texture, microstructure and mechanical properties were investigated. Based on the experimental results, it has been determined that the weld metal (WM) exhibits a duplex microstructure. Ferrite grains, grain boundary austenite, intragranular austenite and widmanstatten austenite and dislocation formations were detected in WM. In addition, ferrite, austenite grains and stacking faults were detected in the base metal (BM) of 2507 super duplex and 317L austenitic stainless steels. While dislocation and stacking fault formations disappeared after PWHT, Cr23C6 precipitates were formed. While a K-S and N-W orientation relationship (OR) was generally seen in the samples with and without PWHT, no OR was seen in the BM of the 317L side. In addition, the PWHT result of the Bain OR seen in the WM of the welded sample was not seen. The findings showed that while the Oxygen core on the WM surface of the sample without PWHT and PWHT was at the highest level at first, it then became zero as it descended deeper. Moreover, it was determined that the levels of Fe, Cr, Ni and Mo nuclei, which were at low levels at first, increased as one went deeper. It has been determined that as the heat input increases, the tensile strength, hardness and bending force decrement, while the toughness values increase in the welded samples. Based on PWHT, there is a reduction in tensile strength, hardness and bending force; [111] orientation to the grains [101] and [001] grains, a rise in the proportion of grains with a Schmid Factor (SF) value greater than 0.4, a decrement in kernel average misorientation (KAM) values and coincidence site lattice (CSL) boundary ratios, (001)<100> and (112)<111> this component has been caused by changes such as the emergence of recrystallized γ grains causing a rise in austenite content, increase in grain size, disappearance of stacking fault occurrences and dislocations. It was determined that the rise in toughness was caused by the emergence of (113)<332> component and the increase in low angle grain boundary ratios.

Knee position sense and knee flexor neuromuscular function are similarly altered after two submaximal eccentric bouts.

This study examined eccentric-induced fatigue effects on knee flexor (KF) neuromuscular function and on knee position sense. This design was repeated across two experimental sessions performed 1week apart to investigate potential repeated bout effects. Sixteen participants performed two submaximal bouts of KF unilateral eccentric contractions until reaching a 20% decrease in maximal voluntary isometric contraction force. Knee position sense was evaluated with position-matching tasks in seated and prone positions at 40° and 70° of knee flexion so that KF were either antagonistic or agonistic during the positioning movement. The twitch interpolation technique was used to assess KF neuromuscular fatigue. Perceived muscle soreness was also assessed. Measurements were performed before, immediately (POST) and 24h after (POST24) each eccentric bout. No repeated bout effect on neuromuscular function and proprioceptive parameters was observed. At POST, central and peripheral factors contributed to the force decrement as shown by significant decreases in voluntary activation level (-3.8 ± 4.8%, p < 0.01) and potentiated doublet torque at 100Hz (-10 ± 15.8%, p < 0.01). At this time point, position-matching errors significantly increased by 1.7 ± 1.9° in seated position at 40° (p < 0.01). At POST24, in presence of muscle soreness (p < 0.05), although KF neuromuscular function had recovered, position-matching errors increased by 0.6 ± 2.6° in prone position at 40° (p < 0.01). These results provide evidence that eccentric-induced position sense alterations may arise from central and/or peripheral mechanisms depending on the testing position.

Fatigue Measured in Dynamic Versus Isometric Modes After Trail Running Races of Various Distances

Fatigue has previously been investigated in trail running by comparing maximal isometric force before and after the race. Isometric contractions may not entirely reflect fatigue-induced changes, and therefore dynamic evaluation is warranted. The aim of the present study was to compare the magnitude of the decrement of maximal isometric force versus maximal power, force, and velocity after trail running races ranging from 40 to 170km. Nineteen trail runners completed races shorter than 60km, and 21 runners completed races longer than 100km. Isometric maximal voluntary contractions (IMVCs) of knee extensors and plantar flexors and maximal 7-second sprints on a cycle ergometer were performed before and after the event. Maximal power output (Pmax; -14% [11%], P < .001), theoretical maximum force (F0; -11% [14%], P < .001), and theoretical maximum velocity (-3% [8%], P = .037) decreased significantly after both races. All dynamic parameters but theoretical maximum velocity decreased more after races longer than 100km than races shorter than 60km (P < .05). Although the changes in IMVCs were significantly correlated (P < .05) with the changes in F0 and Pmax, reductions in IMVCs for knee extensors (-29% [16%], P < .001) and plantar flexors (-26% [13%], P < .001) were larger (P < .001) than the reduction in Pmax and F0. After a trail running race, reductions in isometric versus dynamic forces were correlated, yet they are not interchangeable because the losses in isometric force were 2 to 3 times greater than the reductions in Pmax and F0. This study also shows that the effect of race distance on fatigue measured in isometric mode is true when measured in dynamic mode.

Profound insight into tractor energy dissipation through inevitable interaction inside wheel-soil interface for the period of plowing works

Inevitable interaction between soil and pneumatic tires of an off road vehicle in a rectilinear motion leads to experience of energy dissipation. Rolling resistance force and longitudinal slip of wheels are responsible for the energy dissipation inside wheel-soil interface. Attention of current study is paid to methodically elucidate the energy dissipation by wheels of front wheel assist tractor for the period of plowing works. To achieve this goal, the energy dissipation dependency on draft force requirement of plow implement as well as tractor drive mode (rear wheel drive (RWD) and front wheel-assisted drive (FWAD)) was quantitatively ascertained. Results disclosed that the energy dissipation ascended in domain of 3.18-17.66 and 3.14-21.04 MJ/ha for the RWD and FWAD mode, respectively, as the draft force requirement proliferated from 3.85 to 23.04 kN. Whilst, energy expenditure for plowing works ranged from 12.45 to 74.85 MJ/ha. It means that the energy dissipation was approximately equal to quarter of energy expenditure for plowing works. When the draft force requirement augmented, portion of the energy dissipation caused by the slip of the wheels raised in range of 12.15-82.51% and thereby, that of rolling resistance force of the wheels dropped in range of 87.85-17.49%. Hence, to reduce the energy dissipation, for both drive modes at the higher draft force requirement, activation of slip control system for preservation of the slip of the wheels in a particular range is sufficient enough. For both drive modes at the lower draft force requirement, not only activation of the system, but also decrement of rolling resistance force of the wheels by overinflating the tires must be prescribed. In case of the FWAD mode, overinflating the tires must be accomplished based on generation of driving lead of front wheels in allowable range.

Microgripper Based on Simple Compliance Configurations, Improved by Using Parameterization

The design of a novel electrothermal microgripper device is shown, which is based on an improved chevron type actuator developed considering their elements parameterization, whose resistive model is also provided. The performance of the microgripper’s parameters, such as displacement, force, and temperature distribution, with convection for the voltage range from 0 up to 5 V, is evaluated through numerical and analytical simulation. Microgripper design was also improved with aid of parameterization. The effect on the microgripper performance due to its thickness is also analyzed, finding a considerable increment in force, when thickness increases. Its main advantage is given by the simplicity of the compliance arrangement of the microgrippers jaws. Considering convection, when 5 V are applied, 37.72 °C was generated at the jaw’s tips of the Improved Microgripper 2 (IMG2), implemented with silicon, this relatively low temperature increases its capabilities of application. When the IMG2 is implemented with polysilicon, its response is competitive comparing with a more complex microgripper, increase of displacement (50%) is shown, but a decrement of force (30%). The diameters allowed for the subjection objects are found between 84.64 µm and 108 µm, with weights lower than 612.2 µg. Some tests of subjection were performed using microcylinders of Au, glass ceramic, polycarbonate and carbon fiber, showing a permissible stress on them, considering its Young’s modulus, as well as the total reaction force induced. All simulations were done on Ansys software. The results demonstrate the feasibility of the future microgripper fabrication.

Acute Neuromuscular Electrical Stimulation (NMES) With Blood Flow Restriction: The Effect of Restriction Pressures.

Neuromuscular electrical stimulation (NMES) combined with blood flow restriction (BFR) has been shown to improve muscular strength and size better than NMES alone. However, previous studies used varied methodologies not recommended by previous NMES or BFR research. The present study investigated the acute effects of NMES combined with varying degrees of BFR using research-recommended procedures to enhance understanding and the clinical applicability of this combination. Randomized crossover. Physiology laboratory. A total of 20 healthy adults (age 27 [4]y; height 177 [8]cm; body mass 77 [13]kg). Six sessions separated by at least 7 days. The first 2 visits served as familiarization, with the experimental conditions performed in the final 4 sessions: NMES alone, NMES 40% BFR, NMES 60% BFR, and NMES 80% BFR. Maximal voluntary isometric contraction, muscle thickness, blood pressure, heart rate, rating of perceived exertion, and pain were all recorded before and after each condition. The NMES 80% BFR caused greater maximal voluntary isometric contraction decline than any other condition (-38.9 [22.3]N·m, P < .01). Vastus medialis and vastus lateralis muscle thickness acutely increased after all experimental conditions (P < .05). Pain and ratings of perceived exertion were higher after NMES 80% BFR compared with all other experimental conditions (P < .05). No cardiovascular effects were observed between conditions. The NMES combined with 80% BFR caused greater acute force decrement than the other conditions. However, greater perceptual ratings of pain and ratings of perceived exertion were observed with NMES 80% BFR. These acute observations must be investigated during chronic interventions to corroborate any relationship to changes in muscle strength and size in clinical populations.

Intraoperative Hemidiaphragm Stimulation Offsets Slow‐Twitch Muscle Fiber Atrophy and Contractile Dysfunction

Prolonged mechanical ventilation induces considerable atrophy of slow‐ and fast‐twitch fibers in the human diaphragm. In pilot work, we detected early atrophy and specific force decrement in fast‐twitch fibers of patients following an average 4.6‐hour thoracic operations. We now present interim data from a larger controlled trial of diaphragm stimulation as a means to reduce slow‐ and fast‐twitch‐ fiber atrophy and metabolic derangements that occur with mechanical ventilation. We hypothesized that intraoperative hemidiaphragm would mitigate both slow‐ and fast‐twitch fiber contractile dysfunction in the human diaphragm. Twelve adults (6F, age 62 ±12 years) consented to participate in this IRB‐approved study. During cardiothoracic surgical procedures, one hemidiaphragm underwent 1 minute of electrical phrenic nerve stimulation every 30 minutes (30 twitches/minute, 1.5 milliseconds pulse duration, amplitude variable to produce a strong twitch contraction, up to 25 mA), while the contralateral hemidiaphragm remained inactive. Thirty minutes following the last stimulation bout, biopsies were obtained from both hemidiaphragms. Muscle fibers were buffered, permeabilized, and separated for evaluation of cross‐sectional area and force‐generating properties. A mixed effects model with random intercept for each subject was used to evaluate the fixed effects of stimulation and fiber type on single‐fiber force mechanics, and significance was p&lt;0.05. Subjects underwent an average 7 ±2 hemidiaphragm stimulations at 17 ±2 mA, and biopsies were collected after 4.75 ±1.03 hours of mechanical ventilation. Cross‐sectional area of unstimulated slow‐twitch fibers was significantly smaller than stimulated slow‐twitch fibers (5632 ±349 vs 4619 ±247 μM2), with no effect on fast‐twitch fibers (5631 ±350 vs 5768 ±351 μM2), [p&lt;0.005]. Likewise, stimulation resulted in significantly higher specific force of slow‐twitch (106 ±6 vs 97 ±6 kN/m2), but not fast‐twitch diaphragm single fibers (126 ±6 vs 130 ±6 kN/m2), [p&lt;0.001]. Stimulation did not appreciably alter fiber calcium‐sensitivity properties of either fiber type. Our data suggest that stimulation attenuates diaphragm slow‐twitch fiber atrophy and contractile dysfunction induced by mechanical ventilation. Fiber type‐specific changes with short periods of mechanical ventilation may be attributed to predominant slow fiber recruitment during tidal breathing.Support or Funding InformationThis work was supported by National Institute of Arthritis and Musculoskeletal and Skin Diseases, award R01 AR072328

Comparison of Two Low-Profile Prosthetic Retention System Interfaces: Preliminary Data of an In Vitro Study

In recent years, a major research goal of companies has been to create mechanical components suitable for rehabilitation that are safer and more reliable. Evaluating their biomechanical features could be a way to improve them. The purpose of this study was to evaluate the different biomechanical features of low-profile retentive systems (Rhein®). Two different attachment systems were tested: OT Equator® Smart Box and Locator® R-TX. Once a machine was created for the simulation of the connection and disconnection of the attacks in a combined manner, it was possible to evaluate these parameters over time. Attachments were mounted in two different configurations of the divergence angle: 10° and 50°. The drop retention force proved to be stable over time. The Locator® R-TX attachment experienced a more rapid decrement of the retention force than the OT Equator® Smart Box. Both tested systems experienced a high drop in retention; this drop tended to stabilize after 1.5 years of use, and it was correlated with the divergence angle. The OT Equator® Smart Box system underwent this loss of retention more gradually than the Locator® R-TX. This study demonstrates preliminary results from a bioengineering and biomechanical point of view, providing useful information for the continuous improvement of these devices and, therefore, for the quality of patients’ oral health.

Inorganic phosphate, protons and diprotonated phosphate may contribute to the exacerbated muscle fatigue in older adults

Inorganic phosphate, protons and diprotonated phosphate may contribute to the exacerbated muscle fatigue in older adults

Effects of electrochemical corrosion characteristis on electrochemical cold drawing of Q235 steel bar

In this work, the electrochemical corrosion behaviors of Q235 steel bar in tap water, NaCl, NaCl+HCl and H2SO4 aqueous solutions, and their effects on electrochemical cold drawing (ECD) were investigated, as well as on cold drawing in air for comparison. The results indicate that the higher the corrosion rate and the more uniform the corrosion are, the larger the decrement of drawing force of ECD is, and the drawing force in the H2SO4 solution is decreased by 39% compared with DIA. Previous electrochemical corrosions in these solutions can slightly soften the steel surface layer, but further obviously decrease the drawing force of ECD, and these two phenomena are all proportional to the corrosion rate. The surface softening of the previous electrochemical corrosions is verified by positron annihilation technology to be mainly attributed to the formation of vacancies. The further decrease of the drawing force should be chiefly contributed to the relaxation effect of the already formed vacancies on dislocations. But the effects of the electrochemical corrosions occurred during ECD are very limited because the time for corrosions is quite short. So the decrease in drawing force might be ascribed to the additional dislocation flux originated from the simultaneous effects of current, chemical corrosion and stress during ECD.

Blood Flow Restriction Alters Motor Unit Behavior During Resistance Exercise.

We aimed to determine whether blood flow restriction (BFR) alters the characteristics of individual motor units during low-intensity (LI) exercise. Eight men (26.0±3.8 yrs) performed 5 sets of 15 knee extensions at 20% of one-repetition maximum (with and without BFR). Maximal isometric voluntary contractions (MVC) were performed before and after exercise to quantify force decrement. Submaximal isometric voluntary contractions were additionally performed for 18 s, matching trapezoidal target-force trajectories at 40% pre-MVC. EMG activity was recorded from the vastus lateralis muscle. Then, signals were decomposed to extract motor unit recruitment threshold, firing rates and action potential amplitudes (MUAP). Force decrement was only seen after LI BFR exercise (-20.5%; p<0.05). LI BFR exercise also induced greater decrements in the linear slope coefficient of the regression lines between motor unit recruitment threshold and firing rate (BFR: -165.1±120.4 vs. non-BFR: -44.4±33.1%, p<0.05). Finally, there was a notable shift towards higher values of firing rate and MUAP amplitude post-LI BFR exercise. Taken together, our data indicate that LI BFR exercise increases the activity of motor units with higher MUAP amplitude. They also indicate that motor units with similar MUAP amplitudes become activated at higher firing rates post-LI BFR exercise.

Dynamic bimanual force control in chronic stroke: contribution of non-paretic and paretic hands.

Dynamic force modulation is critical for performing skilled bimanual tasks. Unilateral motor impairments after stroke contribute to asymmetric hand function. Here, we investigate the impact of stroke on dynamic bimanual force control and compare the contribution of each hand to a bimanual task. Thirteen chronic stroke and thirteen healthy control participants performed bimanual, isometric finger flexion during visually guided, force tracking of a trapezoidal trajectory with force increment and decrement phases. We quantified the accuracy and variability of total force from both hands. Individual hand contribution was quantified with the proportion of force contributed to total force and force variability of each hand. The total force output was 53.10% less accurate and 56% more variable in the stroke compared with the control group. The variability of total force was 91.10% greater in force decrement than increment phase. In stroke group, the proportion of force and force variability contributed by each hand differed across the two phases. During force decrement, the proportion of force contributed by the non-paretic hand reduced and force variability of the non-paretic hand increased, compared with the increment phase. The control group showed no differences in each hand's contribution across the two force phases. In conclusion, dynamic bimanual force modulation is impaired after stroke, with greater deficits in force decrement than force increment. The non-paretic and paretic hands adapt differentially to dynamic bimanual task constraints. During force decrement, the non-paretic hand preferentially assumes force modulation, while the paretic hand produces steady force to meet the force requirements.

Bimanual force control differs between increment and decrement

BackgroundEveryday bimanual tasks require increasing and decreasing forces to manipulate objects. Optimal bimanual coordination during force increment and decrement is essential to complete a bimanual task. However, little is known about the differences in bimanual control during force increment and decrement. The purpose of this study was to 1) investigate whether task performance and bimanual coordination differ between force increment and force decrement in a bimanual task, and 2) determine the contribution of bimanual coordination to task performance during force increment and force decrement. MethodsSeventeen right-handed young adults (24.10 ± 3.09 years) performed following tasks involving bimanual isometric index finger flexion: 1) maximum voluntary contractions and 2) visually-guided force tracking involving gradual force increment and decrement. The force tracking task involved controlled force increment and decrement while tracking a trapezoid trajectory. The task goal was to match the target force with the total force, i.e., sum of forces produced by both hands as accurately as possible. We quantified bimanual task performance with the accuracy and variability of total force in force increment and decrement phases. We measured bimanual coordination between two hand forces by computing time-series cross-correlation coefficient and amplitude of coherence in 0–1 Hz. ResultsWe found decreased accuracy and increased variability of the total force in decrement compared with increment phase. Further, the cross-correlation coefficient and coherence amplitude were greater during force decrement than force increment phase. Finally, cross-correlation coefficient and coherence in 0.5–1 Hz predicted the accuracy and variability of total force. ConclusionsWe provide evidence that task performance is reduced during force decrement as compared with force increment, suggesting that force release is more challenging than force generation in bimanual tasks. Further, bimanual coupling of the forces was better during force decrement than force increment. Overall, the coordination of forces from both hands influences the task performance across combined increment and decrement phases. Specifically, decoupling of forces produced by both hands facilitates error compensation strategy to reach the task goal. Together, these findings highlight that the bimanual control of forces is task-dependent and emphasize the importance of collaboration between hands in achieving a common task goal. These results may have implications for understanding changes in bimanual control with aging and neurological disorders.

Effects of Small-Sided Game Variation on Changes in Hamstring Strength.

Madison, G, Patterson, SD, Read, P, Howe, L, and Waldron, M. Effects of small-sided game variation on changes in hamstring strength. J Strength Cond Res 33(3): 839-845, 2019-Small-sided games (SSGs) are commonly used by soccer practitioners to condition players. This form of exercise can result in fatigue, potentially exposing the muscle to injury risk. The purpose of this study was to determine the effects of SSG variations on hamstring torque in semiprofessional soccer players. In a counterbalanced cross-over design, 10 male semiprofessional soccer players took part in both small relative area (3 vs. 3; 300 m) and large relative area (4 vs. 4; 1000 m) SSGs. The games comprised 6 × 4-minute bouts, with 90-second recovery. Both movement and heart rate responses were monitored by global positioning systems (GPS) and hamstring isometric torque was measured pre-training and post-training using a NordBord. There were differences (p < 0.05) between the small and large relative area games for peak hamstring force decrement (5.78 N and -13.62 N, respectively) and mean hamstring force decrement at 90° (11.11 N and -4.78 N, respectively). The number of accelerations was related to (r = 0.46, p = 0.039) reduced hamstring peak torque at 90°. In conclusion, larger relative area SSGs elicited the greatest internal and external loads, resulting in decrements in hamstring force. The number of accelerations performed in the session increases the likelihood of hamstring fatigue and can be controlled with the relative pitch area.

Recovery from acidosis is a robust trigger for loss of force in murine hypokalemic periodic paralysis.

Periodic paralysis is an ion channelopathy of skeletal muscle in which recurrent episodes of weakness or paralysis are caused by sustained depolarization of the resting potential and thus reduction of fiber excitability. Episodes are often triggered by environmental stresses, such as changes in extracellular K+, cooling, or exercise. Rest after vigorous exercise is the most common trigger for weakness in periodic paralysis, but the mechanism is unknown. Here, we use knock-in mutant mouse models of hypokalemic periodic paralysis (HypoKPP; NaV1.4-R669H or CaV1.1-R528H) and hyperkalemic periodic paralysis (HyperKPP; NaV1.4-M1592V) to investigate whether the coupling between pH and susceptibility to loss of muscle force is a possible contributor to exercise-induced weakness. In both mouse models, acidosis (pH 6.7 in 25% CO2) is mildly protective, but a return to pH 7.4 (5% CO2) unexpectedly elicits a robust loss of force in HypoKPP but not HyperKPP muscle. Prolonged exposure to low pH (tens of minutes) is required to cause susceptibility to post-acidosis loss of force, and the force decrement can be prevented by maneuvers that impede Cl- entry. Based on these data, we propose a mechanism for post-acidosis loss of force wherein the reduced Cl- conductance in acidosis leads to a slow accumulation of myoplasmic Cl- A rapid recovery of both pH and Cl- conductance, in the context of increased [Cl]in/[Cl]out, favors the anomalously depolarized state of the bistable resting potential in HypoKPP muscle, which reduces fiber excitability. This mechanism is consistent with the delayed onset of exercise-induced weakness that occurs with rest after vigorous activity.

Design and simulation of innovative foam-filled Lattice Composite Bumper System for bridge protection in ship collisions

In this paper, an innovative Foam-filled Lattice Composite Bumper System (FLCBS) with fiber-reinforced polymer (FRP) skins and a foam-web core is proposed as protective structures for bridge piers against ship collision. FLCBS consists of FRP face sheets, FRP lattice webs and polyurethane (PU) foam cores, which can be manufactured at the same time by vacuum assisted resin infusion process (VARIP). FLCBS exhibits good energy-absorbing and highly designable properties. The design, performance evaluation, manufacture and installation of FLCBS for a real prestressed concrete continuous girder bridge were introduced. A detailed numerical model for performance evaluation was developed with the explicit finite element (FE) software package ANSYS/LS-DYNA. An experimental study of two foam-filled lattice composite panels under low-velocity impact was conducted to verify the numerical model for FLCBS. The corresponding FE models of the composite panels generally showed good agreement, compared to the impact load history and mid-span deflection history of experimental results. Based on the validated numerical model, both head-on collision cases and lateral collision cases with and without FLCBS were investigated. Simulation of different impact angles, velocities and locations were carried out. Numerical results indicate the obvious advantages of FLCBS by comparing peak impact force and impact duration. Significantly decrement of the peak impact force and effectively prolonged impact process indicate the superior performance of FLCBS. The modular fabrication and replacement, easy and time-efficient installation and highly designable structure make FLCBS very attractive as bridge protection in ship collisions.

High-threshold motor unit firing reflects force recovery following a bout of damaging eccentric exercise.

Exercise-induced muscle damage (EIMD) is associated with impaired muscle function and reduced neuromuscular recruitment. However, motor unit firing behaviour throughout the recovery period is unclear. EIMD impairment of maximal voluntary force (MVC) will, in part, be caused by reduced high-threshold motor unit firing, which will subsequently increase to recover MVC. Fourteen healthy active males completed a bout of eccentric exercise on the knee extensors, with measurements of MVC, rate of torque development and surface electromyography performed pre-exercise and 2, 3, 7 and 14 days post-exercise, on both damaged and control limb. EIMD was associated with decreased MVC (235.2 ± 49.3 Nm vs. 161.3 ± 52.5 Nm; p <0.001) and rate of torque development (495.7 ± 136.9 Nm.s-1 vs. 163.4 ± 163.7 Nm.s-1; p <0.001) 48h post-exercise. Mean motor unit firing rate was reduced (16.4 ± 2.2 Hz vs. 12.6 ± 1.7 Hz; p <0.01) in high-threshold motor units only, 48h post-exercise, and common drive was elevated (0.36 ± 0.027 vs. 0.56 ± 0.032; p< 0.001) 48h post-exercise. The firing rate of high-threshold motor units was reduced in parallel with impaired muscle function, whilst early recruited motor units remained unaltered. Common drive of motor units increased in offset to the firing rate impairment. These alterations correlated with the recovery of force decrement, but not of pain elevation. This study provides fresh insight into the central mechanisms associated with EIMD recovery, relative to muscle function. These findings may in turn lead to development of novel management and preventative procedures.

The Effect of Repetitive Rugby Scrummaging on Force Output and Muscle Activity

During rugby scrummaging, front row forwards encounter high levels of force that has been suggested to cause transient fatigue and is likely to reduce subsequent performance. However, little is known about the effect of repetitive scrummaging on force output and onset of fatigue. Twelve male front row forwards (21.5±2.3 yr; height 185.7±4.4 cm; body mass 108.5±7.1 kg) each performed three sets of five maximal-effort isometric scrums for 10 s, with 40 s rest separating each repetition; 2 min recovery was provided between each set. Force output and electromyography (EMG) of the right medial gastrocnemius (MG), biceps femoris (BF), gluteus maximus (GM), erector spinae (ES), rectus abdominis (RA), external oblique (EO), internal oblique (IO), and rectus femoris (RF) were assessed. There was no significant force decrement from performing 15 scrums and no fatigue was detected from EMG median frequency and mean amplitude. For training and practice purposes, coaches and trainers can be confident that 15 individual repetitive static scrums against a machine are unlikely to cause a reduction in force production and promote fatigue. However, the effect of rugby-related activities in conjunction with scrummaging requires further research to determine if transient fatigue is causal to scrummaging for subsequent performance.