Eccentric Force Research Articles

Dynamic modeling and response analysis of rub-impact rotor system with squeeze film damper under maneuvering load

With the development of the aero-engine rotor system towards accurate modeling, it is necessary to establish a general rotor system model considering many important strong nonlinear factors. In this paper, based on the finite element method, a 6-node rub-impact rotor system model including the newly-built blade-casing rub-impact force, climbing maneuvering load, nonlinear Hertz contact force of rolling bearing, rotor eccentric unbalance force, gravity field and squeeze film damper oil film force is established. The dynamic responses of the system under different rub-impact stiffness, different oil film clearance and different bearing clearance are analyzed in detail by means of bifurcation diagram, three-dimensional spectrum diagram and axis trajectory diagram. Through comparative analysis, it is found that the change of the oil film clearance has the greatest influence on the rub-impact response of the system, followed by the bearing clearance, and the change of the rub-impact stiffness has the least influence on the rub-impact response of the system. The study of this paper can provide a theoretical basis for the rub-impact response of the rub-impact rotor system with squeeze film damper under climbing maneuvering flight.

Study on neutral axis location of common bolts of flush end-plate connections

The external load eccentricity and the end-plate thickness were employed as the research parameters, and experimental studies on the tensile properties of the bolts with flush end-plate in the beam-column joints was conducted under the action of eccentric pulling force (group A) or pure bending moment (group B). Through the analysis of failure mode, the force distribution of bolts and the offset ratio of the neutral axis, the variations of neutral axis location for common bolts with flush end-plate connections were investigated. Based on the load-displacement curves and the joints tensile performance, the finite element (FE) analysis was used for the two sets of tests. A good agreement exists between the numerical results and the experimental results. The neutral axis of bolts continuously moved continuously to the compression zone as the load increased. Increasing the bolts eccentric distance could significantly increase the bolt axial force increment and accelerate the offset of neutral axial to compression zone. Increasing the thickness of end-plate could reduce the bolt axial force increment and delay the offset of neutral axial to compression zone. The simplified elastic calculation method is more conservative in predicting the stress state of common bolted end-plate connections Under the action of eccentric pulling force or pure bending moment. The modified formulae for the bolt force for flush end-plate connections are presented, and the modified results agree well with the experimental results. Under the premise of ensuring the safety of the structure, the modified formulae provide a more accurate description of the stress state of the common bolted end-plate connections, while requiring less material and reducing the joint cost. It is expected that this study will serve as a reference for the structural design and construction of common bolted end-plate connections.

Dynamic Characteristic Analysis of Permanent Magnet Brushless DC Motor System with Rolling Rotor

With the wide application of permanent magnet brushless DC motors (BLDCMs) in home appliances and electric vehicles, there is increasing demand for BLDCMs with low vibration and noise. This paper aims to study the dynamic characteristics of a type of BLDCM with a rolling rotor. Firstly, a dynamic model of a BLDCM with eighteen degrees of freedom (18 DOFs) is built, for which the electromagnetic force and the oil-film force of the sliding bearing are considered. Then, the system responses are solved by Runge–Kutta numerical method, and the effects of the rotational speed, bearing backlash and eccentric distance of the rolling rotor on the dynamic response are analyzed in detail. The time history, frequency plot, axis trajectory diagram and phase portrait are introduced to discuss the dynamic behavior of the motor system. Analysis results show that eccentric force increases obviously with increasing rotational speed or eccentric distance, which can change the dynamic response through suppressing the electromagnetic force. The effect of bearing clearance on the rotor and stator is negatively correlated. Therefore, system parameters should be determined properly to improve the running performance of the motor system. Numerical results can provide a useful guide for the design and vibration control of such motor systems.

Anatomic total shoulder glenoid component inclination affects glenohumeral kinetics during abduction: a cadaveric study.

Although typically favorable in outcome, anatomic total shoulder arthroplasty (aTSA) can require long-term revision. The most common cause for revision is glenoid loosening, which may result from eccentric cyclic forces and joint translations. "Rocking" of the glenoid component may be exacerbated by the joint geometry, such as glenoid inclination and version. Restoration of premorbid glenoid inclination may be preferable, although laboratory and computational models indicate that both superior inclination and inferior inclination have benefits. This discrepancy may arise because previous studies were limited by a lack of physiological conditions to test inclination. Therefore, a cadaveric shoulder simulator with 3-dimensional human motion was used to study joint contact and muscle forces with isolated changes in glenoid inclination. Eight human cadaveric shoulders were tested before and after aTSA. Scapular-plane abduction kinematics from human subjects were used to drive a cadaveric shoulder simulator with 3-dimensional scapulothoracic and glenohumeral motion. Glenoid inclination was varied from -10° to +20°, whereas compressive, superior-inferior shear, and anterior-posterior shear forces were collected with a 6-df load cell during motion. Outputs also included muscle forces of the deltoid and rotator cuff. Data were evaluated with statistical parametric mapping repeated-measures analysis of variance and t tests. Inferior glenoid inclination (-10°) reduced both compressive and superior-inferior shear forces vs. neutral 0° inclination by up to 40%, and even more when compared with superior inclination (P<.001). Superior inclinations (+10° and +20°) tended to increase deltoid and rotator cuff forces vs. neutral 0° inclination or inferior inclination, on the order of 20%-40% (P≤.045). All force metrics except anterior-posterior shear were lowest for inferior inclination. Most aTSA muscle forces for neutral 0° inclination were not significantly different from native shoulders and decreased 45% and 15% in the posterior deltoid and supraspinatus, respectively (P≤.003). Joint translations were similar to prior reports in aTSA patients and did not differ between any inclinations or compared with native shoulders. Joint reaction forces were similar to those observed in human subjects with instrumented aTSA implants, providing confidence in the relative magnitude of our results. Inferior inclination reduces overall forces in the shoulder. Superior inclinations increase the muscle effort required for the shoulder to achieve similar motion, thus increasing the forces exerted on the glenoid component. These results suggest that a preference toward aTSA glenoid components in inferior inclination may reduce the likelihood of glenoid loosening by reducing excessive muscle and joint contact forces.

Normative Values For Vertical Jump Kinematic Sequencing By Sex And Collegiate Sport

Establishing normative data specific to sex and sport participation can improve performance appraisal, and may aid in identification of limitations. PURPOSE: To generate kinematic sequencing norms specific to each sex in various sports. METHODS: We tested 543 D1 athletes representing 15 sports using SpartaTrac technology. Subjects executed 6 vertical jumps on a force plate to generate a “Movement Signature” consisting of Load (eccentric force development during the downward phase), Explode (force output during the transitional phase), and Drive (magnitude and duration of concentric force during the upward phase). Athletes were stratified by sex and sport, and normative values were calculated. Multivariate tests estimated differences in these values between groups. RESULTS: Across the total sample, Load was 49.8 ± 10.2, Explode was 47.0 ± 9.9, and Drive was 55.5 ± 10.2; there was wide variance between different men’s sports (p<0.001) and women’s sports (p<0.001) sports. Among men, Load was highest in baseball (53.3 ± 10.3), basketball (52.6 ± 8.9), and soccer (52.5 ± 10.0); it was lowest in tennis (49.0 ± 10.7), water polo (49.1 ± 6.4), and swimming (50.4 ± 9.7) Explode was highest in basketball (53.2 ± 10.2), baseball (52.4 ± 8.6), and soccer (50.8 ± 8.3); it was lowest in water polo (44.0 ± 6.9), tennis (44.9 ± 5.9), and swimming (46.5 ± 8.5). Drive was highest in swimming (60.0 ± 9.9), water polo (54.6 ± 9.5), and baseball (54.0 ± 9.2); it was lowest in tennis (51.4 ± 14.0), basketball (52.8 ± 9.1), and soccer (53.2 ± 7.9). Among women, Load was highest in basketball (55.4 ± 17.1), volleyball (51.0 ± 8.7), and field hockey (48.0 ± 9.0); it was lowest in cross country (40.9 ± 5.8), soccer (45.9 ± 7.1), and water polo (46.1 ± 6.6). Explode was highest in basketball (53.0 ± 12.3), volleyball (48.1 ± 6.6), and field hockey (47.3 ± 8.8); it was lowest in water polo (34.9 ± 5.4), cross country (41.6 ± 8.8), and swimming (41.9 ± 6.6). Drive was highest in water polo (62.9 ± 10.4), volleyball (59.6 ± 7.4), and swimming (57.9 ± 9.9); it was lowest in basketball (51.3 ± 11.2), field hockey (53.0 ± 7.8), and soccer (54.6 ± 8.8). CONCLUSIONS: Ground reaction forces vary between sport populations. Normative values may aid in the customization of training programs for athletes whose signatures differ markedly from expected performances.

Effects of High and Low Training Volume with the Nordic Hamstring Exercise on Hamstring Strength, Jump Height, and Sprint Performance in Female Football Players: A Randomised Trial.

The evidence-based hamstring strengthening programme for prevention of hamstring injuries is not adopted by football teams because of its high training volume. This study on female football players investigated if high-volume training with the Nordic hamstring exercise is more effective on hamstring strength, jump height, and sprint performance than low-volume training. We also examined the time course of changes in muscle strength during the intervention period. Forty-five female football players were randomised to a high- (21 sessions, 538 total reps) or low-volume group (10 sessions, 144 total reps) and performed an 8-week training intervention with the Nordic hamstring exercise during the preseason. We tested hamstring strength (maximal eccentric force with NordBord and maximal eccentric torque with isokinetic dynamometer), jump height, and 40 m sprint before and after the intervention. The NordBord test was also performed during training weeks 4 and 6. Both groups increased maximal eccentric force (high-volume: 29 N (10%), 95% CI: 19-38 N, p < 0.001, low-volume: 37 N (13%), 95% CI: 18-55 N, p = 0.001), but there were no between-group differences (p = 0.38). Maximal eccentric torque, jump height, and sprint performance did not change. Maximal eccentric force increased from the pretest to week 6 (20 N (7%), 95% CI: 8 to 31 N, p < 0.001), but not week 4 (8 N (3%), 95% CI: -2 to 18 N, p = 0.22). High training volume with the Nordic hamstrings exercise did not lead to greater adaptations in strength, jump height, or speed than a low-volume programme. Players in both groups had to train for at least 6 weeks to improve maximal eccentric force significantly.

Stable vs. variable eccentric load. Do they induce different training and physical performance outcomes?

ABSTRACT Since most movements on the field require athletes to produce forces in variable and unpredictable contexts, the use of training programs based on identical repetitions of an exercise may not be optimal for movement transference. Therefore, the aim of this study was to analyse the effects of unexpected eccentric load variability during resistance training in team sport players. Sixty-three men were randomly allocated to two experimental groups (Variable (VTG) and stable (STG) training group) and control group: (CG) volunteered to participate in this study. Experimental groups trained with the same average load of half-squat exercise twice a week for six weeks using rotary inertial devices (RIDs) with (VTG) and without (STG) an unexpected variability of the load. The squat force was measured for every session with force plates. Counter-movement jump (CMJ), sprint, and change of direction performances were measured pre and post-test. CMJ performance improved for VTG (p = .014; ES = 0.7) and STG (p = .005; ES = 0.79) but not for CG. Exposure to high eccentric forces with RIDs lead athletes to improve physical performance in the trained force vector but, since RIDs induce in high levels of variability per se, increasing the level of variability of the exercise will not add benefits to physical performance and training outcomes. HIGHLIGHTS The relevance of the study is to analyse if increasing the variability of the load will improve the athlete output to different physical performance tests. The exposure to eccentric overload with RIDs during the squat exercise allows to greater CMJ height improvements than those seen in the literature with RIDs without overloading the eccentric contraction. RIDs induce in high level of variability per se. Therefore, increasing the level of variability of the exercise will not add benefits to physical performance and training outcomes.

The Influence of Weekly Sprint Volume and Maximal Velocity Exposures on Eccentric Hamstring Strength in Professional Football Players.

Background: Hamstring strains are the most common moderate-major severity injuries in football. The majority of hamstring injuries occur during sprinting, with low eccentric hamstring strength being associated with an elevated risk. Objective: To examine the relationship between sprinting and eccentric hamstring strength by monitoring total weekly sprint distance and weekly efforts > 90% and >95% of maximum velocity. Methods: Fifty-eight professional male footballers were observed over one-and-a-half seasons. Players’ running was monitored during training and matches using GPS, and eccentric hamstring strength was measured weekly. Results: Weekly sprint distance (ρ = −0.13, p < 0.01) and weekly efforts >90% of maximum velocity (ρ = −0.08, p = 0.01) both displayed significant inverse relationships with the percentage change in eccentric hamstring strength; weekly efforts >95% of maximum velocity showed no relationship with hamstring strength (ρ = −0.02, p = 0.45). Only weekly efforts >90% of maximum velocity significantly influenced the mean percentage change in eccentric hamstring force, F(3,58) = 3.71, p = 0.01, with significant differences occurring when comparing 7–8 sprint efforts with 0–2 efforts (0.11%, p = 0.03) and 5–6 efforts (0.12%, p = 0.03). Conclusions: Eccentric hamstring strength levels significantly decrease when 7–8 weekly sprint efforts are completed at >90% of maximum velocity. Monitoring weekly sprint loading at velocities > 90% of maximum velocity may be valuable to help to reduce the risk of hamstring injuries in professional football.

Quantitative Evaluation Index for Analysis of Assembly Effect on Shield Tunnel Segment Structures

The local force and deformation of shield tunnel will be transferred and redistributed owing to the discontinuous segmental structure. This study develops quantitative evaluation index to explore the influence of the axial compression ratio, eccentricity, and longitudinal force on the assembly effect. A full-scale test is conducted on a single segment and staggered assembled segmental linings. First, the results show that the assembly effect has an obvious redistribution influence on the force and deformation of the segment. Second, the influence of eccentricity on the bending moment is much larger than that of the axial force. The stiffness between rings varied at different positions and the larger the stiffness difference, the more obvious the assembly effect. The results demonstrate that the design and mechanical performance of the shield tunnel under different embedded conditions can be optimized by increasing the longitudinal force, which moderates the assembly effect and ensures the structure reaches the highest bearing capacity.

Eccentric Force Velocity Profiling: Motor Control Strategy Considerations and Relationships to Strength and Jump Performance.

Barker, L, Siedlik, J, Magrini, M, Uesato, S, Wang, H, Sjovold, A, Ewing, G, and Harry, JR. . Eccentric force velocity profiling: motor control strategy considerations and relationships to strength and jump performance. J Strength Cond Res 37(3): 574-580, 2023-Currently, no studies exist on the eccentric force-velocity profile (eFVP) during drop landings from increasing drop heights, which may reveal an athlete's braking capacity and control strategies. Therefore, the purpose of this study was to assess the eFVP during bilateral drop landings from increasing drop heights. A secondary purpose was to explore and determine relevant relationships between the eFVP and common metrics like relative strength and jumping performance. Overall, 19 recreationally trained athletes from the university completed a 1-reptition maximum back squat, countermovement jumps, squat jumps, drop jumps, and drop landings from 0.3 to 1.52-m box heights in 0.15-m increments. Average force and velocity from the peak drop landing trial was used to generate an eFVP. The mean linear eFVP was -6.65x + 14.73, and the mean second order polynomial eFVP was -1.37x 2 - 25.84x + 0.17. The second-order polynomial fit the data better with large effect ( dunb = 1.05, p < 0.05). No significant correlations between the eFVP coefficients and the strength and jumping measurements were observed. Future research could investigate how training can influence the eFVP. Eccentric force production during landing may be a unique quality that requires specific development strategies, such has fast or slow eccentric training.

Origin of the lateral return error in a five-axis ultraprecision machine tool and its influence on ball-end milling surface roughness

Precision ball-end milling is widely used in the manufacture of complex microstructures. The purpose of this paper is to explore the origin of the lateral return error in an ultraprecision five-axis machine tool and its influence on a precision ball-end milling surface. First, the origin of the lateral return error of the linear axis is analyzed. Theoretical analysis and experimental results show that the root of the lateral return error is the gap variation between the slider and the guide, which is decided by the eccentric force moment acting on the hydrostatic guideway. The eccentric force moment relies on the external force and motor eccentricity. In general, the larger the eccentric force moment, the more the gap variation, and resultantly the greater the lateral return error. In addition, the lateral return error is inversely proportional to the oil supply pressure. Because the cutting forces are small enough, the cutting parameters, such as feed speed, cutting depth and cutting stepover, have little effect on the error. Secondly, the lateral return error has an important impact on the yield surface roughness, which will significantly increase the surface roughness and reduce the surface quality. Finally, the lateral return error of the hydrostatic guideway can be suppressed by compensating the machining program. The experimental results show that the surface quality of the compensated surface is significantly improved.

How velocity impacts eccentric force generation of fully activated skinned skeletal muscle fibers in long stretches

Ecccentric muscle contractions are fundamental to everyday life. They occur markedly in jumping, running, and accidents. Following an initial force rise, stretching of a fully activated muscle can result in a phase of decreasing force ("Give") followed by force redevelopment. However, how the stretch velocity affects "Give" and force redevelopment remains largely unknown. We investigated the force produced by fully activated single-skinned fibers of rat extensor digitorum longus muscles during long stretches. Fibers were pulled from length 0.85 to 1.3 optimal fiber length at a rate of 1%, 10%, and 100% of the estimated maximum shortening velocity. "Give" was absent in slow stretches. Medium and fast stretches yielded a clear "Give." After the initial force peak, forces decreased by 11.2% and 27.8% relative to the initial peak force before rising again. During the last half of the stretch (from 1.07 to 1.3 optimal fiber length, which is within the range of the expected descending limb of the force-length relationship), the linear force slope tripled from slow to medium stretch and increased further by 60% from medium to fast stretch. These results are compatible with forcible cross-bridge detachment and redevelopment of a cross-bridge distribution, and a viscoelastic titin contribution to fiber force. Accounting for these results can improve muscle models and predictions of multibody simulations.NEW & NOTEWORTHY Eccentric muscle contractions are part of our daily lives. We found that force increased monotonically during slow stretches of fully activated muscle fibers, whereas higher stretch velocities resulted in an increasing drop in force after an initial increase and a final steeper rise in force. Cross-bridges cannot explain the observed force traces. This requires a viscoelastic non-cross-bridge contribution. Considering these results can improve muscle models and predictions of multibody simulations.

Deep Insight into the Pinch Effect in a Tundish with Channel‐Type Induction Heater

Tundish with channel‐type induction heater plays an important role in the continuous casting of high‐quality steel. Electromagnetic pinch is a unique phenomenon in the tundish with channel type induction heater. To have a deep insight into the electromagnetic pinch effect, numerical simulation is applied to investigate the magnetohydrodynamic flow in the tundish. Research results show that the magnetic field, flow field, and temperature field predicted by the mathematical model agree well with the experimental data. Induced current density, magnetic flux density, electromagnetic force, and Joule heat in two channels are symmetrically distributed and are greater than that in the receiving and distributing chambers. The eccentric electromagnetic force leads to the single‐/double‐recirculation zones in the channel. There are larger separation areas in the channel enclosed with the iron core than in the other channel, and it is easy for molten steel to separate from the channel wall surface at the top of the channel. Different forces play different roles in the pinch condition, and the order of importance is as follows: electromagnetic force, stress, gravity, supporting force, centrifugal force, and thermal buoyancy.

Associations between lower limb eccentric muscle capability and change of direction speed in basketball and tennis players.

BackgroundThe ability to perform a quick and rapid change of direction (CoD) is an important determinant of success in a variety of sports. Previous studies have already highlighted that eccentric strength is a dominant predictor of CoD. However, these studies evaluated eccentric strength through a limited number of outcome measures and used small sample sizes.MethodsA total of 196 athletes participated in the study. The aim of our study was to investigate: (1) the correlation between eccentric outcome measures derived from different tests (Nordic hamstring exercise (NHE), countermovement jump (CMJ) and flywheel (FW) squats), (2) the association between eccentric outcome measures and CoD 90°, CoD 180°; and (3) proportion of explained variance in CoD performance.ResultsVery large associations (r = 0.783, p < 0.001) were observed between peak torque during NHE (NHEPT) and force impulse during the eccentric phase of CMJ (CMJFI). Small to moderate correlations were calculated between peak eccentric force in flywheel squats and peak eccentric force in CMJ (r = 0.220–035, p < 0002). All eccentric CMJ outcome measures and NHEPT were reported as moderate negative associations with both CoD tests. Eccentric measures explained 25.1% of the variance in CoD 90° (CMJPF, NHEPT, F0.125 –peak eccentric force during FW squats with 0.125 kg m2 load), while the same outcome measures explained 37.4% of the variance for CoD 180°.ConclusionOur results suggest that different measures of eccentric strength specifically contribute to CoD performance. Therefore, for successful CoD performance, different aspects of eccentric strength training should be considered in testing and training (maximal eccentric strength, eccentric-concentric actions with fast execution).

Changes in Propeller Shaft Behavior by Fluctuating Propeller Forces during Ship Turning

It is known that a ship’s shafting system can be adversely affected by hull deformation, variations in the engine power, the propeller load, and eccentric propeller thrusts, thereby increasingly affecting the behavior of the shaft’s movement. A deformed shafting system may also lead to a potential risk of bearing damage by causing a change in the local load of the rear part of the after-stern tube bearing of the propeller shaft. With this concern, a series of previous studies were focused on optimizing the effects of hull deformation by securing a proper level of propulsion shaft stability and optimizing the relative inclination angle and oil film retention based on a quasi-static state, that is, Rules for the Classification of Steel Ships and experiences of shipyards. However, despite our efforts to resolve this issue, marine accidents involving stern tube bearing damage have continued to occur under relatively unattended ship motions in a transient state, that is, a quasi-static state that can possibly cause sudden stern flow field changes. Therefore, to improve the stability of the propulsion shaft, it is necessary to understand ship motions and the conditions of shafting systems in a dynamic state and a transient state when the system is designed. From this point of view, this study investigated the effect of changes in eccentric propeller forces on the motion of the propeller shaft in a representative transient state of a 50,000 deadweight tonnage tanker by means of the strain gauge method and a displacement sensor. The research findings demonstrate that propeller thrust fluctuations have a direct effect on the shaft stability by significant changes in the shaft motion that can lead to unbalanced supporting loads on the stern tube bearing. These results clearly reproduce the cause of damage to the ship in which the accident occurred at a reliable level and will be a reference for establishing pragmatic guidelines for preventing damage to similar ships in the future.

Construction analysis of Guiyang Metro passing across Guiyang Railway Station in karst zone

The Huo-Sha Tunnel of the Guiyang Metro Line 1 in China runs from the Guiyang Railway Station to the Shachong road station, and it passes close to the railway station platform. The minimum distance between the crown and pile foundation is only 2.19 m, and the settlements of existing buildings are strictly controlled, such as pile foundations. Existing comprehensive measures, such as super pipe shed support, surrounding rock reinforcement, inverted arch grouting, and lining strengthening, are stringently adopted to prevent the influence of construction on the operation of station buildings and in-service railways, high-speed railways, and other buildings. In this study, the changing laws related to existing building subsidence, subgrade settlement, station longitudinal displacement variation, and the development trends of lining safety, stability, eccentricity, compression types, and internal force of the lining are analyzed. The following results are presented. The Huo-Sha Tunnel was gradually excavated. The roadbed settlement of the Guiyang Railway Station platform gradually increased, but it did not exceed the existing railway deformation control standards for the subsurface excavation section. The longitudinal distribution of the existing railway settlement is approximately a normal curve after the construction of the secondary lining. The outcome is consistent with the law of the surface settlement trough proposed by Peck, and the range is −50 to 50 m. The stress of the lining is primarily concentrated at the arch waist and crown, the compression types include large and small eccentricities, and the axial force value gradually increases symmetrically from the crown to the arch footing, while the maximum axial force appears at the arch footing, and all control indicators meet the corresponding control standards. The subgrade settlement of the skew section adheres to the deformation control standards of the existing railway of subsurface excavation sections after the stabilization of the secondary lining deformation.

Kinematic and Kinetic Characteristics of Repetitive Countermovement Jumps with Accentuated Eccentric Loading.

Two methods for challenging the musculoskeletal and nervous systems to better exploit the stretch-shortening cycle (SSC) mechanism during plyometric training are reactive strength exercises and accentuated eccentric loading (AEL). Combining repetitive, reactive jumping with AEL poses a novel approach, in which the effects of both methods may be combined to elicit a unique stimulus. This study compared kinematic, kinetic, and electromyographic variables between a control (CON1) and two AEL conditions (AEL2 and AEL3). Additionally, non-reactive and reactive jumps performed within these sets were compared. Participants performed two sets of six countermovement jumps (CMJ) under each loading condition. AEL3 had moderate to large positive effects (es) on peak and mean eccentric force (es = 1.1, 0.8, respectively; both p < 0.01), and eccentric loading rate (es = 0.8, p < 0.01), but no effect on concentric variables or muscle activation intensity. The effects of AEL2 were similar but smaller. With or without AEL, there were moderate to large positive effects associated with reactive CMJ (second jump in a set, compared to the first) on peak and mean eccentric velocity (es = 1.7, 0.8, respectively; both p < 0.01), peak and mean eccentric force (es = 1.3, 1.2, p < 0.01), eccentric loading rate (es = 1.3, p < 0.01) and muscle activity (es = 1.8–1.9, p < 0.01). Concentric variables did not differ. Thus, the flight phase and act of landing during reactive jumps elicited greater increases in eccentric forces, loading rates, and muscle activity than AEL. Nonetheless, kinetic variables were greatest when AEL was combined with reactive jumping. Considering the limitations or complexity associated with most AEL protocols, sets of repetitive (reactive) CMJ may be more pragmatic for augmenting eccentric kinetic variables and neuromuscular stimuli during training.

The Effects of Fatigue on Muscle Synergies in the Shoulders of Baseball Players.

Muscle synergies are defined as the central nervous system's organizational structure for movement. Muscle synergies change after muscular fatigue, with certain synergies assuming the primary role to compensate for fatigue within another muscle synergy. Owing to the high eccentric forces imposed upon the external rotators (ie, posterior rotator cuff), pitchers that continue to throw while fatigued are at a significantly higher risk of shoulder and/or elbow injury; however, the neuromuscular compensation strategies of baseball players in response to fatigue are currently unknown. Players would utilize the same muscle synergy structure following external rotation (ER) fatigue; however, muscle coefficients of nonfatigued muscles would increase (ie, compensate for the external rotators) after fatigue. Cross-sectional study conducted in a controlled, laboratory setting. Nine players from an intercollegiate competitive club baseball team voluntarily participated in this study. Surface electromyography was used on 14 muscles of the glenohumeral and scapulothoracic joints of the dominant arm during a reaching protocol. Players completed a baseline reaching protocol (prefatigue), then an ER fatigue protocol until maximum concentric ER was reduced by 40%, and finally repeated the same reaching protocol (postfatigue). Principal component analysis was used to extract muscle synergies, the variance accounted for (VAF) of each synergy, and muscle coefficients. Prefatigue was compared with postfatigue using paired t tests for all dependent variables. Four muscle synergies were extracted for both pre- and postfatigue. The VAF for the ER/abduction synergy decreased significantly (prefatigue, 34.6%; postfatigue, 32.4%; P = 0.03), showing a decreased reliance on ER/abduction during the reaching task after fatigue. Within synergy 1, the pectoralis major muscle coefficient (-0.489 vs -0.552; P = 0.01; effect size = 1.68) decreased significantly from prefatigue to postfatigue, indicating that the pectoralis major assumed more of an antagonist role during ER/abduction. Within synergy 2 (forward reaching), there were no significant changes in VAF or muscle coefficients observed. For the third synergy, muscle coefficients increased for the serratus anterior (P = 0.02) and middle deltoid (P = 0.01), whereas in the fourth synergy, the pectoralis major (P = 0.01) increased and teres major (P = 0.01) and biceps brachii (P = 0.05) muscle coefficients decreased. The decreased VAF of the ER/abduction synergy after fatigue indicate that other muscles within that synergy could not fully compensate to maintain function. Interestingly, the changes in muscle coefficients suggest that players relied less on the internal rotation (IR) synergy and more on the cross-body synergy following fatigue. This may be due to imbalances between ER and IR while maintaining balance between cross-abduction and adduction. Clinicians may consider implementing low-load, high-repetition training programs to develop posterior shoulder endurance and prolong the onset of muscular fatigue.

Anthropometric, Physiological, and Physical Profile of Elite Snowboarding Athletes

ABSTRACTA successful snowboarding performance is a multifactorial accomplishment resulting from a complex interaction of the major fitness components. This review aimed to examine the anthropometric, physiological, and physical profiles of elite snowboarding athletes. A database search was performed in the China National Knowledge Infrastructure, PubMed, SPORT Discus, and Web of Science. The findings indicate that (a) elite speed-based and skill-based athletes generally had low body fat percentages. The height and body mass of men speed-based athletes are slightly larger than those of skill-based athletes. (b) The energy system predominantly engaged in is anaerobic metabolism. Muscle oxidative metabolism could help to promote recovery in the repeated high-intensity runs. (c) Top levels of lower-body strength, especially the eccentric force production and explosive concentric power production of athletes, have a determinant role in rapid riding and turning, jump amplitude, trick difficulty, and successful landing. The take-off velocity of skill-based snowboarding and the starting and turning speed of speed-based snowboarding also have a major effect on success in competitions. Further research is warranted to investigate the anthropometry, the relative energy system contributions from aerobic and anaerobic metabolism, and the effect of physical training on performance while considering the possible differentiated effects of different snowboarding disciplines.

Comparison of Test Methods for Determining the Tensile Strength of Soil and Weak Rocks

Tensile strength is a key parameter governing tensile cracking and subsequent failure of soil or rock mass. Existing methods for measuring tensile strength are mainly designed for hard materials and come with inherent problems. As such, they are continuously being adapted and improved by the scientific community. In line with this effort, we recently developed two new tensile test methods for application to soil and weak rocks, namely, the inner hole fracturing test (IHFT) and horizontal compression test (HCT). In this study, we compared the performance of these newly developed methods and the three most commonly used approaches for tensile strength determination, namely, the uniaxial direct tensile test (UDTT), Brazilian test (BT) and three-point bending test (TPBT). Results show that IHFT and HCT exhibit distinct advantages over the three conventional methods when testing soil and weak rocks: first, IHFT and HCT can overcome the eccentric force problem that is a main challenge in UDTT and BT, and second, results obtained from these tests are highly reproducible and stable. Between IHFT and HCT, the latter is found more suitable for routine laboratory testing because of simpler and easier procedure, more stable and reliable results and uniform stress distribution within specimens.