Horizontal Impulse Research Articles

Effects of applying a multivariate training program on physical fitness and tactical performance in a team sport taught during physical education classes.

A multivariate training program could be a pedagogical choice to improve physical and tactical performance in a team sport taught during physical education classes at different levels of education. Thus, the aim of this study was to verify the effects of applying a multivariate training program on physical fitness and tactical performance during the teaching of a basketball didactic unit in basic and secondary education. Seventy-five students from a Portuguese school, with an average age of 15.02 ± 1.31 years, included forty-two students from basic school and thirty-three students from secondary school. The FITescola® test battery was used to assess physical fitness (i.e., sit-ups, push-ups, horizontal impulse, shuttle test, 40 m sprint, agility 4 × 10 m). The Game Performance Assessment Instrument (GPAI) was used to assess students' tactical performance for each player's game performance during a 20-minute 3 vs. 3 match. The GPAI variables were decision making index (DMI), skill execution index (SEI), support actions index (SI), and adaptability index (AI). During a basketball didactic unit teaching, the students were randomly divided into two groups, a control group that will not carry out the training program and an experimental group that will carry out a strength training program, high intensity explosive exercises and activities based on small-sided games (SSG) for 6 weeks. The two groups were evaluated in two moments: before the application of the training program and after the application of the training program regarding changes in physical fitness and tactical performance. The independent samples t-test (samples from two groups) and paired sample Test (for the same group) were applied for pre and post-assessment comparisons. All indexes present significant differences between basic and secondary students in the pre- and post-assessment tests with small effects (t = -6.54 to -4.82, Δ = -27.57 to -0.16, p<0.05-p< 0.001, d = 0.78-1.05). The results allow to conclude that in a school environment, a well-structured multivariate training program can effectively improve students' tactical skills, increasing their physical conditioning levels.

Technical Skills Influences on Front Crawl Tumble Turn Performance in Elite Female Swimmers.

The objective of this research was to compare technical skills composed of kinematic and kinetic variables in the complex motor task of a tumble turn between 9 elites and 9 sub-elite female swimmers. The best tumble turn among three attempts was analyzed using a three-dimensional underwater protocol. A total of 37 kinematic variables were derived from a Direct Linear Transformation algorithm for 3D reconstruction, and 16 kinetic variables measured by a piezoelectric 3D force platform. Data were analyzed by Student's t-test and effect size statistics. Pearson correlations were applied to the data of the eighteen swimmers to relate the association of 53 kinematic, kinetic variables to the performance of the tumble-turn (3 meters Round Trip Time, 3m RTT). The approach and the whole turn times were faster for elite swimmers compared to sub elites (1.09±0.06 vs. 1.23±0.08 sec, and 2.89±0.07 vs. 3.15±0.11 sec.), as well as the horizontal speeds of the swimmers' head 1 m before the rotation (1.73±0.13 vs. 1.57±0.13 m/sec.), at the end of the push-off on force platform (2.55±0.15 vs. 2.31±0.22 m/sec.) and 3 m after the wall (2.01±0.19 vs. 1.68±0.12 m/sec.). Large differences (|d| > 0.8) in favor of the elite swimmers were identified for the index of upper body extension at the beginning of the push-off, the lower limb extension index at the end of push-off, and among the kinetic variables, the horizontal impulse and lateralization of the push-off. Correlations for the whole group revealed a moderate to strong relationship between 6 body extension indices and 3mRTT performance. For the kinetic variables, the correlations indicated the fastest swimmers in 3mRTT showed large lateral impulse during placement (r=0.46), maximum horizontal force during the push-off (r=0.45) and lateralization of the push-off (r=0.44) (all p<0.05). Elite female swimmers had higher approach and push-off speeds, were more streamlined through the contact, and showed a higher horizontal impulse and lateralization of the push-off, than their sub-elite counterparts.

The Bearing Capacity of Compressed Corrosion-Damaged Reinforced Concrete Elements under Lateral Pulse Loading

This article addresses the relevant problem of the stress–strain behavior of compressed reinforced concrete columns under lateral pulse loading. A simplified engineering method of analyzing the limit value of lateral pulse loading P, depending on longitudinal force N acting on the column, is developed. The proposed method involves the construction of the P-N curve that has three portions. Portion 1 describes the plastic deformation of concrete and rebars of that part of the structure that is mostly in bending. Portion 2 describes the state of the column that can trigger the brittle failure of the concrete along the normal section, and Portion 3 describes the high compression of the column that predominantly triggers its shear failure. For Portions 1 and 2, analytical relationships are obtained using equilibrium equations. Corrosive damage is taken into account in the analytical model as a reduction in the strength and deformability characteristics of the material. A conventional local corrosion spot can be considered; it can be located both in and outside of the area of action of lateral pulse loading. The results obtained using the proposed model were compared with the results of numerical studies and a full-scale experiment. As a result of testing the developed engineering technique, it was found that it provides a safety margin for corrosion-damaged elements of 0.20–0.8 of the ultimate value of horizontal impulse at operational values of compressive force. The spot corrosion damage considered in the paper leads to a 10–60% strength reduction in compressed columns, depending on their location.

The Acute Effects of Plyometric Exercises on Sprint Performance and Kinematics

Background: Post-activation potentiation refers to the acute and temporary enhancement of performance in explosive movements after performing a conditioning activity, such as plyometrics. The current study aimed to investigate the acute effects of horizontal leg bounding on 30 m acceleration performance, 5 m split times, and sprint kinematics (step frequency and length, flight and contact time). Methods: Fourteen young sprinters, nine females and five males, performed two experimental conditions and one control condition in randomized and crossover orders. The experimental conditions included 3 × 10 repetitions of alternate-leg horizontal bounding or 3 × 5 repetitions of single-leg horizontal bounding for each leg. Active recovery was performed in the control condition. A 30 m sprint test was executed before and 5 min after each condition. Results: Sprint times at 5 m (p = 0.014) and 10 m (p = 0.041) were improved after performing alternate-leg horizontal bounding. Additionally, an increase in running velocity (p = 0.017) and step frequency (p = 0.028) was observed in the 0–5 m segment of the sprint. Sprint performance and kinematics showed no significant differences after performing single-leg horizontal bounding. Conclusions: Alternate-leg horizontal bounding, which is a sprint-specific exercise that emphasizes a horizontal impulse, can be used effectively to improve performance in the initial phase of sprint acceleration.

Stability of Monotone, Nonnegative, and Compactly Supported Vorticities in the Half Cylinder and Infinite Perimeter Growth for Patches

We consider the incompressible Euler equations in the half cylinder $ \mathbb{R}_{>0}\times\mathbb{T}$. In this domain, any vorticity which is independent of $x_2$ defines a stationary solution. We prove that such a stationary solution is nonlinearly stable in a weighted $L^{1}$ norm involving the horizontal impulse, if the vorticity is non-negative and non-increasing in $x_1$. This includes stability of cylindrical patches $\{x_{1}<\alpha\},\; \alpha>0$. The stability result is based on the fact that such a profile is the unique minimizer of the horizontal impulse among all functions with the same distribution function. Based on stability, we prove existence of vortex patches in the half cylinder that exhibit infinite perimeter growth in infinite time.

Biomechanics of World-Class 800 m Women at the 2017 IAAF World Championships.

The 800 m race challenges the aerobic and anaerobic energy systems, and athletes adopt a technique that allows for running efficiency as well as sprinting speeds. The aim of this novel study was to compare important kinematic variables between the two laps of the 2017 IAAF World Championships women's final. Video data (150 Hz) were collected of all eight finalists on both laps at a distance approximately 50 m from the finish line along the home straight. Running speed, step length, cadence, temporal variables, sagittal plane joint angles, and shank angle at initial contact were measured. Running speed was faster on lap 2 (p = 0.008) because of large increases in cadence (p = 0.012). These higher cadences resulted in large decreases in contact times (p = 0.031) and in flight times (p = 0.016) on lap 2. Greater knee flexion and ankle plantarflexion (p ≤ 0.039) at initial contact on lap 2 seemed partly responsible for shorter swing times (p = 0.016), and which accompanied a decrease in shank angle at initial contact from lap 1 (7°) to a more vertical position on lap 2 (4°) (p = 0.008). Coaches should note that the need for higher cadence, horizontal impulse production during shorter contact times, and the adoption of forefoot striking require strength and neural system training to allow for athletes to increase cadence during the sprint finish. Increasing cadence (and not step length) was the driving factor for faster finishing speeds in the women's 800 m.

A new impact model for the flexible rocking oscillator

AbstractMotivated by the rocking motion of non‐rigid structures during earthquakes, this paper investigates the rocking behaviour of laterally flexible oscillators. Previous studies in this area utilised ad hoc assumptions to consider transitions between different motion phases at impact. Some models explored direct transitions between two rocking phases assuming that elastic translational velocities remain the same, while others assumed the dissipation of all vertical momentum after a rocking phase, leading to a full contact phase upon impact. In this paper, an improved impact model is proposed, where consistent mechanical principles are used to determine the phase transition criteria. The model departs from the principle that the spring and damper elements of the oscillator cannot transfer horizontal impulses over infinitesimally small durations. Alongside other mechanical constraints, this principle yields a series of momentum equations, which are solved to determine the post‐impact velocities when transiting from one rocking phase to another. Through these equations, it is shown that a transition to full contact implies a specific locus of the vertical impulse from the support medium. Analytical and numerical investigations are then conducted to comparatively evaluate the new impact model. It is demonstrated that the new impact model yields equivalent results to established rigid rocking impact models for effectively rigid structures and resolves long‐standing issues associated with excessive energy dissipation observed in previous models. Time histories of free rocking motion and stability analyses under pulse excitations are used to illustrate and generalise the findings for a range of geometries.

Agreement Between Caloric and Horizontal Video Head Impulse Testing in School-Aged Children Presenting With Dizziness.

The purpose of this investigation was to determine the relationship between the horizontal semicircular canal video head impulse test (hvHIT) and the caloric test in a sample of school-age children evaluated due to complaints of dizziness or imbalance. In adults, these two tests provide different but complementary information regarding vestibular function and/or pathology. However, the most common causes of vestibular loss in children are different than those in adults, and it is not clear how these two tests of horizontal semicircular canal function are related in a pediatric population. This is a retrospective study of electronic records of pediatric patients (< 18 years old) who were referred for vestibular function testing and completed both hvHIT and caloric testing. Agreement between both test results was evaluated by examining measures of sensitivity, specificity, and calculating Cohen's kappa. Results from 117 patients (mean age = 13.7 years, age range: 8-17 years, 37 males) were analyzed. Most patients (97/115 [84.3%]) demonstrated both normal hvHIT and caloric test results. Compared to caloric testing, the hvHIT had poor sensitivity (38.9%) and excellent specificity (98.0%). Cohen's kappa analysis revealed moderate agreement between the methods (coefficient = 0.49 [0.24, 0.73]). Where disagreement between the two tests occurred, the final diagnosis was not contingent on either test result. There is a moderate agreement between the hvHIT and the caloric test in pediatric patients presenting to a tertiary care medical center for a vestibular evaluation. Most patients in this cohort presented with normal findings. The specificity of hvHIT was excellent, although the sensitivity was low. This may be related to the low prevalence of peripheral vestibular system disease in this study cohort, and these results do not generalize to centers that serve populations of children where vestibular disorders are more prevalent (e.g., children with sensorineural hearing loss).

The Biomechanical Characterization of the Turning Phase during a 180° Change of Direction.

The aim of this study was to characterize the turning phase during a modified 505 test. Forty collegiate basketball students, divided into faster and slower performers and high-playing-level and low-playing-level groups, were evaluated for the force-time characteristics (braking and/or propulsive phase) of the penultimate foot contact (PFC), final foot contact (FFC), and first accelerating foot contact (AFC), and for completion time and approach velocity. Based on the composition of the AFC, trials were classified as braking/propulsive or only propulsive. Regression analysis for the prediction of completion time was performed. The AFC contributed to reacceleration through shorter contact times and step length, and lower braking force production (p < 0.05). Faster performers and the high-playing-level group demonstrated (p < 0.05): lower completion times, higher approach velocities, longer steps length in the PFC and FFC, greater braking forces and impulses in the PFC; greater braking and propulsive forces, braking impulses, lower contact times in the FFC; greater braking and propulsive horizontal forces, horizontal impulses, lower contact times and vertical impulses in the AFC. Kinetic variables from only the FFC and AFC and approach velocity predicted 75% (braking/propulsive trials) and 76.2% (only-propulsive trials) of completion times. The characterization of the turning phase demonstrated the specific contribution of each foot contact and the possible implications for training prescription.

Effects of biofeedback on whole lower limb joint kinematics and external kinetics

ABSTRACT Biofeedback (BFb) is a useful tool to accelerate the skill development process. Limited research has applied BFb to the whole lower-limb in a complex skill therefore the aim of this research was to assess the effectiveness of a biofeedback intervention targeting whole lower limb kinematics. Thirty-two healthy participants were randomized to a BFb (n = 16) and a Control group (n = 16). Participants visited a motion capture laboratory on three occasions during one week, and returned for retention testing at 4–6 weeks. Following introduction to a novel lunge-touch task, visual BFb on lower limb joint kinematic extension angular velocities (ω) and timing were provided following each lunge. BFb was effective in increasing Hipω (F = 3.746, p = 0.03) and Kneeω (F = 10.241, p = 0.01). Peak Ankleω remained unchanged (F = 1.537, p = 0.23, η2 = 0.05), however Peak Ankleθ (F = 10.915, p < 0.001, η2 = 0.27) and AnkleROM (F = 9.543, p < 0.001, η2 = 0.24) significantly increased. Despite kinematic changes, there were no significant changes in any external kinetics. No significant correlations were found between Hipω, Kneeω or Ankleω and horizontal impulse (ImpulseY: r = 0.20, p = 0.26; r = −0.11, p = 0.24; and r = 0.22, p = 0.28, respectively). Findings demonstrate that BFb can be used to alter multiple kinematic variables in a complex skill, but do not necessarily alter associated kinetic variables not directly targeted by BFb.

Changes to horizontal force-velocity and impulse measures during sprint running acceleration with thigh and shank wearable resistance

ABSTRACT This study determined the effects of two wearable resistance (WR) placements (i.e. thigh and shank) on horizontal force-velocity and impulse measures during sprint running acceleration. Eleven male athletes performed 50 m sprints either unloaded or with WR of 2% body mass attached to the thigh or shank. In-ground force platforms were used to measure ground reaction forces and determine dependent variables of interest. The main findings were: 1) increases in sprint times and reductions in maximum velocity were trivial to small when using thigh WR (0.00–1.93%) and small to moderate with shank WR (1.56–3.33%); 2) athletes maintained or significantly increased horizontal force-velocity mechanical variables with WR (effect size = 0.32–1.23), except for theoretical maximal velocity with thigh WR, and peak power, theoretical maximal velocity and maximal ratio of force with shank WR; 3) greater increases to braking and vertical impulses were observed with shank WR (2.72–26.3% compared to unloaded) than with thigh WR (2.17–12.1% compared to unloaded) when considering the entire acceleration phase; and, 4) no clear trends were observed in many of the individual responses. These findings highlight the velocity-specific nature of this resistance training method and provide insight into what mechanical components are overloaded by lower-limb WR.

PROFILE AND CLASSIFICATION OF HANDGRIP AND HORIZONTAL JUMP IN SCHOOLCHILDREN PHYSICAL EDUCATION CLASS PRACTITIONERS AND SPORTS PRACTITIONERS

The aim of the study was to compare and classify the body mass index, handgrip, and horizontal jump of adolescents. The study included 48 individuals, aged 14 to 16 years, subdivided into four groups: a) students practicing physical education classes (GPEF, n=15); b) soccer players (GPF, n=14); c) athletics practitioners (runners, throwers, and jumpers) (GPA, n=11); and d) judo practitioners (GPJ, n=8). We collected data on body mass, stature, body mass index (BMI), handgrip test, and horizontal impulse. For statistical analysis, the Kruskal Wallis ANOVA was applied, followed by Dunn Post Hoc, with a significance level &lt; 0.05. BMI and horizontal jump data were compared and classified using Proesp-Br tables (2012) and handgrip using percentiles from the study of Serrano et al. (2009). The GPA, GPJ, and GPF groups presented longer activity duration when compared to the GPEF. The variables BMI, handgrip, and horizontal jump demonstrated no statistical differences between the groups. Handgrip was rated higher than the 50th percentile for the GPF, GPA, and GPJ groups. The horizontal jump in the GPJ and GPEF groups was classified as weak and reasonable, while the results of the GPF and GPA groups were good and excellent. It was found that the relative frequency, in percentage, of individuals who practice sports, points to better performance in the muscular strength of the upper and lower limbs.

The Kinematic and Kinetic Development of Sprinting and Countermovement Jump Performance in Boys.

BackgroundThe aim of the study was to examine the kinematics and kinetics of sprint running and countermovement jump performance between the ages of 8–9, and 11–12 years old boys in order to understand the developmental plateau in performance.Methods18 physically active boys (Age: 10.1 ± 1.6), in an under 9 years old (U9) and an under 12 years old (U12) group performed 15 m sprints and countermovement jumps. A 3D motion analysis system (200 Hz), synchronized with four force platforms (1,000 Hz), was used to collect kinematic and kinetic data during the first stance phase of the sprint run and the countermovement jump.ResultsThe U12 group had a significantly greater height (U9: 1.364 ± 0.064 m; U12: 1.548 ± 0.046 mm), larger mass (U9: 30.9 ± 3.5 kg; U12: 43.9 ± 5.0 kg), superior sprint performance over 0–5 m (U9: 1.31 ± 0.007 s; U12: 1.23 ± 0.009 s) and 0–15 m (U9: 3.20 ± 0.17 s; U12: 3.01 ± 0.20 s), and increased jump height (U9: 0.17 ± 0.06 m; U12: 0.24 ± 0.10 m) than the under nine group. During the first stance phase of the sprint the U12 group had a significantly greater vertical (U9: 0.22 ± 0.02 BW/s; U12: 0.25 ± 0.03 BW.s) and horizontal impulse (U9: 0.07 ± 0.02 BW/s; U12: 0.09 ± 0.03 BW.s) than the U9 group. When performing a countermovement jump the U12 group had a significantly greater mean average eccentric force (U9: 407.3 ± 55.0 N; U12: 542.2 ± 65.1 N) and mean average concentric force (U9: 495.8 ± 41.3 N; U12: 684.0 ± 62.1 N). Joint kinematics for the countermovement jump were significantly different between age groups for the ankle range of motion (U9: 80.6 ± 17.4°; U12: 64.1 ± 9°) and knee minimum joint angle (U9: −5.7 ± 3.9°; U12: 0.0 ± 4.4°). Conclusion: The study demonstrates for the first time that the development of physically active boys between the ages of 8–9 to 11–12 years increased the ground reaction forces and impulses during sprint running and countermovement jumps, but that sprint running technique had not developed during this period. Furthermore, countermovement jump technique was still emerging at the age of 8–9 years old. Practitioners need to implement on-going fine-grained sprint running and CMJ technique sessions to ensure that the increased force producing capabilities that come with age are appropriately utilized.

Wave-in-deck experiments with focussed waves into a solid deck

This paper examines impact forces resulting from wave-in-deck processes from two separate series of experiments: one with a generic solid deck model, and the other with a combined jacket and deck model, both were conducted with and without an I-beam grillage in-place below the solid deck. A range of inundation levels from 2.1 to 7.1 cm at 1:80 scale is considered. The focus is on global impact forces, which are considered more relevant for integrity assessment of overall bottom-founded structures when survivability is in question and local slamming is not addressed. The objective is to characterise the resultant impact forces as well as to investigate whether there is any interaction between the flows through the jacket and hitting the deck. Focussed wave groups were generated to impinge on the models which were suspended from a carriage over a towing tank. The motion of the support carriage mimics uniform current in-line with the incident waves. Both undisturbed surface elevations as well as impact force time histories were measured. From the first series of tests, a large increase in peak forces as well as high frequency oscillations (force spikes) is observed with the grillage in-place. As soon as the jacket model is in-place for the second series of tests, albeit with a different mounting support arrangement, such a large difference vanishes, which could likely be due to the effect of frequency-dependent transfer functions. We provide experimental evidence of the presence of the jacket in modifying the wave impact on the deck through a significant reduction in the total horizontal impulse. The effects of current on the wave impact forces are also investigated. A simple analytical model based on a momentum argument is used to describe the scaling of horizontal peak force with currents and inundation levels. Finally, the importance of the short duration force spikes as well as vertical impact loads on a real structure at full-scale is discussed based on the same analytical model.

Influence of forefoot bending stiffness on American football performance and metatarsophalangeal joint bending angle

ABSTRACT Changes in forefoot bending stiffness have been shown to affect metatarsophalangeal peak bending angles as well as athletic performance. Increasing bending stiffness tends to reduce peak bending angles, which could potentially reduce hyperextension injuries such as turf toe. Limited information is available, however, on the efficacy of increasing forefoot bending stiffness on large-sized athletes such as those that participate in American Football, with prior studies being conducted on smaller athlete populations. Therefore, the purpose of this study was to determine the influence of increased forefoot bending stiffness on metatarsophalangeal joint extension and athletic performance of grid-iron football players. Ten varsity grid-iron football players performed four National Football League combine movements in a motion capture laboratory in three footwear conditions of varying bending stiffness: Soft (12.7 N/mm), Control (23.8 N/mm), Stiff (42.2 N/mm). None of the footwear conditions significantly altered the maximum metatarsophalangeal bending. Therefore, to reduce metatarsophalangeal hyperextension injuries in American football players a greater amount of forefoot bending stiffness may be required. Performance differences were present only during the five-metre sprint acceleration, with athletes having an improved performance in the Control and Stiff conditions. This improved performance was due to an increased horizontal impulse and improvements in power generation at the ankle joint.

Key performance indicators and leg positioning for the kick-start in competitive swimmers

ABSTRACT The aim of the study was to (1) assess the test–retest reliability of a novel performance analysis system for swimming (KiSwim) including an instrumented starting block and optical motion capture system, (2) identify key performance indicators (KPI) for the kick-start, (3) determine the most beneficial position of the strong leg and (4) investigate the effect of acute reversal of leg positioning. During three sessions, kick-starts of 15 competitive swimmers were investigated. Eighteen kinematic and kinetic parameters showed high reliability (ICC>0.75) from which principal component analysis identified seven KPI (i.e., time to 15 m, time on-block, depth at 7.5 m, horizontal take-off velocity, horizontal impulse back plate, horizontal peak force back plate and vertical peak force front plate). For the preferred start position, the back plate showed a higher horizontal peak force (0.71 vs. 0.96 x body mass; p < 0.001) and impulse (0.191 vs. 0.28Ns/BW; p < 0.001) compared to front plate. Acute reversal of the leg position reduced performance (i.e., increased time to 15 m and reduced horizontal take-off velocity). However, plate-specific kinetic analysis revealed a larger horizontal peak force (p < 0.001) and impulse (p < 0.001) for the back compared to the front plate in any start position investigated. Therefore, swimmers are encouraged to position the strong leg in the back.

What goes up must come down: Consequences of jump strategy modification on dance leap take-off biomechanics

ABSTRACT Chronic foot and ankle injuries are common in dancers; understanding how lower extremity loading changes in response to altered task goals can be beneficial for rehabilitation and injury prevention strategies. The purpose of this study was to examine mechanical demands during jump take-offs when the task goal was modified to focus on either increasing jump distance or increasing jump height. It was hypothesized that a jump strategy focused on height would result in decreased energetic demands on the foot and ankle joints. Thirty healthy, experienced female dancers performed saut de chat leaps while travelling as far as possible (FAR) or jumping as high as possible (UP). Ground reaction force (GRF) impulses and peak sagittal plane net joint moments and sagittal plane mechanical energy expenditure (MEE) of the metatarsophalangeal (MTP), ankle, knee, and hip joints were calculated. During take-off, vertical and horizontal braking GRF impulses were greater and horizontal propulsive GRF impulse was lower in the UP condition. MEE at the MTP, ankle, and hip joints was lower in UP, and MEE at the knee was higher in UP. These results suggest that a strategy focused on height may be helpful in unloading the ankle and MTP joints during dance leaps.

Billiard Physics: Motion of a Cue Ball after Hit by a Cue Stick Horizontally

Billiards and pool games provide an excellent medium to discuss several physics principles including Newton’s laws of motion, friction, impulse-momentum theorem, work-energy theorem, kinematics and dynamics of rotational motion, and rolling motion. This paper provides a complete analysis of the motion of a cue ball that is given a sharp impulse, which is parallel to the horizontal surface. The horizontal impulse, in this work, is restricted to be in the plane passing through the ball’s center of mass and the contact point such that it sets the ball in horizontal motion with no sidespin. The motion of the cue ball until it eventually starts to roll without slipping is analyzed for all different impulse points on the surface of the ball. These analyses provide physics instructors a set of exciting examples on the rolling motion.

Principal Component Analysis of the Associations Between Kinetic Variables in Cutting and Jumping, and Cutting Performance Outcome.

Welch, N, Richter, C, Moran, K, and Franklyn-Miller, A. Principal component analysis of the associations between kinetic variables in cutting and jumping, and cutting performance outcome. J Strength Cond Res 35(7): 1848-1855, 2021-The primary aim of this study was to determine which features within the ground reaction force (GRF) trace during cutting are related to performance outcome in different angled cuts. The secondary aim was to understand the relationship between GRF features in a series of maximum strength, explosive strength, and reactive strength tests, and cutting performance outcome. Twenty-five male intercounty Gaelic football players (23.5 ± 4.2 years, 183 ± 6 cm, and 83 ± 6.9 kg) participated in the study. Subjects completed 110 and 45° cutting tasks, single leg squat jumps, drop landings, drop jumps, and isometric midthigh pulls. A principal component (PC) analysis and simulation approach were applied to the data and correlations between PCs and cutting performance outcome measured. Lower vertical to horizontal impulse ratios (r = -0.70 to -0.46) in both cuts and greater forces over the first 50 ms of ground contact (r = -0.44) in the 110° cut correlated with enhanced cutting performance outcomes. Greater reactive strength index and height in the drop jump (r = -0.51 and -0.54) and greater impulses over the first 25 ms of ground contact in the drop landing (r = 0.49 and 0.70) correlated with enhanced cutting performance outcomes. These results highlight the importance of greater horizontal and rapid force production in cutting and greater reactive strength qualities to enhance cutting performance.

Simulation and Experimental Study on the Discontinuous Dynamic Impact on Unidirectional Confined Coal‐Rock Damage

The deformation and the damage characteristics of coal and rock under different restraint static load and different impact velocity were studied with the FEM software LS‐DYNA. Based on this, the influence of impact load on the damage of coal rock under one‐dimensional constraint condition was studied by using the self‐developed constrained pendulum impact dynamic loading test device and ultrasonic testing device. The results show that the deformation of coal and rock increases with the increase of impact velocity and decreases firstly and then remains constant with the increase of constraint static load; when the constraint static load exceeds the compression strength, the displacement increases rapidly and the coal collapses at a strike; the damage quantity of coal and rock has a cumulative effect, and the damage quantity increases with the increase of impact number; when the constraint static load is identical, the increase of impulse contributes to microcrack propagation, and the damage quantity of coal and rock accelerates with the increase of single impact impulse; when the impulse is identical, the constraint static load restrains the microcrack propagation, and the damage quantity of coal and rock decelerates with the increase of constraint static load; the complete damage quantity range of coal and rock is 0.65∼0.75. In order to fully destroy the coal and rock, if the single horizontal impulse is greater, the number of shocks needed is less; if the constrained static load is greater, the more impact times are needed.