Resultant Ground Reaction Force Research Articles

The stiff plate location into the shoe influences the running biomechanics

ABSTRACT The changes in running biomechanics induced by an increased longitudinal bending stiffness (stiff plates added into the shoes) have been well investigated, but little is known concerning the effects of the stiff plate location into the shoe on running biomechanics. Fourteen male recreational runners ran at two participant-specific running speeds (3.28 ± 0.28 m/s and 4.01 ± 0.27 m/s) with two shoe conditions where a stiff plate was added either in high (under the insole) or low location (between the midsole and outsole). Ground reaction forces, lower limb joint angles, net joint torques and work, as well as alignment between the resultant ground reaction force and the leg were analysed. Among the running speeds performed by the runners, the high location significantly decreased propulsive ground reaction forces, increased metatarsophalangeal joint dorsiflexion and ankle plantarflexion, induced an increased alignment between the resultant ground reaction force and the runner’s leg, thus decreasing all the lower limb joint torques and the positive work at the knee joint compared to the low location. The results suggested that the high stiff plate location into the shoe should be considered for running performance perspectives, but care should be taken to not alter the perceived comfort and/or increase injury risks.

Differences and mechanisms underpinning a change in the knee flexion moment while running in stability and neutral footwear among young females

BackgroundHigher peak external knee flexion moments (KFM) during running has been observed in healthy people wearing athletic footwear compared to barefoot, which may increase risk of knee pathologies such as patellofemoral pain. Currently, no studies have examined whether stability and neutral style athletic shoes influence the peak KFM differently, or explored the underlying biomechanical mechanisms by which footwear alters peak KFM in young females.MethodsLower limb biomechanics of sixty girls aged between 10 and 25 years old were collected while running in footwear (both stability and neutral) and barefoot. The external peak KFM, sagittal plane kinematics, sagittal plane knee ground reaction force (GRF) lever arm and sagittal plane resultant GRF magnitude were analysed by repeated measures Analysis of Variance. Linear mixed models were fit to identify predictors of a change in peak KFM, and to determine if the effects of these predictors differed between footwear conditions.ResultsThe peak KFM was higher wearing both shoe styles compared to barefoot (p < 0.001), while no between-shoe differences were found (p > 0.05). Both shoes also increased kinematic variables at the hip, knee, and ankle (p < 0.05). When all these variables were entered into the mixed model, only a change in the knee-GRF lever arm was predictive of a change in peak KFM wearing shoes compared to barefoot (p < 0.001).ConclusionThese findings provide evidence that stability and neutral shoes increase peak KFM compared to barefoot, which is associated with a change in the knee-GRF lever arm rather than a change in lower limb kinematics. Future studies may consider manipulating footwear characteristics to reduce the knee-GRF lever arm in an effort to reduce peak KFM and the potential risk of patellofemoral pain.

The feasibility of predicting ground reaction forces during running from a trunk accelerometry driven mass-spring-damper model.

BackgroundMonitoring the external ground reaction forces (GRF) acting on the human body during running could help to understand how external loads influence tissue adaptation over time. Although mass-spring-damper (MSD) models have the potential to simulate the complex multi-segmental mechanics of the human body and predict GRF, these models currently require input from measured GRF limiting their application in field settings. Based on the hypothesis that the acceleration of the MSD-model’s upper mass primarily represents the acceleration of the trunk segment, this paper explored the feasibility of using measured trunk accelerometry to estimate the MSD-model parameters required to predict resultant GRF during running.MethodsTwenty male athletes ran at approach speeds between 2–5 m s−1. Resultant trunk accelerometry was used as a surrogate of the MSD-model upper mass acceleration to estimate the MSD-model parameters (ACCparam) required to predict resultant GRF. A purpose-built gradient descent optimisation routine was used where the MSD-model’s upper mass acceleration was fitted to the measured trunk accelerometer signal. Root mean squared errors (RMSE) were calculated to evaluate the accuracy of the trunk accelerometry fitting and GRF predictions. In addition, MSD-model parameters were estimated from fitting measured resultant GRF (GRFparam), to explore the difference between ACCparam and GRFparam.ResultsDespite a good match between the measured trunk accelerometry and the MSD-model’s upper mass acceleration (median RMSE between 0.16 and 0.22 g), poor GRF predictions (median RMSE between 6.68 and 12.77 N kg−1) were observed. In contrast, the MSD-model was able to replicate the measured GRF with high accuracy (median RMSE between 0.45 and 0.59 N kg−1) across running speeds from GRFparam. The ACCparam from measured trunk accelerometry under- or overestimated the GRFparam obtained from measured GRF, and generally demonstrated larger within parameter variations.DiscussionDespite the potential of obtaining a close fit between the MSD-model’s upper mass acceleration and the measured trunk accelerometry, the ACCparam estimated from this process were inadequate to predict resultant GRF waveforms during slow to moderate speed running. We therefore conclude that trunk-mounted accelerometry alone is inappropriate as input for the MSD-model to predict meaningful GRF waveforms. Further investigations are needed to continue to explore the feasibility of using body-worn micro sensor technology to drive simple human body models that would allow practitioners and researchers to estimate and monitor GRF waveforms in field settings.

Does an increase in energy return and/or longitudinal bending stiffness shoe features reduce the energetic cost of running?

This study focused on the effects of shoe energy return and shoe longitudinal bending stiffness on the energetic cost and biomechanics of running. The energetic cost of running and biomechanical variables altering running economy (ground contact times, stride frequency, vertical and leg stiffness, ground reaction force impulses, alignment between the resultant ground reaction force and the leg) were measured for nineteen male recreational runners. Participants ran overground under their ventilatory anaerobic threshold (10.8 ± 1.1kmh-1 on average) using four shoe prototypes with features combining low or high magnitudes of energy return and longitudinal bending stiffness. Neither the energy return, nor the longitudinal bending stiffness, or the interaction of these shoe features altered the energetic cost of running. High energy return shoes induced significant increased ground contact time from 274.5 ± 18.3 to 277.1 ± 18.7ms, and significant decreased stride frequency from 1.34 ± 0.05 to 1.33 ± 0.05Hz. High bending stiffness shoes induced significant increased ground contact time from 273.8 ± 18.2 to 277.9 ± 18.7ms, significant increased vertical stiffness from 23.2 ± 3.4 to 23.8 ± 3.0kNm-1, and significant decreased net vertical impulse from 245.4 ± 17.2 to 241.7 ± 17.5BWms. Increased energy return and longitudinal bending stiffness induced subtle changes in the running biomechanics, but did not induce any decrease in the energetic cost of running.

Understanding the track and field sprint start through a functional analysis of the external force features which contribute to higher levels of block phase performance

ABSTRACTThis study aimed to identify the continuous ground reaction force (GRF) features which contribute to higher levels of block phase performance. Twenty-three sprint-trained athletes completed starts from their preferred settings during which GRFs were recorded separately under each block. Continuous features of the magnitude and direction of the resultant GRF signals which explained 90% of the variation between the sprinters were identified. Each sprinter’s coefficient score for these continuous features was then input to a linear regression model to predict block phase performance (normalised external power). Four significant (p < 0.05) predictor features associated with GRF magnitude were identified; there were none associated with GRF direction. A feature associated with greater rear block GRF magnitudes from the onset of the push was the most important predictor (β = 1.185), followed by greater front block GRF magnitudes for the final three-quarters of the push (β = 0.791). Features which included a later rear block exit (β = 0.254) and greater front leg GRF magnitudes during the mid-push phase (β = 0.224) were also significant predictors. Sprint practitioners are encouraged, where possible, to consider the continuous magnitude of the GRFs produced throughout the block phase in addition to selected discrete values.

An analysis of the three-dimensional kinetics and kinematics of maximal effort punches among amateur boxers

ABSTRACTThe purpose of this study was to quantify the 3D kinetics and kinematics of six punch types among amateur boxers. Fifteen males (age: 24.9 ± 4.2 years; stature: 1.78 ± 0.1 m; body mass: 75.3 ± 13.4 kg; boxing experience: 6.3 ± 2.8 years) performed maximal effort punches against a suspended punch bag during which upper body kinematics were assessed via a 3D motion capture system, and ground reaction forces (GRF) of the lead and rear legs via two force plates. For all variables except elbow-joint angular velocity, analysis revealed significant (P < 0.05) differences between straight, hook and uppercut punches. The lead hook exhibited the greatest peak fist velocity (11.95 ± 1.84 m/s), the jab the shortest delivery time (405 ± 0.15 ms), the rear uppercut the greatest shoulder-joint angular velocity (1069.8 ± 104.5°/s), and the lead uppercut the greatest elbow angular velocity (651.0 ± 357.5°/s). Peak resultant GRF differed significantly (P < 0.05) between rear and lead legs for the jab punch only. Whilst these findings provide novel descriptive data for coaches and boxers, future research should examine if physical and physiological capabilities relate to the key biomechanical qualities associated with maximal punching performance.

Reliability and Performance Changes with the Addition of a Cognitive Task to Static and Dynamic Postural Stability Testing

Concussions are an unfortunate consequence of sports participation. They affect motor control, neurocognitive performance, and recent reports indicate they increase the risk of lower extremity musculoskeletal injury (LEMI) upon return to sport. The increased risk of secondary LEMI may indicate the need to establish a test that is predictive of LEMI risk following return to sport. PURPOSE: Assess the between-session reliability and the effects of adding a cognitive task to static and dynamic postural stability testing. METHODS: Twelve healthy, physically active subjects (Age: 22.3 ± 2.9 years, Height: 174.4 ± 7.5 cm, Weight: 154.5 ± 28.0 lbs) participated. Subjects underwent static and dynamic postural stability testing with and without the addition of a cognitive task (Stroop task) on two separate days. Static postural stability was assessed with a single-leg balance task under eyes open (with and without the addition of the Stroop task) and eyes closed conditions. Variability of each ground reaction force component was averaged across three trials for each of the static postural stability conditions. Dynamic postural stability testing consisted of forward jump over a hurdle with a one-legged landing performed with and without the addition of the Stroop task. A stability index was calculated based on the resultant ground reaction force and each of its components. Interclass correlation coefficients (ICC, 2,1) were calculated to determine the between-session reliability of each testing condition. Comparisons were made across the static conditions and between the dynamic postural stability tasks. RESULTS: The addition of a cognitive load proved to have moderate to excellent between-session reliability for the majority of variables calculated during static (ICC values 0.74 – 0.81) and dynamic postural stability testing (ICC values 0.77 – 0.80; ML=0.800, V=0.774, DPSI=0.781). No significant differences were observed between the postural stability tasks (with or without the Stroop task). CONCLUSION: Postural stability tasks with the addition of a cognitive load prove to have moderate to excellent reliability in a healthy population. These results provide new evidence on the feasibility of dual-task postural stability testing when examining risk of LEMI following return to sport.

The contribution analysis of knee compression bandage and arm swing control on maximum horizontal distance in standing long jump.

The aim of the study was to analyze the effect of knee compression bandage and arm swing control on the maximum horizontal distance and ground reaction force variables in standing long jump. Adult male (n=8; mean age, 22.75±1.98 years; mean heights, 1.77±0.03 m; mean weights, 71.82±12.87 kg) participated in the experiment. The results obtained from variables of ground reaction force (GRF) in medial lateral, anterior posterior (AP), vertical (V) direction, resultant GRF (RGRF), decay rate (%), and maximum horizontal distance (MHD) were as follows; MHD, AP GRF, VGRF, RGRF (AP-V), and ratio of load reduction (%) showed more effective result under both wearing of knee compression bandage and arm swing. In analysis of main effects, MHD, AP GRF, VGRF, and RGRF showed more increased value in case of wearing of knee compression bandage than as was not. While arm swing during standing long jump (SLJ) works as important factor to MHD, showed more effective factor in exercise rehabilitation, injury prevention and swell treatment etc. in wearing of worn knee compression bandage. Therefore it was assumed that arm swing and wearing of knee compression bandage may improve the performance of SLJ. Also, the characteristics of knee compression bandage suggest that it can be a great help for those participating in exercise rehabilitation.

Do rotational shear-cushioning shoes influence horizontal ground reaction forces and perceived comfort during basketball cutting maneuvers?

BackgroundCourt shoe designs predominantly focus on reducing excessive vertical ground reaction force, but shear force cushioning has received little attention in the basketball population. We aimed to examine the effect of a novel shoe-cushioning design on both resultant horizontal ground reaction forces and comfort perception during two basketball-specific cutting movements.MethodsFifteen university team basketball players performed lateral shuffling and 45-degree sidestep cutting at maximum effort in basketball shoes with and without the shear-cushioning system (SCS). Paired t-tests were used to examine the differences in kinetics and comfort perception between two shoes.ResultsSCS shoe allowed for larger rotational material deformation compared with control shoes, but no significant shoe differences were found in braking phase kinetics during both cutting movements (P = 0.35). Interestingly, a greater horizontal propulsion impulse was found with the SCS during 45-degree cutting (P < 0.05), when compared with the control. In addition, players wearing SCS shoes perceived better forefoot comfort (P = 0.012). During lateral shuffling, there were no significant differences in horizontal GRF and comfort perception between shoe conditions (P > 0.05).DiscussionThe application of a rotational shear-cushioning structure allowed for better forefoot comfort and enhanced propulsion performance in cutting, but did not influence the shear impact. Understanding horizontal ground reaction force information may be useful in designing footwear to prevent shear-related injuries in sport populations.

The effects of task execution variables on resultant vertical ground reaction force acting on foot sole during squat lifting

The main objective of this study was to investigate the effects of task execution variables on the value and point of application of the resultant vertical ground reaction force acting on the foot sole during squat lifting. This can be of particular importance since it may help to recognize the roles of the foot sole parts in bearing the support force during industrial and labor lifting activities. Twelve healthy men lifted a box with 4, 8 and 12 kg masses at fast and slow speeds. Ground reaction force and center of pressure were measured using two parallel Kistler force-plates. In addition, a Vicon system and a 3D linked segment model were used to define the movement pattern. Significant increases resulted for the peak vertical ground reaction force with the increases in the lift speed (P < 0.05) and box weight (P < 0.05). As well a significant increase resulted for the range of center of pressure location with the increase in the box weight (P < 0.05). Moreover, significant interactions (P < 0.05) between the lift speed and box weight were detected. The front and rear parts of the foot had the most important roles in bearing the support force during the beginning and final phases of movement, respectively. Finally, it was concluded that the value and point of application of vertical ground reaction force are seemingly dependent on the momentum of the subject plus box.

Ontogenetic changes in foot strike pattern and calcaneal loading during walking in young children

The assumption that the morphology of the human calcaneus reflects high and cyclical impact forces at heel strike during adult human walking has never been experimentally tested. Since a walking step with a heel strike is an emergent behavior in children, an ontogenetic study provides a natural experiment to begin testing the relationship between the mechanics of heel strike and calcaneal anatomy. This study examined the ground reaction forces (GRFs) of stepping in children to determine the location of the center of pressure (COP) relative to the calcaneus and the orientation and magnitude of ground reaction forces during foot contact. Three-dimensional kinematic and kinetic data were analyzed for 18 children ranging in age from 11.5 to 43.1 months. Early steppers used a flat foot contact (FFC) and experienced relatively high vertical and resultant GRFs with COP often anterior to the calcaneus. More experienced walkers used an initial heel contact (IHC) in which GRFs were significantly lower but the center of pressure remained under the heel a greater proportion of time. Thus, during FFC the foot experienced higher loading, but the heel itself was relatively wider and the load was distributed more evenly. In IHC walkers load was concentrated on the anterior calcaneus and a narrower heel, suggesting a need for increased calcaneal robusticity during development to mitigate injury. These results provide new insight into foot loading outside of typical mature contact patterns, inform structure-function relationships during development, and illuminate potential causes of heel injury in young walkers.

Ground reaction forces produced by two different hockey skating arm swing techniques

The arm swing in hockey skating can have a positive effect on the forces produced by each skate, and the resulting velocity from each push off. The main purpose of this study was to measure the differences in ground reaction forces (GRFs) produced from an anteroposterior versus a mediolateral style hockey skating arm swing. Twenty-four elite-level female hockey players performed each technique while standing on a ground-mounted force platform, and all trials were filmed using two video cameras. Force data was assessed for peak scaled GRFs in the frontal and sagittal planes, and resultant GRF magnitude and direction. Upper limb kinematics were assessed from the video using Dartfish video analysis software, confirming that the subjects successfully performed two distinct arm swing techniques. The mediolateral arm swing used a mean of 18.38° of glenohumeral flexion/extension and 183.68° of glenohumeral abduction/adduction while the anteroposterior technique used 214.17° and 28.97° respectively. The results of this study confirmed that the mediolateral arm swing produced 37% greater frontal plane and 33% less sagittal plane GRFs than the anteroposterior arm swing. The magnitudes of the resultant GRFs were not significantly different between the two techniques; however, the mediolateral technique produced a resultant GRF with a significantly larger angle from the direction of travel (44.44°) as compared to the anteroposterior technique (31.60°). The results of this study suggest that the direction of GRFs produced by the mediolateral arm swing more closely mimic the direction of lower limb propulsion during the skating stride.

A data set with kinematic and ground reaction forces of human balance.

This article describes a public data set containing the three-dimensional kinematics of the whole human body and the ground reaction forces (with a dual force platform setup) of subjects who were standing still for 60 s in different conditions, in which the subjects’ vision and the standing surface were manipulated. Twenty-seven young subjects and 22 old subjects were evaluated. The data set comprises a file with metadata plus 1,813 files with the ground reaction force (GRF) and kinematics data for the 49 subjects (three files for each of the 12 trials plus one file for each subject). The file with metadata has information about each subject’s sociocultural, demographic, and health characteristics. The files with the GRF have the data from each force platform and from the resultant GRF (including the center of pressure data). The files with the kinematics contain the three-dimensional positions of 42 markers that were placed on each subject’s body and 73 calculated joint angles. In this text, we illustrate how to access, analyze, and visualize the data set. All the data is available at Figshare (DOI: 10.6084/m9.figshare.4525082), and a companion Jupyter Notebook presents programming code to access the data set, generate analyses and other examples. The availability of a public data set on the Internet that contains these measurements and information about how to access and process this data can potentially boost the research on human postural control, increase the reproducibility of studies, and be used for training and education, among other applications.

A generalisable methodology for stability assessment of walking aid users

To assist balance and mobility, older adults are often prescribed walking aids. Nevertheless, surprisingly their use has been associated with increased falls-risk. To address this finding we first need to characterise a person's stability while using a walking aid. Therefore, we present a generalisable method for the assessment of stability of walking frame (WF) users. Our method, for the first time, considers user and device as a combined system.We define the combined centre of pressure (CoPsystem) of user and WF to be the point through which the resultant ground reaction force for all feet of both the WF and user acts if theresultant moment acts only around an axisperpendicular tothe ground plane.We also define the combined base of support (BoSsystem) to be the convex polygon formed by the boundaries of the anatomical and WF feet in contact with the ground and interconnecting lines between them. To measure these parameters we have developed an instrumented WF with a load cell in each foot which we use together with pressure-sensing insoles and a camera system, the latter providing the relative position of the WF and anatomical feet. Software uses the resulting data to calculate the stability margin of the combined system, defined as the distance between CoPsystem and the nearest edge of BoSsystem. Our software also calculates the weight supported through the frame and when each foot (of user and/or frame) is on the floor. Finally, we present experimental work demonstrating the value of our approach.

A comparison of the ground reaction force frequency content during rearfoot and non-rearfoot running patterns

Running with a non-rearfoot pattern has been claimed to reduce injury risk because the impact peak in the vertical ground reaction force (GRF) is visually absent in the time-domain compared with a rearfoot pattern. However, running results in a rapid deceleration of the lower extremity segments immediately following initial contact with the ground, regardless of footfall pattern. Therefore, the frequency content of the GRF is expected to contain evidence of this collision. The purpose of the present study was to characterize the waveform components of the GRF generated during the impact phase by habitual rearfoot and habitual non-rearfoot pattern groups using the continuous wavelet transform. Twenty rearfoot and 20 non-rearfoot participants ran over-ground at a standardized speed with their habitual footfall pattern. The continuous wavelet transform was performed on the resultant GRF vector and the vertical GRF. GRF signals generated by the non-rearfoot pattern group during early stance had maximum signal power of 15.4±9.1Hz occurring at 23.1±6.3% of stance, which is within the 10–20Hz range previously associated with impact in rearfoot runners. Maximum signal power occurred earlier in the impact phase (11.5±1.5%) and with a higher frequency (27.2±3.9Hz) in the rearfoot pattern group verses the non-rearfoot pattern group (P<0.05). While the impact force transient may not appear as a prominent feature within the time-domain GRF with a non-rearfoot pattern, the results indicate that both footfall patterns generate frequencies associated with the impact peak in the resultant and vertical GRF.

How do rocker-soled shoes influence the knee adduction moment in people with knee osteoarthritis? An analysis of biomechanical mechanisms

The primary objective was to examine mechanisms behind previously observed changes in the knee adduction moment (KAM) with rocker-soled shoes, in participants sub-grouped according to whether they experienced an immediate decrease, or increase, in peak KAM. In subgroups where frontal plane knee ground reaction force (GRF) lever-arm emerged as a significant predictor, a secondary aim was to examine biomechanical factors that contributed to change in this parameter. Thirty individuals with symptomatic, radiographic knee osteoarthritis (OA) underwent 3D gait analysis in unstable rocker-soled shoes and non-rocker-soled shoes. Multiple regression analyses, within each subgroup, examined relationships between changes in frontal plane knee-GRF lever arm and frontal plane resultant GRF magnitude and changes in peak KAM and KAM impulse between shoe conditions. In the subgroup that decreased peak KAM with rocker-soled shoes (n=23), change in knee-GRF lever arm and frontal plane GRF magnitude at peak KAM together were significant predictors of change in peak KAM; however, only change in mean knee-GRF lever arm significantly predicted change in KAM impulse. Decreased medial GRF magnitude, increased lateral trunk lean towards the stance limb and reduced varus/increased valgus hip-knee-ankle angle were associated with a lower knee-GRF lever arm in this group, with rocker-soled shoes. In contrast, none of the independent variables predicted changes in KAM in the subgroup who increased peak KAM with rocker-soled shoes (n=7).

External kinetics of the kettlebell snatch in amateur lifters.

BackgroundKettlebell lifting has gained increased popularity as both a form of resistance training and as a sport, despite the paucity of literature validating its use as a training tool. Kettlebell sport requires participants to complete the kettlebell snatch continuously over prolonged periods of time. Kettlebell sport and weightlifting involve similar exercises, however, their traditional uses suggest they are better suited to training different fitness qualities. This study examined the three-dimensional ground reaction force (GRF) and force applied to the kettlebell over a 6 min kettlebell snatch set in 12 kettlebell-trained males.MethodsDuring this set, VICON was used to record the kettlebell trajectory with nine infrared cameras while the GRF of each leg was recorded with a separate AMTI force plate. Over the course of the set, an average of 13.9 ± 3.3 repetitions per minute were performed with a 24 kg kettlebell. Significance was evaluated with a two-way ANOVA and paired t-tests, whilst Cohen’s F (ESF) and Cohen’s D (ESD) were used to determine the magnitude.ResultsThe applied force at the point of maximum acceleration was 814 ± 75 N and 885 ± 86 N for the downwards and upwards phases, respectively. The absolute peak resultant bilateral GRF was 1,746 ± 217 N and 1,768 ± 242 N for the downwards and upwards phases, respectively. Bilateral GRF of the first and last 14 repetitions was found to be similar, however there was a significant difference in the peak applied force (F (1.11) = 7.42, p = 0.02, ESF = 0.45). Unilateral GRF was found have a significant difference for the absolute anterior–posterior (F (1.11) = 885.15, p < 0.0001, ESF = 7) and medio-lateral force vectors (F (1.11) = 5.31, p = 0.042, ESF = 0.67).DiscussionOver the course of a single repetition there were significant differences in the GRF and applied force at multiple points of the kettlebells trajectory. The kettlebell snatch loads each leg differently throughout a repetition and performing the kettlebell snatch for 6 min will result in a reduction in peak applied force.

Technique Selection 'the Coaches Challenge' Influencing Injury Risk During the First Contact Hand of the Round off Skill in Female Gymnastics.

The importance of technique selection on elbow injury risk has been identified for the key round off skill in female gymnastics, with a focus on the second contact limb. The aim of this study was to shift the focus to the first contact limb and investigate the biomechanical injury risk during parallel and T-shape round-off (RO) techniques. Seven international-level female gymnasts performed 10 trials of the RO to back-handspring with parallel and T-shape hand positions. Synchronized kinematic (3D motion analysis system; 247 Hz) and kinetic (two force plates; 1235 Hz) data were collected for each trial. The t-test with effect size statistics determined differences between the two techniques. No significant differences were found for vertical, anterior posterior and resultant ground reaction force, elbow joint kinematics and kinetics. Specifically, the results highlighted that change in technique in RO skills did not influence first contact limb elbow joint mechanics and therefore, injury risk. The findings of the present study suggest the injury potential of this skill is focused on the second limb during the parallel technique of this fundamental gymnastic skill.

Mechanical Alterations Associated with Repeated Treadmill Sprinting under Heat Stress

PurposeExamine the mechanical alterations associated with repeated treadmill sprinting performed in HOT (38°C) and CON (25°C) conditions.MethodsEleven recreationally active males performed a 30-min warm-up followed by three sets of five 5-s sprints with 25-s recovery and 3-min between sets in each environment. Constant-velocity running for 1-min at 10 and 20 km.h-1 was also performed prior to and following sprinting.ResultsMean skin (37.2±0.7 vs. 32.7±0.8°C; P<0.001) and core (38.9±0.2 vs. 38.8±0.3°C; P<0.05) temperatures, together with thermal comfort (P<0.001) were higher following repeated sprinting in HOT vs. CON. Step frequency and vertical stiffness were lower (-2.6±1.6% and -5.5±5.5%; both P<0.001) and contact time (+3.2±2.4%; P<0.01) higher in HOT for the mean of sets 1–3 compared to CON. Running distance per sprint decreased from set 1 to 3 (-7.0±6.4%; P<0.001), with a tendency for shorter distance covered in HOT vs. CON (-2.7±3.4%; P = 0.06). Mean vertical (-2.6±5.5%; P<0.01), horizontal (-9.1±4.4%; P<0.001) and resultant ground reaction forces (-3.0±2.8%; P<0.01) along with vertical stiffness (-12.9±2.3%; P<0.001) and leg stiffness (-8.4±2.7%; P<0.01) decreased from set 1 to 3, independently of conditions. Propulsive power decreased from set 1 to 3 (-16.9±2.4%; P<0.001), with lower propulsive power values in set 2 (-6.6%; P<0.05) in HOT vs. CON. No changes in constant-velocity running patterns occurred between conditions, or from pre-to-post repeated-sprint exercise.ConclusionsThermal strain alters step frequency and vertical stiffness during repeated sprinting; however without exacerbating mechanical alterations. The absence of changes in constant-velocity running patterns suggests a strong link between fatigue-induced velocity decrements during sprinting and mechanical alterations.

Identification of types of landings after blocking in volleyball associated with risk of ACL injury

Landing with a low knee flexion angle after volleyball block jumps may be associated with an increased risk of anterior cruciate ligament (ACL) injury. The aim of the present study was to identify the types of volleyball landings after blocks where the knee flexion angle is found to be under a critical knee flexion angle value of 30° at the instant of the first peak of the ground reaction force (GRF). Synchronized kinematic and kinetic data were collected for each trial. T-tests were used to determine if each knee flexion angle at the instant of the peak GRF was significantly different from the critical value of 30°. A repeated measures ANOVA was used to compare knee flexion angle, time to first peak and the magnitude of the first peak of the resultant GRF and knee stiffness. Significantly lower knee flexion angles were found in the “go” landing (p = .01, ES = 0.6) and the “reverse” landing (p = .02, ES = 0.6) only. The results for knee flexion angle and GRF parameters indicated a significant difference between a “reverse” and “go” and other types of landings, except the “side stick” landing for GRF. The “reverse” and “go” landings may present a risk for ACL injury due to the single-leg landing of these activities that have an associated mediolateral movement.