The Validity of Hawkin Dynamics Wireless Dual Force Plates for Measuring Countermovement Jump and Drop Jump Variables.

Force plate testing is becoming more commonplace in sport due to the advent of commercially available, portable, and affordable force plate systems (i.e., hardware and software). Following the validation of the Hawkin Dynamics Inc. (HD) proprietary software in recent literature, the aim of this study was to determine the concurrent validity of the HD wireless dual force plate hardware for assessing vertical jumps. During a single testing session, the HD force plates were placed directly atop two adjacent Advanced Mechanical Technology Inc. in-ground force plates (the "gold standard") to simultaneously collect vertical ground reaction forces produced by 20 participants (27 ± 6 years, 85 ± 14 kg, 176.5 ± 9.23 cm) during the countermovement jump (CMJ) and drop jump (DJ) tests (1000 Hz). Agreement between force plate systems was determined via ordinary least products regression using bootstrapped 95% confidence intervals. No bias was present between the two force plate systems for any of the CMJ and DJ variables, except DJ peak braking force (proportional bias) and DJ peak braking power (fixed and proportional bias). The HD system may be considered a valid alternative to the industry gold standard for assessing vertical jumps because fixed or proportional bias was identified for none of the CMJ variables (n = 17) and only 2 out of 18 DJ variables.

Predicting Weight Category-Specific Performance Zones for Olympic, World, and European Weightlifting Competitions.

ABSTRACT: Chavda, S, Comfort, P, Lake, JP, Bishop, C, and Turner, AN. Predicting weight category-specific performance zones for Olympic, World, and European weightlifting competitions. J Strength Cond Res 37(10): 2038-2045, 2023-Understanding the total likely required weight category to achieve a specific rank within a specific competition can aid in the long-term and short-term preparation and tactics for performance teams. The primary objective of this investigation was to develop a set of predictive models for new weight categories across 5 performance zones for 3 major weightlifting competitions. Performance total (Ptot) data for top 15 male athletes were obtained from the International Weightlifting Federation website from 1998 to 2020 across the Olympics, and World and European Championships. A second-order polynomial regression was conducted with 95% confidence, and predictive intervals were calculated. The average of the newly contested body mass was then used as the intercept. Predictions were compared against current performances of the new weight categories up to the 2020 Olympics. Results revealed that the models for all competition types varied in their predictive ability for each performance zone, across each new weight category. On average, predicted Ptot displayed a difference from actual Ptot of 3.65 ± 2.51% (12.46 ± 9.16 kg), 0.78 ± 3.29% (2.26 ± 10.08 kg), and -1.13 ± 3.46% (-4.32 ± 11.10 kg) for the Olympics, and World and European Championships, respectively. The results suggest that the predictive models may be a good indicator of future performances; however, the models may have greater efficacy in some weight categories and performance zones than others.

Kinetics and Kinematics of the Free-Weight Back Squat and Loaded Jump Squat.

ABSTRACT: Thompson, SW, Lake, JP, Rogerson, D, Ruddock, A, and Barnes, A. Kinetics and kinematics of the free-weight back squat and loaded jump squat. J Strength Cond Res 37(1): 1-8, 2023-The aim of this study was to compare kinetics and kinematics of 2 lower-body free-weight exercises, calculated from concentric and propulsion subphases, across multiple loads. Sixteen strength-trained men performed back squat 1 repetition maximum (1RM) tests (visit 1), followed by 2 incremental back squat and jump squat protocols (visit 2) (loads = 0% and 30-60%, back squat 1RM). Concentric phase and propulsion phase force-time-displacement characteristics were derived from force plate data and compared using analysis of variance and Hedges' g effect sizes. Intrasession reliability was calculated using intraclass correlation coefficient (ICC) and coefficient of variation (CV). All dependent variables met acceptable reliability (ICC >0.7; CV < 10%). Statistically significant 3-way interactions (load × phase × exercise) and 2-way main effects (phase × exercise) were observed for mean force, velocity (30-60% 1RM), power, work, displacement, and duration (0%, 30-50% 1RM) ( p < 0.05). A significant 2-way interaction (load × exercise) was observed for impulse ( p < 0.001). Jump squat velocity ( g = 0.94-3.80), impulse ( g = 1.98-3.21), power ( g = 0.84-2.93), and work ( g = 1.09-3.56) were significantly larger across concentric and propulsion phases, as well as mean propulsion force ( g = 0.30-1.06) performed over all loads ( p < 0.001). No statistically significant differences were observed for mean concentric force. Statistically longer durations ( g = 0.38-1.54) and larger displacements ( g = 2.03-4.40) were evident for all loads and both subphases ( p < 0.05). Ballistic, lower-body exercise produces greater kinetic and kinematic outputs than nonballistic equivalents, irrespective of phase determination. Practitioners should therefore use ballistic methods when prescribing or testing lower-body exercises to maximize athlete's force-time-displacement characteristics.

Countermovement Jump Standards in Rugby League: What is a "Good" Performance?

ABSTRACT: McMahon, JJ, Lake, JP, Dos'Santos, T, Jones, PA, Thomasson, ML, and Comfort, P. Countermovement jump standards in rugby league: what is a "good" performance? J Strength Cond Res 36(6): 1691-1698, 2022-The countermovement jump (CMJ) is considered an important test in rugby league, and the force platform is the recommended tool for assessing CMJ performance in this cohort. Because of inconsistent methods applied across previous studies, there is currently a lack of understanding of what constitutes a "good" CMJ performance, with respect to the typical CMJ metrics that are reported for rugby league players. The purpose of this study was, therefore, to produce a scale of reference values for the jump height (JH), reactive strength index modified (RSImod), and mean (PPmean) and peak (PPpeak) propulsion power (relative to body mass) for top-level senior rugby league players competing in the global "forward" and "back" positional groups. One hundred four players (55 forwards and 49 backs) from the top 2 tiers of English rugby league performed 3 CMJs on a force platform at the beginning of pre-season training. The JH, RSImod, PPmean, and PPpeak were calculated using criterion methods, and a scale of norm-referenced values (percentiles) was produced for each positional group. The backs outperformed the forwards for each CMJ metric reported, thus supporting the production of position-specific norm-referenced values. When each positional group was separated into quartile subgroups, the respective JH, RSImod, PPmean, and PPpeak values were mostly largely and significantly different both within and between positions. The presented scale of reference values can, therefore, be used to determine the performance standards of rugby league forwards and backs with respect to the most commonly reported CMJ-derived variables for this cohort.

Relationship Between Reactive Strength Index Variants in Rugby League Players.

ABSTRACT: McMahon, JJ, Suchomel, TJ, Lake, JP, and Comfort, P. Relationship between reactive strength index variants in rugby league players. J Strength Cond Res 35(1): 280-285, 2021-Two reactive strength index (RSI) variants exist, the RSI and RSI modified (RSImod), which are typically calculated during the drop jump (DJ) and countermovement jump (CMJ), respectively. Both RSI variants have been used to monitor athletes' ability to complete stretch-shortening cycle actions quickly, but they have never been compared. The purpose of this study was to determine whether they yield relatable information about reactive strength characteristics. Male professional rugby league players (n = 21, age = 20.8 ± 2.3 years, height = 1.82 ± 0.06 m and body mass = 94.3 ± 8.4 kg) performed 3 DJs (30 cm) and CMJs on a force plate. Reactive strength index and RSImod were subsequently calculated by dividing jump height (JH) by ground contact time (GCT) and time to take-off (TTT), respectively. All variables were highly reliable (intraclass correlation coefficient ≥0.78) with acceptable levels of variability (coefficient of variation ≤8.2%), albeit larger variability was noted for DJ variables. Moreover, there was a large relationship between RSI and RSImod (r = 0.524, p = 0.007), whereas very large relationships were noted between JHs (r = 0.762, p < 0.001) and between GCT and TTT (ρ = 0.705, p < 0.001). In addition, RSI (0.90 ± 0.22) was largely and significantly (d = 2.57, p < 0.001) greater than RSImod (0.47 ± 0.08). The DJ-derived RSI yields much larger values than the CMJ-derived RSImod and although a large relationship was noted between them, it equated to just 22% shared variance. These results suggest that the 2 RSI variants do not explain each other well, indicating that they do not assess entirely the same reactive strength qualities and should not be used interchangeably.

Effect of Barbell Load on Vertical Jump Landing Force-Time Characteristics.

ABSTRACT: Lake, JP, Mundy, PD, Comfort, P, McMahon, JJ, Suchomel, TJ, and Carden, P. Effect of barbell load on vertical jump landing force-time characteristics. J Strength Cond Res 35(1): 25-32, 2021-The aim of this study was to quantify the effect that barbell load has on the jump height and force-time characteristics of the countermovement jump (CMJ). Fifteen strength-trained men (mean ± SD: age 23 ± 2 years, mass 84.9 ± 8.1 kg, and height 1.80 ± 0.05 m) performed 3 CMJs with no additional load, and with barbell loads of 25, 50, 75, and 100% of body mass on 2 force plates recording at 1,000 Hz. Propulsion and landing force-time characteristics were obtained from force-time data and compared using analysis of variance and effect sizes. Jump height decreased significantly as load increased (26-71%, d = 1.80-6.87). During propulsion, impulse increased with load up to 75% of body mass (6-9%, d = 0.71-1.08), mean net force decreased (10-43%, d = 0.50-2.45), and time increased (13-50%, d = 0.70-2.57). During landing, impulse increased as load increased up to 75% of body mass (5 to 12%, d = 0.54-1.01), mean net force decreased (13-38%, d = 0.41-1.24), and time increased (20-47%, d = 0.65-1.47). Adding barbell load to CMJ significantly decreases CMJ height. Furthermore, CMJ with additional barbell load increases landing phase impulse. However, while mean net force decreases as barbell load increases, landing time increases so that jumpers are exposed to mechanical load for longer. Practitioners should exercise caution when implementing loaded CMJ to assess their athletes.

Vertical Jump Testing in Rugby League: A Rationale for Calculating Take-Off Momentum.

The purpose of this study was to determine the usefulness of calculating jump take-off momentum in rugby league (RL) by exploring its relationship with sprint momentum, due to the latter being an important attribute of this sport. Twenty-five male RL players performed 3 maximal-effort countermovement jumps on a force platform and 3 maximal effort 20-m sprints (with split times recorded). Jump take-off momentum and sprint momentum (between 0 and 5, 5 and 10, and 10 and 20 m) were calculated (mass multiplied by velocity) and their relationship determined. There was a very large positive relationship between both jump take-off and 0- to 5-m sprint momentum (r = .781, P < .001) and jump take-off and 5- to 10-m sprint momentum (r = .878, P < .001). There was a nearly perfect positive relationship between jump take-off and 10- to 20-m sprint momentum (r = .920, P < .001). Jump take-off and sprint momentum demonstrated good-excellent reliability and very large-nearly perfect associations (61%-85% common variance) in an RL cohort, enabling prediction equations to be created. Thus, it may be practically useful to calculate jump take-off momentum as part of routine countermovement jump testing of RL players and other collision-sport athletes to enable the indirect monitoring of sprint momentum.

The effects of barbell load on countermovement vertical jump power and net impulse.

The aim of this study was to examine the effects of barbell load on countermovement vertical jump (CMJ) power and net impulse within a theoretically valid framework, cognisant of the underpinning force, temporal, and spatial components. A total of 24 resistance-trained rugby union athletes (average ± SD: age: 23.1 ± 3.4 years; height: 1.83 ± 0.05 m; body mass (BM): 91.3 ± 10.5 kg) performed maximal CMJ under 5 experimental conditions in a randomised, counterbalanced order: unloaded, and with additional loads of 25%, 50%, 75%, and 100% of BM. Peak power and average power were maximised during the unloaded condition, both decreasing significantly (P < 0.05) as load increased. Net impulse was maximised with 75% of BM, which was significantly greater (P < 0.05) than the unloaded and 100% of BM conditions. Net mean force and mean velocity were maximised during the unloaded condition and decreased significantly (P < 0.05) as load increased, whereas phase duration increased significantly (P < 0.05) as load increased. As such, the interaction between barbell load and the underpinning force, time, and displacement components should be considered by strength and conditioning coaches when prescribing barbell loads.

Load Absorption Force-Time Characteristics Following the Second Pull of Weightlifting Derivatives.

The purpose of this study was to compare the load absorption force-time characteristics of weightlifting catching and pulling derivatives. Twelve resistance-trained men performed repetitions of the hang power clean (HPC), jump shrug (JS), and hang high pull (HHP) on a force platform with 30, 45, 65, and 80% of their 1-repetition maximum HPC. Load absorption phase duration, mean force, and work were calculated from the force-time data. The HHP produced a significantly longer load absorption phase duration compared with the HPC (p < 0.001; d = 3.77) and JS (p < 0.001; d = 5.48), whereas no difference existed between the HPC and JS (p = 0.573; d = 0.51). The JS produced significantly greater load absorption mean forces compared with the HPC (p < 0.001; d = 2.85) and HHP (p < 0.001; d = 3.75), whereas no difference existed between the HPC and HHP (p = 0.253; d = 0.37). Significantly more load absorption work was performed during the JS compared with the HPC (p < 0.001; d = 5.03) and HHP (p < 0.001; d = 1.69), whereas HHP load absorption work was also significantly greater compared with the HPC (p < 0.001; d = 4.81). The weightlifting pulling derivatives examined in the current study (JS and HHP) produced greater load absorption demands after the second pull compared with the weightlifting catching derivative (HPC). The JS and HHP may be used as effective training stimuli for load absorption during impact tasks such as jumping.

A Comparison of Catch Phase Force-Time Characteristics During Clean Derivatives From the Knee.

Comfort, P, Williams, R, Suchomel, TJ, and Lake, JP. A comparison of catch phase force-time characteristics during clean derivatives from the knee. J Strength Cond Res 31(7): 1911-1918, 2017-The aim of this study was to compare load-absorption force-time characteristics of the clean from the knee (CK), power clean from the knee (PCK), and clean pull from the knee (CPK). Ten collegiate athletes (age 27.5 ± 4.2 years; height 180.4 ± 6.7 cm; mass 84.4 ± 7.8 kg) performed 3 repetitions each of the CK, PCK, and CPK with 90% of their 1 repetition maximum power clean on a force platform. The CK load-absorption duration (0.95 ± 0.35 seconds) was significantly longer compared with the CPK (0.44 ± 0.15 seconds; p < 0.001, d = 2.53), but not compared with the PCK (0.56 ± 0.11 seconds; p > 0.05, d = 1.08), with no differences between PCK and CPK (p > 0.05, d = 0.91). The CPK demonstrated the greatest mean force (2,039 ± 394 N), which was significantly greater than the PCK (1,771 ± 325 N; p = 0.012, d = 0.83), but not significantly different to the CK (1,830 ± 331 N; p > 0.05, d = 0.60); CK and PCK were not different (p > 0.05, d = 0.18). Significantly more load-absorption work was performed during the CK (655 ± 276 J) compared with the PCK (288 ± 109 J; d = 1.75, p < 0.001), but not compared with the CPK (518 ± 132 J; d = 0.80, p > 0.05). Additionally, more load-absorption work was performed during the CPK compared with the PCK (d = 1.90, p = 0.032). Inclusion of the catch phase during the CK does not provide any additional stimulus in terms of mean force or work during the load-absorption phase compared with the CPK, although the CPK may be beneficial in training rapid force absorption because of high force and a short duration.

Magnitude and relative distribution of kettlebell snatch force-time characteristics.

The aim of this study was to compare mechanical output from kettlebell snatch and 2-handed kettlebell swing exercise. Twenty-two men performed 3 sets of 8 kettlebell snatch and 2-handed swing exercise with a 24-kg kettlebell on a force platform. Vertical and horizontal net impulse, mean force, displacement, the magnitude, and rate of work performed displacing the kettlebell-and-lifter center of mass (CM), phase durations and impulse ratio (horizontal to resultant) were calculated from force data. The results of repeated-measures analysis of variance showed that: (a) vertical CM displacement was significantly larger during kettlebell snatch exercise (22 ± 4 vs. 18 ± 5 cm, p = 0.001), and vertical CM displacement was significantly larger than horizontal CM displacement, regardless of exercise (20 ± 3 vs. 7 ± 1 cm, p < 0.0001); (b) the magnitude (253 ± 73 vs. 3 ± 1 J, p < 0.0001) and rate of work (714 ± 288 vs. 11 ± 4 W, p < 0.0001) performed to vertically displace the CM was larger than the horizontal equivalent in both exercises, and the magnitude (5 ± 2 vs. 1 ± 1 J, p < 0.0001) and rate of work (18 ± 7 vs. 4 ± 3 W, p < 0.0001) performed to horizontally displace the CM during 2-handed swing exercise was significantly larger than the kettlebell snatch equivalent; (c) this was underpinned by the magnitude of horizontal impulse (29 ± 7 vs. 18 ± 7 N·s, p < 0.0001) and the impulse ratio (23 vs. 14%, p < 0.0001). These findings reveal that, apart from the greater emphasis, 2-handed swing exercise places on horizontal mechanical output, the mechanical output of the 2 exercises is similar. Research shows that 2-handed swing exercise improves maximum and explosive strength. These results suggest that strength and conditioning coaches should consider using kettlebell snatch and 2-handed swing exercise interchangeably for the ballistic component of athlete strength and conditioning programs.

Power and impulse applied during push press exercise.

The aim of this study was to quantify the load, which maximized peak and mean power, and impulse applied to these loads, during the push press and to compare them to equivalent jump squat data. Resistance-trained men performed 2 push press (n = 17; age: 25.4 ± 7.4 years; height: 183.4 ± 5 cm; body mass: 87 ± 15.6 kg) and jump squat (n = 8 of original 17; age: 28.7 ± 8.1 years; height: 184.3 ± 5.5 cm; mass: 98 ± 5.3 kg) singles with 10-90% of their push press and back squat 1 repetition maximum (1RM), respectively, in 10% 1RM increments while standing on a force platform. Push press peak and mean power was maximized with 75.3 ± 16.4 and 64.7 ± 20% 1RM, respectively, and impulses applied to these loads were 243 ± 29 N·s and 231 ± 36 N·s. Increasing and decreasing load, from the load that maximized peak and mean power, by 10 and 20% 1RM reduced peak and mean power by 6-15% (p ≤ 0.05). Push press and jump squat maximum peak power (7%, p = 0.08) and the impulse that was applied to the load that maximized peak (8%, p = 0.17) and mean (13%, p = 0.91) power were not significantly different, but push press maximum mean power was significantly greater than the jump squat equivalent (∼9.5%, p = 0.03). The mechanical demand of the push press is comparable with the jump squat and could provide a time-efficient combination of lower-body power and upper-body and trunk strength training.

Relative strength explains the differences in multi-joint rapid force production between sexes.

The primary aim of this study was to determine whether relative strength explains the differences in the rapid force production (force developed during first 150-, 200-, and 250 ms) of females and males, and to evaluate the relationships between peak force and rapid force production. Sixty-three team sport athletes (females: n = 25, age = 21.5 ± 1.3 years, stature = 166 ± 5 cm, body mass = 60.65 ± 10.04 kg; males: n = 38, age = 21.9 ± 1.1 years, stature = 178 ± 7 cm, body mass = 76.55 ± 12.88 kg) performed a series of isometric mid-thigh pull (IMTP) trials, with all participants' data used for correlational analysis. After testing, females and males were divided into 20 strength-matched pairs, based on their relative peak force (peak force ∙ body mass). There were no meaningful differences between sexes for relative force at 150 ms (g = 0.007 [95% CI -0.627, 0.648]), 200 ms (g = -0.059 [95% CI -0.695, 0.588]) and 250 ms (g = -0.156 [95% CI -0.778, 0.473]). Similarly, when expressed as a percentage of peak force there were no meaningful differences in force at 150 ms (g = -0.015 [95.0%CI -0.650, 0.680]), 200 ms (g = -0.099 [95.0%CI -0.714, 0.559]) or 250 ms (g = -0.272 [95.0%CI -0.856, 0.328]) between strength-matched females and males. Based on the correlations, there were very large to nearly perfect relationships (r = 0.77-0.94, p <0.001) between peak force and rapid force production, with peak force explaining 59%, 77% and 89% of the variance in force at 150-, 200- and 250 ms, respectively. When comparing females and males, relative strength (based on body weight or a percentage of peak force) should be considered, and practitioners should be aware of the role of peak force in rapid force production.

Mechanical power production assessment during weightlifting exercises. A systematic review.

The assessment of the mechanical power production is of great importance for researchers and practitioners. The purpose of this review was to compare the differences in ground reaction force (GRF), kinematic, and combined (bar velocity x GRF) methods to assess mechanical power production during weightlifting exercises. A search of electronic databases was conducted to identify all publications up to 31 May 2019. The peak power output (PPO) was selected as the key variable. The exercises included in this review were clean variations, which includes the hang power clean (HPC), power clean (PC) and clean. A total of 26 articles met the inclusion criteria with 53.9% using the GRF, 38.5% combined, and 30.8% the kinematic method. Articles were evaluated and descriptively analysed to enable comparison between methods. The three methods have inherent methodological differences in the data analysis and measurement systems, which suggests that these methods should not be used interchangeably to assess PPO in Watts during weightlifting exercises. In addition, this review provides evidence and rationale for the use of the GRF to assess power production applied to the system mass while the kinematic method may be more appropriate when looking to assess only the power applied to the barbell.

Mechanical demands of kettlebell swing exercise.

The aims of this study were to establish mechanical demands of kettlebell swing exercise and provide context by comparing them to mechanical demands of back squat and jump squat exercise. Sixteen men performed 2 sets of 10 swings with 16, 24, and 32 kg, 2 back squats with 20, 40, 60, and 80% 1-repetition maximum (1RM), and 2 jump squats with 0, 20, 40, and 60% 1RM. Sagittal plane motion and ground reaction forces (GRFs) were recorded during swing performance, and GRFs were recorded during back and jump squat performances. Net impulse, and peak and mean propulsion phase force and power applied to the center of mass (CM) were obtained from GRF data and kettlebell displacement and velocity from motion data. The results of repeated measures analysis of variance showed that all swing CM measures were maximized during the 32-kg condition but that velocity of the kettlebell was maximized during the 16-kg condition; displacement was consistent across different loads. Peak and mean force tended to be greater during back and jump squat performances, but swing peak and mean power were greater than back squat power and largely comparable with jump squat power. However, the highest net impulse was recorded during swing exercise with 32 kg (276.1 ± 45.3 N·s vs. 60% 1RM back squat: 182.8 ± 43.1 N·s, and 40% jump squat: 231.3 ± 47.1 N·s). These findings indicate a large mechanical demand during swing exercise that could make swing exercise a useful addition to strength and conditioning programs that aim to develop the ability to rapidly apply force.

Kettlebell swing training improves maximal and explosive strength.

The aim of this study was to establish the effect that kettlebell swing (KB) training had on measures of maximum (half squat-HS-1 repetition maximum [1RM]) and explosive (vertical jump height-VJH) strength. To put these effects into context, they were compared with the effects of jump squat power training (JS-known to improve 1RM and VJH). Twenty-one healthy men (age = 18-27 years, body mass = 72.58 ± 12.87 kg) who could perform a proficient HS were tested for their HS 1RM and VJH pre- and post-training. Subjects were randomly assigned to either a KB or JS training group after HS 1RM testing and trained twice a week. The KB group performed 12-minute bouts of KB exercise (12 rounds of 30-second exercise, 30-second rest with 12 kg if <70 kg or 16 kg if >70 kg). The JS group performed at least 4 sets of 3 JS with the load that maximized peak power-Training volume was altered to accommodate different training loads and ranged from 4 sets of 3 with the heaviest load (60% 1RM) to 8 sets of 6 with the lightest load (0% 1RM). Maximum strength improved by 9.8% (HS 1RM: 165-181% body mass, p < 0.001) after the training intervention, and post hoc analysis revealed that there was no significant difference between the effect of KB and JS training (p = 0.56). Explosive strength improved by 19.8% (VJH: 20.6-24.3 cm) after the training intervention, and post hoc analysis revealed that the type of training did not significantly affect this either (p = 0.38). The results of this study clearly demonstrate that 6 weeks of biweekly KB training provides a stimulus that is sufficient to increase both maximum and explosive strength offering a useful alternative to strength and conditioning professionals seeking variety for their athletes.

Barbell kinematics should not be used to estimate power output applied to the Barbell-and-body system center of mass during lower-body resistance exercise.

The aim of this study was to compare measures of power output applied to the center of mass of the barbell and body system (CM) obtained by multiplying ground reaction force (GRF) by (a) the velocity of the barbell; (b) the velocity of the CM derived from three-dimensional (3D) whole-body motion analysis, and (c) the velocity of the CM derived from GRF during lower-body resistance exercise. Ten resistance-trained men performed 3 maximal-effort single back squats with 60% 1 repetition maximum while GRF and whole-body motion were captured using synchronized Kistler force platforms and a Vicon Motus motion analysis system. Repeated measures analysis of variance of time-normalized kinematic and kinetic data obtained using the different methods showed that the barbell was displaced 13.4% (p < 0.05) more than the CM, the velocity of the barbell was 16.1% (p < 0.05) greater than the velocity of the CM, and power applied to the CM obtained by multiplying GRF by the velocity of the barbell was 18.7% (p < 0.05) greater than power applied to the CM obtained by multiplying the force applied to the CM by its resultant velocity. Further, the velocity of the barbell was significantly greater than the velocity of the trunk, upper leg, lower leg, and foot (p < 0.05), indicating that a failure to consider the kinematics of body segments during lower-body resistance exercise can lead to a significant overestimation of power applied to the CM. Strength and conditioning coaches and investigators are urged to obtain measures of power from the force applied to and the velocity of either the barbell (using inverse dynamics) or CM (GRF or 3D motion analysis). Failure to apply these suggestions could result in continued overestimation of CM power, compromising methodological integrity.

Wearing knee wraps affects mechanical output and performance characteristics of back squat exercise.

The aim of this study was to investigate the effects of wearing knee wraps on mechanical output and performance characteristics of back squat exercise. Ten resistance trained men (back squat 1 repetition maximum [1RM]: 160.5 ± 18.4 kg) performed 6 single back squats with 80% 1RM, 3 wearing knee wraps, 3 without. Mechanical output was obtained from ground reaction force, performance characteristics from digitized motion footage obtained from a single high-speed digital camera. Wearing knee wraps led to a 39% reduction (0.09 compared with 0.11 m, p = 0.037) in horizontal barbell displacement that continued during the lifting phase. Lowering phase vertical impulse remained within 1% across conditions; however, the lowering phase was performed 45% faster (1.13 compared with 1.57 seconds). This demonstrated that vertical force applied to the center of mass during the lowering phase was considerably larger and was likely a consequence of the generation and storage of elastic energy within the knee wrap. Subsequent vertical impulse applied to the center of mass was 10% greater (192 compared with 169 N·s, p = 0.018). Mechanical work involved in vertically displacing the center of mass was performed 20% faster and was reflected by a 10% increase in peak power (2,121 compared with 1,841 W, p = 0.019). The elastic properties of knee wraps increased mechanical output but altered back squat technique in a way that is likely to alter the musculature targeted by the exercise and possibly compromise the integrity of the knee joint. Knee wraps should not be worn during the strength and condition process, and perceived weakness in the knee joint should be assessed and treated.

Identifying and reporting position-specific countermovement jump outcome and phase characteristics within rugby league.

The countermovement jump (CMJ) has been suggested to be an important test of neuromuscular performance for rugby league (RL) players. Identifying force platform-derived CMJ variables that may be more applicable to RL positions (e.g., forwards and backs) has yet to be fully explored in the scientific literature. The aim of this study was to identify RL position-specific CMJ force-time variables. Specifically, we aimed to compare select force-time variables from the countermovement (i.e., the combination of unweighting and braking) and propulsion phases of the CMJ between the global forwards and backs positional groups. We also aimed to compare typical (i.e., jump height) and alternative (i.e., take-off momentum) outcome CMJ variables between positional groups. Finally, we sought to visually present each individual player's CMJ performance alongside the average data to facilitate the interpretation and reporting of the CMJ performances of RL athletes. Twenty-seven forwards and twenty-seven backs who competed in the senior men's English RL Championship, performed three CMJs on a force platform at the beginning of the pre-season training period. There were no significant differences in any countermovement or propulsion phase variable between positions with just small effect sizes noted (P ≥0.09, d ≤0.46). Jump height (and so take-off velocity) was significantly greater for backs with moderate effects displayed (P = 0.03, d = 0.60). Take-off momentum (take-off velocity × body mass) was largely and significantly greater for forwards (P<0.01, d = 1.01). There was considerable overlap of individual player's body mass and CMJ outcome variables across positions, despite significant differences in the mean values attained by each positional group. The results suggest that it may be beneficial for RL practitioners to identify player-specific, or at least position-specific, variables. As a minimum, it may be worthwhile selecting CMJ force-time variables based on what is considered important to individual player's or small clusters of similar players' projected successes during RL competition.