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Christopher Thomas, Paul A. Jones, and Paul Comfort

Purpose:

To determine the reliability of the Dynamic Strength Index (DSI) in college athletes.

Method:

Nineteen male college athletes performed the squat jump (SJ) and isometric midthigh pull (IMTP) to determine peak force, on 2 separate days. Reliability was assessed by intraclass correlation coefficient (ICC), typical error (TE), percentage change in the mean, smallest worthwhile change (SWC), and coefficient of variation (%CV).

Results:

Peak force for the SJ was 2137 ± 499 N and 2781 ± 435 N for the IMTP, resulting in a mean DSI of 0.78 ± 0.19. Peak forces in the SJ (ICC = .99, TE = 57.22 N, change in mean = 0.2%, SWC = 4.7%, CV = 2.6%) and IMTP (ICC = .95, TE = 104.22 N, change in mean = 0.5%, SWC = 3.1%, CV = 3.8%) were considered highly reliable between sessions. However, IMTP peak force was the only variable with an overall TE < SWC. The DSI was also highly reliable (ICC = .97, TE = 0.03, change in mean = −0.3%, SWC = 5.1%, CV = 4.6%) between sessions.

Conclusion:

This study demonstrates that peak force in the SJ and IMTP are reliable, resulting in a reliable assessment of dynamic-force-production capabilities via the DSI. The DSI may be used to guide individualized training interventions and monitor specific adaptations to training. Changes in SJ peak force, IMTP peak force, and DSI were >4.67%, 3.13%, and 5.13%, respectively, identifying meaningful changes in response to training or competition.

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Adam Grainger, Paul Comfort, and Shane Heffernan

Purpose: Partial body cryotherapy (PBC) has been shown to be beneficial for postexercise recovery; however, no study has demonstrated the effectiveness of PBC for recovery following elite rugby union training. Rugby union is a unique sport that involves high-velocity collisions and may induce greater performance decrements than other sports; thus, PBC could be beneficial. The application of PBC in “real world” has rarely been investigated during the competitive phase of a playing season and warranted investigation. Methods: In a counterbalanced sequential research design, professional rugby athletes (n = 18; age 25.4 [4.0] y; training age 7.2 [4.0] y; mass 99.8 [10.6] kg; height 188.3 [6.0] cm) were assigned to a 12-week PBC intervention, washout period (4 wk), and reassessed as their own controls. Self-reported well-being, muscle soreness, sleep quality, and countermovement jump height were assessed before and 40 hours after “real-world” training. Wilcoxon signed-rank tests and Cohen d were used for statistical analysis. Results: No differences were observed between PBC and control conditions (P > .05; d = 0.00–0.14) for well-being (−0.02% [0.08%] vs 0.01% [0.06%]), muscle soreness (−0.01% [0.11%] vs 0.01% [0.16%]), sleep quality (−0.03% [0.14%] vs 0.10% [0.29%]), or countermovement jump height (36.48–36.59 vs 38.13–37.52 cm; P = .54). Conclusions: These results suggest PBC is ineffective for the restoration of selected performance parameters during the performance maintenance phase of the competitive season. To ascertain the appropriation of its use, future investigations should seek to assess the use of cryotherapies at various phases of the elite rugby union competitive season.

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Allan Munro, Lee Herrington, and Paul Comfort

Context:

Injuries to the anterior cruciate ligament (ACL) and patellofemoral joint (PFJ) are a significant problem in female athletes. A number of screening tasks have been used in the literature to identify those at greatest risk of injury. To date, no study has examined the relationship in 2-dimensional (2D) knee valgus between common screening tasks to determine whether individuals exhibit similar movement patterns across tasks.

Objective:

To establish whether frontal-plane projection angle (FPPA) during the single-leg squat (SLS), single-leg land (SLL), and drop jump (DJ) are related.

Design:

Cross-sectional study.

Setting:

University laboratory.

Participants:

52 national-league female football players and 36 national-league female basketball players.

Main Outcome Measures:

2D FPPA during the SLS, SLL, and DJ screening tasks.

Results:

Significant correlations were found between tasks. FPPA in the SLS was significantly correlated with SLL (r = .52) and DJ (r = .30), whereas FPPA in the SLL was also significantly correlated to DJ (r = .33). FPPA was significantly greater in the SLS than in the SLL (P < .001) and DJ (P < .001) and in the SLL than in the DJ (P < .001).

Conclusion:

The results showed that 2D FPPA is correlated across the SLS, SLL, and DJ tasks. However, significantly greater FPPA values in the unilateral tasks suggest that the DJ may not identify risk of injury in sports where primary injury mechanisms are during unilateral loading tasks. Therefore, it is recommended that both unilateral and bilateral tasks be included when screening for ACL and PFJ injury risk.

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Christina Carr, John J. McMahon, and Paul Comfort

Purpose:

Previous research has investigated changes in athletes’ strength, power, and speed performances across the competitive season of many sports, although this has not been explored in cricketers. The aim of this study was to investigate changes in lower-body strength and jump and sprint performances across an English county cricket season.

Methods:

Male cricketers (N = 12; age 24.4 ± 2.3 y, body mass 84.3 ± 9.9 kg, height 184.1 ± 8.1 cm) performed countermovement jumps (CMJs) and 20-m sprints on 4 separate occasions and back-squat strength testing on 3 separate occasions across a competitive season.

Results:

Both absolute (12.9%, P = .005, effect size [ES] = 0.53) and relative lower-body strength (15.8%, P = .004, ES = 0.69) and CMJ height (5.3%, P = .037, ES = 0.42) improved significantly over the preseason training period, although no significant change (1.7%, P > .05) in sprint performance was observed. In contrast, absolute (14.3%, P = .001, ES = 0.72) and relative strength (15.0%, P = .001, ES = 0.77), CMJ height (4.2%, P = .023, ES = 0.40), and sprint performance (3.8%, P = .012, ES = 0.94) declined significantly across the season.

Conclusions:

The results of this study show that neither the demands of the competitive cricket season nor current in-season training practices provide a sufficient stimulus to maintain strength, jump, and sprint performances in these cricketers. Therefore, coaches should implement a more-frequent, higher-load strength-training program across the competitive cricket season.

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Christopher Thomas, Paul Comfort, Paul A. Jones, and Thomas Dos’Santos

Purpose:

To investigate the relationships between maximal isometric strength, vertical jump (VJ), sprint speed, and change-of-direction speed (CoDS) in academy netball players and determine whether players who have high performance in isometric strength testing would demonstrate superior performance in VJ, sprint speed, and CoDS measures.

Method:

Twenty-six young female netball players (age 16.1 ± 1.2 y, height 173.9 ± 5.7 cm, body mass 66.0 ± 7.2 kg) from a regional netball academy performed isometric midthigh pull (IMTP), squat jumps (SJs), countermovement jumps (CMJs), 10-m sprints, and CoDS (505).

Results:

IMTP measures displayed moderate to strong correlations with sprint and CoDS performance (r = –.41 to –.66). The VJs, which included SJs and CMJs, demonstrated strong correlations with 10-m sprint times (r = –.60 to –.65; P < .01) and CoDS (r = –.60 to –.71; P = .01). Stronger players displayed significantly faster sprint (ES = 1.1–1.2) and CoDS times (ES = 1.2–1.7) and greater VJ height (ES = 0.9–1.0) than weaker players.

Conclusion:

The results of this study illustrate the importance of developing high levels of lower-body strength to enhance VJ, sprint, and CoDS performance in youth netball players, with stronger athletes demonstrating superior VJ, sprint, and CoDS performances.

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Thomas Dos’Santos, Christopher Thomas, Paul A. Jones, and Paul Comfort

Purpose:

To investigate the within-session reliability of bilateral- and unilateral-stance isometric midthigh-pull (IMTP) force–time characteristics including peak force (PF), relative PF, and impulse at time bands (0–100, 0–200, 0–250, and 0–300 milliseconds) and to compare isometric force–time characteristics between right and left and dominant (D) and nondominant (ND) limbs.

Methods:

Professional male rugby league and multisport male college athletes (N = 54; age, 23.4 ± 4.2 y; height, 1.80 ± 0.05 m; mass, 88.9 ± 12.9 kg) performed 3 bilateral IMTP trials and 6 unilateral-stance IMTP trials (3 per leg) on a force plate sampling at 600 Hz.

Results:

Intraclass correlation coefficients (ICCs) and coefficients of variation (CVs) demonstrated high within-session reliability for bilateral and unilateral IMTP PF (ICC = .94, CV = 4.7–5.5%). Lower reliability measures and greater variability were observed for bilateral and unilateral IMTP impulse at time bands (ICC = .81–.88, CV = 7.7–11.8%). Paired-sample t tests and Cohen d effect sizes revealed no significant differences for all isometric force–time characteristics between right and left limbs in male college athletes (P >.05, d ≤ 0.32) and professional rugby league players (P > .05, d ≤ 0.11); however, significant differences were found between D and ND limbs in male college athletes (P < .001, d = 0.43–0.91) and professional rugby league players (P < .001, d = 0.27–0.46).

Conclusion:

This study demonstrated high within-session reliability for unilateral-stance IMTP PF, revealing significant differences in isometric force–time characteristics between D and ND limbs in male athletes.

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Paul Comfort, Paul. A. Jones, John J. McMahon, and Robert Newton

The isometric midthigh pull (IMTP) has been used to monitor changes in force, maximum rate of force development (mRFD), and impulse, with performance in this task being associated with performance in athletic tasks. Numerous postures have been adopted in the literature, which may affect the kinetic variables during the task; therefore, the aim of this investigation was to determine whether different knee-joint angles (120°, 130°, 140°, and 150°) and hip-joint angles (125° and 145°), including the subjects preferred posture, affect force, mRFD, and impulse during the IMTP. Intraclass correlation coefficients demonstrated high within-session reliability (r ≥ .870, P < .001) for all kinetic variables determined in all postures, excluding impulse measures during the 130° knee-flexion, 125° hip-flexion posture, which showed a low to moderate reliability (r = .666–.739, P < .001), while between-sessions testing demonstrated high reliability (r > .819, P < .001) for all kinetic variables. There were no significant differences in peak force (P > .05, Cohen d = 0.037, power = .408), mRFD (P > .05, Cohen d = 0.037, power = .409), or impulse at 100 ms (P > .05, Cohen d = 0.056, power = .609), 200 ms (P > .05, Cohen d = 0.057, power = .624), or 300 ms (P > .05, Cohen d = 0.061, power = .656) across postures. Smallest detectable differences demonstrated that changes in performance of >1.3% in peak isometric force, >10.3% in mRFD, >5.3% in impulse at 100 ms, >4.4% in impulse at 200 ms, and >7.1% in impulse at 300 ms should be considered meaningful, irrespective of posture.

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Christopher Thomas, Thomas Dos’Santos, Paul A. Jones, and Paul Comfort

Purpose:

The purpose of this investigation was to determine the reliability of the 30-15 Intermittent Fitness Test (30-15IFT) in semiprofessional soccer players.

Methods:

Fourteen male semiprofessional soccer players performed the 30-15IFT on 2 occasions separated by 7 d. Reliability was assessed by intraclass correlation coefficient (ICC), typical error of measurement expressed as a coefficient of variation (CV), and smallest worthwhile change (SWC) to determine any significant difference between testing sessions.

Results:

Maximal intermittent running velocity (VIFT) demonstrated good reliability (ICC = .80) for between-sessions reliability. The CV was 2.5% for between-sessions reliability of the 30-15IFT. As the SWC (0.70 km/h) falls within the range in which the individual’s true score is likely to lie (1.0 km/h), the usefulness of the VIFT was rated as marginal. Despite the usefulness of the 30-15IFT being deemed marginal, a change in performance as small as 1.0 km/h (2 stages) in VIFT could be considered substantial or real.

Conclusion:

This study demonstrates that VIFT in the 30-15IFT is reliable, resulting in a reliable assessment of team-sport-specific cardiorespiratory fitness, with changes as small as 1.0 km/h (2 stages) in VIFT considered meaningful.

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John J. McMahon, Paul A. Jones, and Paul Comfort

Purpose:

To determine the concurrent validity and reliability of the popular Just Jump system (JJS) for determining jump height and, if necessary, provide a correction equation for future reference.

Methods:

Eighteen male college athletes performed 3 bilateral countermovement jumps (CMJs) on 2 JJSs (alternative method) that were placed on top of a force platform (criterion method). Two JJSs were used to establish consistency between systems. Jump height was calculated from flight time obtained from the JJS and force platform.

Results:

Intraclass correlation coefficients (ICCs) demonstrated excellent within-session reliability of the CMJ height measurement derived from both the JJS (ICC = .96, P < .001) and the force platform (ICC = .96, P < .001). Dependent t tests revealed that the JJS yielded a significantly greater CMJ jump height (0.46 ± 0.09 m vs 0.33 ± 0.08 m) than the force platform (P < .001, Cohen d = 1.39, power = 1.00). There was, however, an excellent relationship between CMJ heights derived from the JJS and force platform (r = .998, P < .001, power = 1.00), with a coefficient of determination (R 2) of .995. Therefore, the following correction equation was produced: Criterion jump height = (0.8747 × alternative jump height) – 0.0666.

Conclusions:

The JJS provides a reliable but overestimated measure of jump height. It is suggested, therefore, that practitioners who use the JJS as part of future work apply the correction equation presented in this study to resultant jump-height values.

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Jessica A. Calderbank, Paul Comfort, and John J. McMahon

Purpose: The aim of the current study was to investigate the relationship between dive distance (DD) and countermovement jump (CMJ) height, track start CMJ height, countermovement broad jump (CMBJ) distance, track start broad jump distance, and isometric midthigh pull peak force and relative peak force. Methods: A total of 27 (11 female and 16 male) regional-national-international-standard swimmers (mean [SD]; age = 19.5 [5.5] y; mass = 69.3 [10.5] kg; height = 1.77 [0.09] m) performed 3 trials of a track start dive, CMJ, track start CMJ, CMBJ, track start broad jump, and isometric midthigh pull. Results: Data were separated into pooled (females and males combined), females, and males. Large to very large correlations were found between DD and all variables tested for pooled data (r = .554–.853, P < .001–.008), with DD-CMBJ displaying the highest correlation (r = .853, P < .001). CMBJ accounted for 70% of the variance in DD. Females demonstrated moderate nonsignificant correlations between DD isometric midthigh pull (r = .379, P < .125). Males demonstrated very large significant correlations between DD-CMJ (r = .761, P < .001). Conclusions: DD demonstrated strong correlations with jump performances and multijoint isometric force production in pooled data. Males showed stronger correlations than females due to being stronger and being able to perform the jumping/strength tasks to a higher standard. Enhanced jump performance and increased maximal force production may, therefore, enhance DD in swimmers.