Familiarization and Reliability of the Isometric Knee Extension Test for Rapid Force Production Assessment

Despite the rising interest in the use of portable force sensors during isometric exercises to inform on neuromuscular performance, the design of practical field-based methods to obtain reliable measures is an ongoing challenge. We aim at identifying the intra-session and test-retest reliability of a rapid, isometric knee extension test to evaluate the maximal voluntary concentric force (MVC), rate of force development (RFD) and impulse following a field-based approach. On two occasions, 14 athletes unfamiliar with the test completed three sets of 2 s ballistic contractions (as fast and hard as possible) with 30 s rest. Raw and filtered data were collected in real time using a portable force sensor. RFD and impulse were highly reliability during “late” phases of the contraction (0–250 ms) since the first session (coefficient of variation (CV) < 9.8%). Earlier phases (0–150 ms) achieved a moderate reliability after one familiarization session (CV < 7.1%). Measures at 0–50 ms did not reach sufficient reliability (CV~14%). MVC was accurately assessed. Dominant limbs were not importantly altered by the familiarization. In opposite, non-dominant limbs showed large variations. New evidence is provided about the positive effects of a single familiarization session to improve the reliability the isometric knee extension test for rapid force production assessment. Coaches and practitioners may benefit of from these findings to conduct practical and reliable assessments of the rapid force production using a portable force sensor and a field-based approach.

GROUND REACTION FORCES OF COMMONLY USED VOLLEYBALL BLOCKING APPROACHES

Aim: The purpose of this study was to quantify ground reaction forces for some of the most commonly utilised volleyball blocking approaches and to examine their kinetic and kinematic characteristics. Basic procedures: The study was comprised of 18 healthy recreationally active women who volunteered to participate. Immediately after completion of the warm-up protocol, subjects performed 5 blocking approaches: stationary blocking approach (SBA), shuffle block to the right (SHBR), shuffle block to the left (SHBL), swing block to the right (SWBR) and swing block to the left (SWBL). In order to allow adequate recovery, each trial was randomly assigned and separated by a 1-2 minute rest interval. A uni-axial force plate with data acquisition system sampling at 1000 Hz was used to measure ground reaction forces. Main findings: SWBR and SWBL unveiled the greatest peak concentric force and rate of force development when compared to SBA, while no difference was observed when compared to SHBR and SHBL. Results: No significant differences were observed in peak landing force, impulse, and vertical jump height between any of the blocking approaches examined in this study. Conclusions: Knowing biomechanical characteristics of some of the most commonly utilised volleyball blocking approaches may help athletes to appropriately respond and quickly adjust to the opponent’s attacking position. Kinetic and kinematic variables are likely to be augmented with an advanced level of competition and can be trained and improved by properly designed and implemented strength and conditioning programmes.

Using Drop Jumps and Jump Squats to Assess Eccentric and Concentric Force-Velocity Characteristics

The purpose of this study was to investigate the eccentric and concentric force-velocity (Fv) characteristics recorded during drop jumps (DJ) from different heights and loaded jump squats (JS) and to determine the number of jumps required to accurately model the eccentric and concentric Fv relationships. Fourteen resistance-trained men (age: 21.9 ± 1.8 years) performed a countermovement jump (CMJ) and DJ from heights of 0.40, 0.60, and 0.80 m. JS with loads equivalent to 0%, 27%, 56%, and 85% 1-repetition maximum were performed in a separate session. Force platforms and a 3-D motion analysis system were used to record the average force ( F ¯ ) and velocity ( v ¯ ) during the absorption (CMJ, DJ40, DJ60, DJ80) and propulsion (JS0, JS27, JS56, JS85) phases of the jumps. Eccentric (absorption phase) and concentric (propulsion phase) Fv characteristics were then calculated and linear regression equations were determined when the number of jumps included was varied. F ¯ during the absorption phase significantly increased from CMJ to DJ60 while v ¯ increased significantly from CMJ to DJ80. The two-point method (CMJ, DJ80) resulted in a significantly lower y-intercept (mean difference [MD]: 0.7 N/kg) and a greater slope (MD: 0.7 Ns/m) for the eccentric Fv characteristics compared to the multiple-point method. F ¯ increased significantly and v ¯ decreased significantly with increasing external load in the JS conditions. The two-point method (JS0, JS85) resulted in a significantly greater y-intercept (MD: 1.1 N/kg) compared to the multiple-point method for the concentric Fv characteristics. Both DJ and loaded JS may provide means of assessing the eccentric and concentric Fv characteristics with only two jumps being required.

The effects of low and moderate doses of caffeine supplementation on upper and lower body maximal voluntary concentric and eccentric muscle force

Despite the growing quantity of literature exploring the effect of caffeine on muscular strength, there is a dearth of data that directly explores differences in erogenicity between upper and lower body musculature and the dose–response effect. The present study sought to investigate the effects of low and moderate doses of caffeine on the maximal voluntary strength of the elbow flexors and knee extensors. Ten nonspecifically strength-trained, recreationally active participants (aged 21 ± 0.3 years) completed the study. Using a randomised, counterbalanced, and double-blind approach, isokinetic concentric and eccentric strength was measured at 60 and 180°/s following administration of a placebo, 3 mg·kg−1 body mass caffeine, and 6 mg·kg−1 body mass caffeine. There was no effect of caffeine on the maximal voluntary concentric and eccentric strength of the elbow flexors, or the eccentric strength of the knee extensors. Both 3 and 6 mg·kg−1 body mass caffeine caused a significant increase in peak concentric force of the knee extensors at 180°/s. No difference was apparent between the 2 concentrations. Only 6 mg·kg−1 body mass caused an increase in peak concentric force during repeated contractions. The results infer that the effective caffeine concentration to evoke improved muscle performance may be related to muscle mass and contraction type. The present work indicates that a relatively low dose of caffeine treatment may be effective for improving lower body muscular strength, but may have little benefit for the strength of major muscular groups of the upper body.

Concurrent validity and test-retest reliability of the barbell hip-thrust exercise

Purpose: The barbell hip thrust (BHT) exercise is growing in popularity as evident by the large increase in research outputs investigating its utility as a training intervention and a testing tool. The aim of this study was to examine the concurrent validity and test-retest reliability of the BHT for the first time. Methods: To establish concurrent validity, the peak force and power measured with the BHT force-velocity profile test of 49 handball athletes, were correlated with peak concentric force of the knee-flexors and hip-extensors measured with an isokinetic device at two different velocities. Test-retest reliability was established by correlating the peak force and power measured with the BHT force-velocity profile test of 20 handball athletes on two separate days. Results: The relationships between the isokinetic testing scores and the BHT force-velocity profile tests were moderate to strong (Pearson r ranges: 0.45-0.86). Test-retest reliability of the BHT force-velocity profile was very high as shown with intra-class correlations of 0.94 and 0.99 for peak force and 0.97 and 0.99 for peak power measures. Conclusion: The BHT force-velocity profile can serve as a tentative substitute in cases that athletes do not have access to an isokinetic device given the moderate to strong correlations between them. Moreover, the BHT force-velocity profile was shown to be very reliable thus providing coaches and scientists a range of day-to-day performance variability in this exercise.

The Effect of a Simulated Soccer Match on Anterior Cruciate Ligament Injury Risk Factors

To investigate the effect of within match fatigue on knee kinematics and jump kinetics in girls’ soccer players, a quasi-experiment time series design was employed collecting data before, after and at 15-min intervals during a 90-min simulated soccer match. 15 girl players (age 13.1±1.4 years) performed a counter movement jump and a single-leg drop jump. Mean concentric force and flight time to contraction time ratio were derived from the counter movement jump. Knee valgus and flexion angles were calculated during the single-leg drop from 3-dimensional motion capture. Subjective ratings of perceived exertion (RPE) and readiness were collected at each time series. Small to large increases in RPE and reductions in readiness were observed throughout the match from baseline. Moderate to large improvements in mean concentric force were shown at 15, 75 and 90-min when compared to baseline. Flight time to contraction time ratio increased moderately at 15 min. Changes in kinematics were typically trivial or unclear however, small increases in knee valgus were shown after 30 min compared to baseline. Subjective measures may provide useful information to understand the physical response of young players to match play.