Isometric knee-extensor torque development and jump height in volleyball players

PURPOSE: The goal of the present study was to determine the contribution of the intrinsic muscle properties and muscle activation of the knee extensors to the maximal rate of unilateral isometric torque development and to relate both factors to maximal bilateral jumping performance in experienced jumpers. On the basis of previous studies, we hypothesized that maximal rate of torque development during maximal effort isometric contractions and jump height would depend on the subjects' ability for maximal muscle activation rather than on the muscle's contractile properties. METHODS: Eleven male elite volleyball players (20 +/- 2 yr, means +/- SD) performed squat jumps starting from a 120 degrees knee angle (SJ120; full extension = 180 degrees ) and countermovement jumps. In addition, maximal voluntary and electrically evoked unilateral isometric knee-extension torque development (120 degrees angle) was obtained. Torque time integral for the first 40 ms after torque onset (TTI40) and (time to) maximal rate of torque development (MRTD) were calculated. Muscle activation was quantified using surface EMG. RESULTS: Voluntary TTI40 was significantly related to the preceding EMG (r2 = 0.83) and negatively related to the time to MRTD (r2 = 0.64). Voluntary MRTD and TTI40 were not related to their respective values obtained during electrical stimulation (r2 < 0.04). Only electrically evoked MRTD was significantly related to jump height (e.g., r2 = 0.70 for SJ120). CONCLUSIONS: As expected initial maximal voluntary isometric torque development correlated with muscle activation and not with muscle contractile speed. However, unexpectedly, only the latter could predict jump performance in skilled jumpers.     

Electrostimulation training effects on the physical performance of ice hockey players

PURPOSE: The aim of this study was to examine the influence of a short-term electromyostimulation (EMS) training program on the strength of knee extensors, skating, and vertical jump performance of a group of ice hockey players. METHODS: Seventeen ice hockey players participated in this study, with nine in the electrostimulated group (ES) and the remaining height as controls (C). EMS sessions consisted of 30 contractions (4-s duration, 85 Hz) and were carried out 3x wk for 3 wk. Isokinetic strength of the knee extensor muscles was determined with a Biodex dynamometer at different eccentric and concentric angular velocities (angular velocities ranging from -120 to 300 degrees .s). Jumping ability was evaluated during squat jump (SJ), countermovement jump (CMJ), drop jump (DJ), and 15 consecutive CMJ (15J). Sprint times for 10- and 30-m skates in specific conditions were measured using an infrared photoelectric system. RESULTS: After 3 wk of EMS training, isokinetic torque increased significantly (P<0.05) for ES group in eccentric (-120 and -60 degrees .s) and concentric conditions (60 and 300 degrees .s), whereas vertical jump height decreased significantly (P<0.05) for SJ (-2.9+/-2.4 cm), CMJ (-2.1+/-2.0 cm), and DJ (-1.3+/-1.1 cm). The 10-m skating performance was significantly improved (from 2.18+/-0.20 to 2.07+/-0.09 s, before and after the 3-wk EMS period, respectively; P<0.05). CONCLUSION: It was demonstrated that an EMS program of the knee extensors significantly enhanced isokinetic strength (eccentric and for two concentric velocities) and short skating performance of a group of ice hockey players.     

Electromyographic and force production characteristics of leg extensor muscles of elite weight lifters during isometric, concentric, and various stretch-shortening cycle exercises

Electromyographic and force production characteristics of leg extensor muscles of elite weight lifters (n = 14) were examined during isometric, concentric, and various stretch-shortening cycle exercises. The data showed that utilization of stored elastic energy of leg extensor muscles was observable in countermovement jumping (CMJ) conditions performed at various loads. These greater (P less than 0.01-0.001) vertical jumping heights obtained in CMJ conditions in comparison to the respective squat jumps (SJ) took place at all loads examined from 0 up to 180 kg with no differences in the average IEMG activity between the concentric phases of the performances. In various dropping jumps (from 20 to 100 cm), the average jumping heights were not, however, greater than in the squat jump. The jumping heights both in SJ and CMJ correlated negatively (r = -0.56 and -0.58; P less than 0.05) with the time of isometric force production. Maximal isometric force correlated (r = 0.61; P less than 0.01) with the thigh girth but not (r = 0.22; NS) with the mean muscle fiber area of the vastus lateralis muscle. The jumping heights in SJ and CMJ performed with higher (100-140 kg) loads correlated more significantly (r = 0.76-0.79; P less than 0.01-0.001) than that of maximal isometric force (r = 0.47-0.67; P less than 0.05-0.01) with the results in the Olympic weight lifting. The present findings indicate the important role of effective recoil of the elastic energy in the muscle during the stretch-shortening cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Is the effect of a countermovement on jump height due to active state development?

PURPOSE: To investigate whether the difference in jump height between countermovement jumps (CMJ) and squat jumps (SJ) could be explained by a difference in active state during propulsion. METHODS: Simulations were performed with a model of the human musculoskeletal system comprising four body segments and six muscles. The model's only input was STIM, the stimulation of muscles, which could be switched "off" or "on." After switching "on," STIM increased to its maximum at a fixed rate of change (dSTIM/dt). For various values of dSTIM/dt, stimulation switch times were optimized to produce a maximum height CMJ. From this CMJ, the configuration at the lowest height of the center of gravity (CG) was selected and used as static starting configuration for simulation of SJ. Next, STIM-switch times were optimized to find the maximum height SJ. RESULTS: Simulated CMJ and SJ closely resembled jumps of human subjects. Maximum jump height of the model was greater in CMJ than in SJ, with the difference ranging from 0.4 cm at infinitely high dSTIM/dt to about 2.5 cm at the lowest dSTIM/dt investigated. The greater jump height in CMJ was due to a greater work output of the hip extensor muscles. These muscles could produce more force and work over the first 30% of their shortening range in CMJ, due to the fact that they had a higher active state in CMJ than in SJ. CONCLUSION: The greater jump height in CMJ than in SJ could be explained by the fact that in CMJ active state developed during the preparatory countermovement, whereas in SJ it inevitably developed during the propulsion phase, so that the muscles could produce more force and work during shortening in CMJ.     

Effects of plyometric and weight training on muscle-tendon complex and jump performance

PURPOSE: The purpose of this study was to investigate the effects of plyometric and weight training protocols on the mechanical properties of muscle-tendon complex and muscle activities and performances during jumping. METHODS: Ten subjects completed 12 wk (4 d.wk(-1)) of a unilateral training program for plantar flexors. They performed plyometric training on one side (PT; hopping and drop jump using 40% of 1RM) and weight training on the other side (WT; 80% of 1RM). Tendon stiffness was measured using ultrasonography during isometric plantar flexion. Three kinds of unilateral jump heights using only ankle joint (squat jump: SJ; countermovement jump: CMJ; drop jump: DJ) on sledge apparatus were measured. During jumping, electromyographic activities were recorded from plantar flexors and tibial anterior muscle. Joint stiffness was calculated as the change in joint torque divided by the change in ankle angle during eccentric phase of DJ. RESULTS: Tendon stiffness increased significantly for WT, but not for PT. Conversely, joint stiffness increased significantly for PT, but not for WT. Whereas PT increased significantly jump heights of SJ, CMJ, and DJ, WT increased SJ only. The relative increases in jump heights were significantly greater for PT than for WT. However, there were no significant differences between PT and WT in the changes in the electromyographic activities of measured muscles during jumping. CONCLUSION: These results indicate that the jump performance gains after plyometric training are attributed to changes in the mechanical properties of muscle-tendon complex, rather than to the muscle activation strategies.     

Effects of power and flexibility training on vertical jump technique

PURPOSE: The purpose of this study was to assess the effects of power and flexibility training on countermovement and drop jump techniques. METHOD: All jumps were executed with the goal of attaining maximum height and no restrictions were placed on the magnitude of countermovement or ground contact time. Subjects underwent initial testing followed by random allocation to one of four groups: power training to increase vertical jump height (P), stretching to increase flexibility (S), a combination of power and stretch training (PS), and a control group (C). Training lasted for 10 wk, followed by retesting. Jump height was calculated in addition to the following technique variables: eccentric lower-limb stiffness produced during the countermovement phase, magnitude of countermovement, and in the case of the drop jumps, ground contact time. RESULTS: Groups PS, P, and S all increased countermovement jump (CMJ) height, but only groups PS and P increased drop jump height (DJ30, DJ60, and DJ90 for drop jumps performed from 30-, 60-, and 90-cm drop heights). The technique changes associated with power training were increases in magnitude of countermovement (CMJ, DJ30, DJ60, and DJ90) and increases in ground contact time (DJ30 and DJ60). In addition, the eccentric lower-limb stiffness produced during the countermovement phase of the jumps increased for CMJ and decreased for DJ30, DJ60, and DJ90. Stretching appeared to have no significant effect on CMJ or drop jump technique. CONCLUSION: The results of this study show that when the training goal is maximum jump height alone, it is likely that drop jump technique will change in the direction of a lower eccentric leg stiffness, greater depth of countermovement, and a longer ground contact time, whereas for a countermovement jump eccentric leg stiffness and the depth of countermovement will both increase. It is proposed that these technique changes are a result of attempting to optimize a complex combination of factors involved in jumping (e.g., utilization of elastic energy, Golgi tendon organ inhibition, and contractile component contribution).     

Effects and mechanisms of strength training in children

It has been demonstrated that strength training can be organized in children in a safe and effective way. However, there is limited data regarding its impact on muscle hypertrophy. This study investigated the effects of a high-intensity strength training (HIS) on knee extensor/flexor strength, countermovement (CMJ) jumping height, postural control, soft lean mass and muscle cross-sectional area (CSA) of the dominant leg in prepubertal children. Thirty-two children participated in this study and were assigned to an intervention (INT; N=17) or a control class ( N=15). The INT participated in 10 weeks of weight-machine based HIS integrated in physical education. Pre/post tests included the measurements of peak torque of the knee extensors/flexors at 60 and 180°/s, CMJ jumping height, postural sway, soft lean mass of the leg by bioelectrical impedance analysis, and CSA (m. quadriceps) by magnetic resonance imaging. HIS resulted in significant increases in knee extensor/flexor peak torque (60°/s and 180°/s). HIS did not produce significant changes in CMJ jumping height, postural sway, soft lean mass, and CSA. Although HIS was effective at increasing peak torque of the knee extensors/flexors in children, it was unable to affect muscle size. It appears that neural factors rather than muscle hypertrophy account for the observed strength gains in children.     

Electromyostimulation and plyometric training effects on jumping and sprint time

This study compared the effects of four-week training periods of electromyostimulation (EMS), plyometric training (P), or combined EMS and P training of the knee extensor muscles on 20 m sprint time (ST), jumping ability (Squat jump [SJ] and Countermovement jump [CMJ]), maximal isometric strength (MVC), and muscle cross-sectional area (CSA). Forty subjects were randomly assigned to one of the four treatment groups: electromyostimulation (EG), plyometric (PG), combined EMG, and P (EPG), that took place 4 times per week, and a control group (CG). Subjects were tested before and after the training program, as well as once more after 2 wk of detraining. A significant improvement (p < 0.05) in ST was observed after training (2.4 %) in EG while a significant slowing (p < 0.05) was observed (- 2.3 %) in EPG. Significant increases in EPG (p < 0.05) were observed in SJ (7.5 %) and CMJ (7.3 %) after training, while no significant changes in both jumps were observed after training and detraining for EG. A significant increase (p < 0.05) in MVC was observed after training (9.1 %) and after detraining (8.1 %) in EG. A significant increase (p < 0.05) in MVC was observed after training (16.3 %) in EPG. A significant increase (p < 0.01) in CSA was observed after training in EG (9.0 %) and in EPG (7.1 %). EMS combined with plyometric training increased the jumping height and sprint run in physically active men. In addition, EMS alone or EMS combined with plyometric training leads to increase maximal strength and to some hypertrophy of trained muscles. However, EMS training alone did not result in any improvement in jumping explosive strength development or even interfered in sprint run.     

Resistance training in older women: effect on vertical jump and functional performance

AIM: Few studies have determined the effects of a heavy resistance training on vertical jump performance in older adults. The purpose of the present study was to evaluate the effects of a 12 week heavy resistance training in vertical jump and functional performance in healthy, inactive, older women. METHODS: Seventeen sedentary older women (aged 53-69 years) were assigned either to a control (C, n=8), or to a heavy resistance training (RT, n=9). The RT group trained knee extensors and flexors at 80% of 1-repetition maximum (1-RM), 3 days per week for 12 weeks. Both groups were evaluated in 1-RM lower body strength, squat jump (SJ) and countermovement jump (CMJ) height, and chair rising time, before and after the training period. RESULTS: After the training period, RT significantly improved (P<0.001) the 1-RM lower body strength, SJ, CMJ height, and chair rising by 68.9%, 24.5%, 21.7%, and 13%, respectively. Significant correlations were observed among the percent (%) change in 1-RM lower body strength and SJ (r=0.849, P<0.001), CMJ height (r=0.797, P<0.001), and chair rising time (r=-0.917, P<0.001). CONCLUSIONS: Muscle power improved after a short-term heavy resistance training, as measured by vertical jump and chair rising performance, in inactive older adults. The gains in muscle strength may contribute to the gains in chair rising time, SJ and CMJ height.     

Effects of jump training with negative versus positive loading on jumping mechanics

We examined the effects of jump training with negative (-30% of the subject's body weight (BW)) VS. positive loading (+30% BW) on the mechanical behaviour of leg extensor muscles. 32 men were divided into control (CG), negative loading (NLG), or positive loading training group (PLG). Both training groups performed maximal effort countermovement jumps (CMJ) over a 7-week training period. The impact of training on the mechanical behaviour of leg extensor muscles was assessed through CMJ performed with external loads ranging from -30% BW to +30% BW. Both training groups showed significant ( P≤0.013) increase in BW CMJ height (NLG: 9%, effect size (ES)=0.85, VS. PLG: 3.4%, ES=0.31), peak jumping velocity ( V(peak); NLG: 4.1%; ES=0.80, P=0.011, VS. PLG: 1.4%, ES=0.24; P=0.017), and depth of the countermovement (Δ H(ecc); NLG: 20%; ES=-1.64, P=0.004, VS. PLG: 11.4%; ES=-0.86, P=0.015). Although the increase in both the V(peak) and Δ H(ecc) were expected to reduce the recorded ground reaction force, the indices of force- and power-production characteristics of CMJ remained unchanged. Finally, NLG (but not PLG) suggested load-specific improvement in the movement kinematic and kinetic patterns. Overall, the observed results revealed a rather novel finding regarding the effectiveness of negative loading in enhancing CMJ performance which could be of potential importance for further development of routine training protocols. Although the involved biomechanical and neuromuscular mechanisms need further exploration, the improved performance could be partly based on an altered jumping pattern that utilizes an enhanced ability of leg extensors to provide kinetic and power output during the concentric jump phase.     

Multi-segment coordination: fatigue effects.

PURPOSE: The aim of this study was to investigate the segmental coordination of vertical jumps under fatigue. METHODS: Twelve subjects performed maximal countermovement jumps with and without fatigue, which was imposed by maximal continuous jumps in place until their maximal jump height corresponded to 70% of the nonfatigued condition. Video, ground reaction forces, and electromyographic signals were recorded to analyze the segmental coordination of countermovement jumps before (CMJ1) and after (CMJ2) fatigue. The magnitude of joint extension initiation, peak joint angular velocity, and peak net power around the ankle, knee, and hip joints and their respective times were determined. RESULTS: CMJ2 was characterized by a longer contact time, which was accompanied with an earlier movement initiation and several differences (P < 0.05) in the variables used to describe coordination. When the movement duration was normalized with respect to the contact phase duration, the differences between CMJ1 and CMJ2 were not sustained. A consistent pattern was indicated, in which the segmental coordination did not differ between jump conditions. When the magnitude of the muscle activation was set aside, a remarkably consistent muscle activation time was noticed between conditions. CONCLUSIONS: It was indicated that countermovement jumps were performed with a consistent well-timed motion of the segments. A "common drive," which acts without the knowledge of the muscle properties, was suggested as mediating and controlling the muscle activation timing between agonist-antagonist muscle pairs.     

Does plyometric training improve vertical jump height? A meta‐analytical review

The aim of this study was to determine the precise effect of plyometric training (PT) on vertical jump height in healthy individuals. Meta-analyses of randomised and non-randomised controlled trials that evaluated the effect of PT on four typical vertical jump height tests were carried out: squat jump (SJ); countermovement jump (CMJ); countermovement jump with the arm swing (CMJA); and drop jump (DJ). Studies were identified by computerised and manual searches of the literature. Data on changes in jump height for the plyometric and control groups were extracted and statistically pooled in a meta-analysis, separately for each type of jump. A total of 26 studies yielding 13 data points for SJ, 19 data points for CMJ, 14 data points for CMJA and 7 data points for DJ met the initial inclusion criteria. The pooled estimate of the effect of PT on vertical jump height was 4.7% (95% CI 1.8 to 7.6%), 8.7% (95% CI 7.0 to 10.4%), 7.5% (95% CI 4.2 to 10.8%) and 4.7% (95% CI 0.8 to 8.6%) for the SJ, CMJ, CMJA and DJ, respectively. When expressed in standardised units (ie, effect sizes), the effect of PT on vertical jump height was 0.44 (95% CI 0.15 to 0.72), 0.88 (95% CI 0.64 to 1.11), 0.74 (95% CI 0.47 to 1.02) and 0.62 (95% CI 0.18 to 1.05) for the SJ, CMJ, CMJA and DJ, respectively. PT provides a statistically significant and practically relevant improvement in vertical jump height with the mean effect ranging from 4.7% (SJ and DJ), over 7.5% (CMJA) to 8.7% (CMJ). These results justify the application of PT for the purpose of development of vertical jump performance in healthy individuals.     

Cross-validation of three jump power equations

The vertical jump-and-reach score is used as a component in the estimation of peak mechanical power in two equations put forth by Lewis and Harman et al. PURPOSE: The purpose of the present study was to: 1) cross-validate the two equations using the vertical jump-and-reach test, 2) develop a more accurate equation from a large heterogeneous population, 3) analyze gender differences and jump protocols, and 4) assess Predicted Residual Sum of Squares (PRESS) as a cross-validation procedure. METHODS: One hundred eight college-age male and female athletes and nonathletes were tested on a force platform. They performed three maximal effort vertical jumps each of the squat jump (SJ) and countermovement jump (CMJ) while simultaneously performing the vertical jump-and-reach test. Regression analysis was used to predict peak power from body mass and vertical jump height. RESULTS: SJ data yielded a better power prediction equation than did CMJ data because of the greater variability in CMJ technique. The following equation was derived from SJ data: Peak Power (W) = 60.7x (jump height cm]) +45.3x(body mass [kg])-2055. This equation revealed greater accuracy than either the Lewis or previous Harman et al. equations and underestimated peak power by less than 1%, with a SEE of 355.0 W using SJ protocol. The use of one equation for both males and females resulted in only a slight (5% of power output) difference between genders. Using CMJ data in the SJ-derived equation resulted in only a 2.7% overestimation of peak power. Cross-validation of regression equations using PRESS reveals accurate and reliable R2 and SEE values. CONCLUSIONS: The SJ equation is a slightly more accurate equation than that derived from CMJ data. This equation should be used in the determination of peak power in place of the formulas developed by both Harman et al. and Lewis. Separate equations for males and females are unnecessary.     

Chronic effects of different frequencies of local vibrations

The aim of this study was to compare the chronic effects of different frequencies of mechanical vibrations, applied in the direction of the resultant muscle forces' vector addition, on the performance of the lower extremities. After performing the maximal voluntary contraction (MVC), squat jump (SJ), countermovement jump (CMJ), multiple vertical jumps and running speed tests, 55 male volunteers were distributed into 4 groups: an Isometric group, an 8-Hz group, a 26-Hz group and a Control group. After a 4-week training period, the same tests were performed. The training groups reached a significant increase of the MVC, but the increases of the 8-Hz (23.2%) and 26-Hz (22.2%) groups were higher than the Isometric group (12.1%). A significant increase was observed between SJ and CMJ values measured at the pre-test and the post-test stages in the 8-Hz (SJ=11.1%; CMJ=8.7%) and 26-Hz groups (SJ=9.6%; CMJ=7.5%). No differences were observed between 8-Hz and 26-Hz groups. The application of vibrations of 8?Hz and 26?Hz, directed to the resulting muscle forces, was able to increase the performance of the lower extremities. This kind of local vibration did not produce positive effects on multiple vertical jumps or running speed.     

Reliability of jumping performance in active men and women under different stretch loading conditions

BACKGROUND: To determine the reliability of squatting jumps (SJ), counter-movement jumps (CMJ) and drop jumps (DJ) tests, as well as the reliability of the optimal dropping height during drop jumping. METHODS: Jumping performance was assessed in 8 male and 9 female physical education students. Their age, weight and height (mean +/- SD) were 23.9 +/- 2.1 years, 72.0 +/- 12.1 kg, 174.3 +/- 10.4 cm, and 23.1 +/- 2.0 years, 54.8 +/- 4.9 kg, 160.1 +/- 5.0 cm for the males and females, respectively. The jumping performance was determined on six different testing days. On each testing day, squatting jumps (SJ) and counter-movement jumps (CMJ) were performed as well as drop jumps (DJ) from heights between 20 and 100 cm. The dropping height given the maximum attained height was registered as the optimal dropping height (ODH). After a 15 min rest period, a 30 sec hopping test (HT) was performed and the mean height attained (MHT) as well as the number of jumps executed (NHT) were recorded. The height attained was computed from the flight time, which was measured with a digital timer (+/- 0.001 sec) connected to a resistive platform. RESULTS: The pooled coefficients of variation in percentage were 5.4 (SJ), 6.3 (CMJ), 6.2 (DJ), 31.9 (ODH), 3.1 (NHT) and 6.7 (MHT). A parabolic relationship between dropping height and attained height was found (r = 0.39-0.43, p < 0.001). The ODH was 48.2 +/- 14.0 cm and 62.9 +/- 21.3 cm for females and males, respectively (p < 0.05). Multiple regression analysis showed than ODH can be predicted from the SJ with a standard error of 9 cm. CONCLUSIONS: The variability of the assessment of jumping performance is similar to that reported for other variables used in the assessment of physical fitness. In contrast, the assessment of the optimal dropping height is less reliable.     

Effects of plyometric training on jumping performance in junior basketball players.

BACKGROUND: Although the plyometric training has proved its efficiency, it remains generally unknown whether a limited amount of plyometric training could improve movements in subjects who already demonstrate high level of performance. METHODS: Three different training regimens were performed in order to study effects of plyometric training on elite junior basketball players. While control group (CG) participated only in the regular midseason training activity, another two groups performed a limited amount of plyometric training employing drop jumps from the height of either 50 cm (EG-50) or 100 cm (EG-100). The height of the maximal vertical jump (CMJ), as well as the maximal voluntary force (F) and the rate of force development (RFD) of hip and knee extensors were tested prior to and after the training. RESULTS: An increase in CMJ (4.8 and 5.6 cm in EG-50 and EG-100, respectively), as well as in F of hip extensors and RFD of knee extensors was observed in both experimental groups, while no significant changes were recorded in CG. When the pretest scores were used as a covariate, both experimental groups demonstrated higher increase in CMJ and RFD of knee extensors then CG. However, no differences were observed between EG-50 and EG-100. The multiple correlation between four isometric parameters and CMJ revealed R2=0.29. CONCLUSIONS: A limited amount of plyometric training could improve jumping performance in elite junior basketball players and this improvement could be partly related with an increase in F of hip extensors and RFD of knee extensors. However, neither of the two initial heights of the applied drop jumps proved to be more effective.     

Training effects of amortization phase with eccentric/concentric variations--the vertical jump.

The purpose of this study was to find if the performance of the vertical jump is improved by the result of increasing the force, velocity, and muscle adaptation(s). For the experiment, there were four groups (Gr): three groups did the training programs and the fourth group was the control group (CG). In order to establish improvements in force and velocity, the groups performed three variations using eccentric (Ecc) and concentric (Con) movement. Finally, the leg presses, squat jumps (SJ) and countermovement jumps (CJ) were used to determine the changes in the muscle adaptations before and after the training program. For measuring the force, four force transducers were used. In determining the velocity, a displacement transducer was used. A surface EMG was used to measure the muscle activations during all of the tests that were performed. After eight weeks of training, Gr 1 and Gr 2 improved the isometric force and the power during the Con. movement for the leg press. For the SJ, Gr. 1 and 2 increased their height, muscle activity, force, and power, but only group one improved the velocity. However, group three improved the isometric force and power for the leg presses, the squat jump and the countermovement jump. For both of the jumps, they also increased velocity, muscle activity and height. There were no changes observed for group four. Although the three combinations of Ecc/Con movements existed during the training program, the force, velocity and muscle adaptations increased, but not all of the improvements were of the same degree.     

Relationship between vastus lateralis muscle ultrasound echography,knee extensors rate of torque development,and jump height in professional soccer athletes

Purpose

This study aimed to investigate the relationship between knee extensors maximum voluntary contraction (MVC) torque and rate of torque development (RTD) with jump performance and with echography intensity (EI) from the vastus lateralis muscle (VL).

Methods

We assessed the MVC torque and RTD from knee extensors by a standard isokinetic device from 16 male professional soccer players (25.5?±?3.9 years). Counter-movement jump (CMJ) and squat jump (SJ) height were calculated from their flight times. EI was determined from VL ultrasound images’ grayscale histogram acquired in the middle of the tight. Correlation between variables was investigated by the Pearson correlation coefficient.

Results

We observed a VL EI of 26.4?±?7.8 a.u., and the SJ and CMJ heights were 36.0?±?3.0 and 36.2?±?4.1 cm, respectively. There was a significant correlation between the MVC torque and absolute late-phase RTD (r?=?0.67 and 0.76—RTD₂₀₀ and RTD₂₅₀, respectively), between the CMJ height and absolute RTD₅₀ (r?=?0.50), and between the normalized early-phase RTD and SJ height (r?=?0.53–0.60—RTD₅₀ and RTD₁₅₀). Additionally, normalized RTD₂₀₀ (r?=?? 0.51) and RTD₂₅₀ (r?=?? 0.56) were negatively correlated with EI.

Conclusions

Such results suggest that athletes with the ability to produce torque explosively in the very beginning (between 50 and 150 ms) of knee extension can jump higher. Also, the muscle quality assessed by the VL IE seems to be more important to maximum strength and later periods of torque rise (>?200 ms).

     

The effects of a prolonged running exercise on strength characteristics

The aim of this study was to examine concentric, isometric, and eccentric strength reductions in the quadriceps muscle following a prolonged running exercise. Before and after a 2 h run (28.4+/-1.4 km) peak torque (PT) of the knee extensors at angular velocities of -120, -90, -60, 0, 60, 120, 180, 240 degrees x s(-1) using an isokinetic dynamometer, electromyographic (EMG) activity of the vastus lateralis (VL) and vastus medialis (VM) muscles and height of a counter movement jump were recorded in twelve well-trained triathletes. Counter movement jump performances decreased by 10% and PT values were all significantly lower (p < 0.01) at each angular velocity following the run. The torque loss was significantly (p < 0.01) greater under eccentric contractions (from 18 to 21%) than under concentric ones (from 11 to 14%). EMG activity (RMS) was lower in both VL and VM muscles after the 2 h run but no difference existed in RMS losses between concentric and eccentric contractions. The present results demonstrate that 1) a prolonged running exercise more greatly affects eccentric force production in the quadriceps muscle, and 2) this specificity seems to be due to an impairment of the muscular contractile mechanism rather than a modification to the neural input.     

Movement competency and measures of isometric and dynamic strength and power in boys of different maturity status

An understanding of how movement competency, strength, and power interact with natural growth and maturation is required in order to determine meaningful changes with developing athletes. Isometric and dynamic testing in youth athletes provides insight into the natural development of the force-velocity (F-V) spectrum. Two-hundred and six young male athletes, aged 9-17 years of age, were grouped according to stage of maturation based on their maturity offset which was determined as number of years from peak height velocity (PHV). All participants performed the back-squat assessment (BSA), isometric mid-thigh pull (IMTP), countermovement jump (CMJ), and squat jump (SJ) tests. Absolute and scaled force-time variables were collected from the IMTP, CMJ, and SJ. No significant differences were observed between maturational groups for squat movement competency (P > .05). One-way ANOVA with Bonferroni post-hoc analysis revealed that increasing maturity led to significant, moderate to large increases in allometrically scaled peak force (PF_allo) for all tests (P < .05). Multiple stepwise linear regression models revealed IMTP PF_allo significantly predicted 34.8% and 41.3% of variance in SJ and CMJ jump height, respectively (P < .05). Natural growth and maturation induces positive adaptations to movement competency as well as isometric and dynamic strength and power. Trends from the IMTP, SJ, and CMJ tests indicate the largest differences in strength, and power may occur around the adolescent growth spurt despite the large variation in rates of change within the circa-PHV group.