Effect of different stretching strategies on the kinetics of vertical jumping in female volleyball athletes

Purpose： The present study aimed to examine the effect of static stretching （SS） and a sport-specific dynamic stretching （DS） session at two specific post-stretch time intervals in highly trained female athletes （age 19.90 ± 1.60 years; height 1.80 ±0.06 m; mass 76.87 ± 9.95 kg） on kinetic parameters of peak force, time-to-takeoff, and rate of force development. Methods： The data were collected over 3 days （randomized within subject design with control session）. Following each stretch session （SS vs. DS vs. control） of equal duration （7 min total： 30 s per targeted muscle group） participants performed countermovement jumping on a force platform at 1 and 15 rain after stretching. Results： The DS session significantly improved upon kinetic variables of rate of force development, peak force, and time-to-takeoffrelative to SS at 1 min after stretching. No significant effect was found at 15 min. Conclusion： Together these findings suggest that when training and competing to jump quickly and maximally the female athlete should incorporate DS instead of SS as part of their pre-competition warm-up, but conduct performance within 15 min of their warm-up to elicit maximal gains

Force Distribution Model of Motor Units of Leg Extensor Muscles

The paper aims to defme the lawfulness （model）, by which one can reliably estimate the distribution of motor units＇ force from the moment when leg extensor muscles start to generate the force until the moment when the maximal level is achieved. The study included 110 participants. To assess the contractile characteristics of leg extensors, standardized equipment and standardized isometric test in sitting position with the angle of the knee joint of 125° were used. The participants were instructed to exert their maximal force as quickly as possible. Using descriptive statistics, cluster analysis and fitting the exponential model of the distribution of force of leg extensor motor units in the whole range of force generation is defined. The model has the following form： y = 0.2051e3.3855x, where y is the motor units force expressed in daN （decanewtons）, x is the time expressed in s （seconds）. It provides an understanding of the control of multivariate motor unit recruitment and distribution of their force during sports movements as well as training programming for the adoption of forms for conlrolling force distribution of motor units, the development of their maximum force and their involvement speed.

Sport specificity background affects the principal component structure of vertical squat jump performance of young adult female athletes

Purpose： Long-term training specificity is thought to alter performance in tests evaluating strength and power production capability. The aim of the present study was to provide additional information to the limited existing knowledge concerning the possible differences of the force/time profile of squat jumping among different groups of young female athletes. Methods： One hundred and seventy-three adult women （20.1 ± 2.8 years, 1.71 ± 0.09 m, 65.6 ± 10.3 kg, mean± SD for age, height, and mass, respectively） engaged in track and field （TF）, volleyball （VO）, handball （HA）, basketball （BA）, and physical education students （PE） executed maximal squat jumps （SQJ） on a force plate. Pearson＇s correlation was used to identify the relationship between SQJ performance, the anthropometric characteristics and the biomechanical parameters. Differences concerning the biomechanical parameters among groups were investigated with analysis of variance, while the force- （FPD） or time- （TPD） dependency of SQJ execution was examined using principal components analysis （PCA）. Results： SQJ was unrelated to body height but significantly correlated with body mass （r = -0.26, p = 0.001）. TF jumped higher and produced larger peak body power output compared to all the other groups （p 〈 0.05）. All athletes were superior to PE since they performed the SQJ with a longer （p 〈 0.05） vertical body center of mass trajectory during the propulsion phase. PCA results revealed that TF significantly differentiated than the other groups by relying on FPD. Conclusion： Various different profiles of FPD and TPD were detected due to different sporting background in young female athletes. Since TF superiority in SQJ was relied on the larger power production and a greater FPD, female indoor team sport athletes are suggested to execute jumping exercises adopting the jumping strategies utilized by TE

Peak Torque as an Indicator of Rapid Torque Production during Screening Examinations

Although the ability to produce force rapidly is an indispensable characteristic of optimal health and performance, screening for this very critical parameter of strength is difficult because of clinician time constraints. The purpose this study was to investigate relationships between peak torque （PT） and rate of torque development （RTD） at 0-30, 0-50, 0-100, and 0-200 ms in female collegiate soccer athletes. Seventeen female collegiate soccer athletes were recruited. Isometric PT and RTD were collected at the hip abductors （AB）, hip adductors （AD）, knee extensors （KE） and knee flexors （KF）. The coefficients of determination were calculated to evaluate the association between PT and RTD. Normalized AB, AD and KF PT were significantly correlated to RTD at 0-30, 0-50, 0-100 and 0-200 ms, while KEPT was only significantly correlated to RTD at 0-100 and 0-200 ms. The results of this study indicate that PT is a viable, indirect indicator of early late phase RTD at separate time intervals at the AB, AD and KF. However, it is likely that other physiological factors coupled with PT are required to provide information on the rapid force production capabilities of the KFs and KEs based on the percent of common variance observed.

Effects of Two Isometric Strength Training Methods on Jump and Sprint Performances:A Randomized Controlled Trial

Purpose Isometric strength training (IST) with rapid non-sustained contraction (RIST) is effective in improving the ability to generate force rapidly.However,the neuromuscular adaptation of IST with sustained contraction (SIST) and RIST is not known.Therefore,the aim of the study was to compare the neuromuscular adaptations of RIST with SIST.Methods Thirty-three national floorball players (23.9 ± 3.1 years old;1.69± 0.08 m;64.6± 11.1 kg) were recruited for this study.Pre-and post-test included countermovement jump (CMJ),30-m sprint (TT30),isometric squat at 90° (ISqT90) and 120° (ISqT120) knee angles.They were randomly assigned to either control (Con) (n =9),RIST (n =12) or SIST (n =12)group and performed 12 sessions of intervention training.All groups performed the same sets of exercises,but RIST and SIST had to perform ISqT with and without sustained contraction,respectively.Results Time × group effect for CMJ height (P =0.01,η2p=0.25),peak force (PF) (P =0.03,η2p =0.22) and rate of force development (RFD) (P =0.02,η2p =0.22) obtained from ISqT 120 were noted.A main effect for time was observed in CMJ height,PF obtained from ISqT90 and ISqT120,and RFD obtained from ISqT90 (P < 0.01,0.27 < η2p < 0.57).There was greater improvement in TT30 (P =0.043,d =3.00),ISqT90 PF (P =0.034,d =3.12),ISqT 120 PF (P =0.003,d =4.54) and ISqT120 RFD (P=0.033,d=1.36) in the SIST than the Con group.Conclusion SIST was more effective in improving strength and dynamic performance as compared to RIST,making it a viable training method to enhance dynamic performances.

An examination of acute cross-over effects following unilateral low intensity concentric and eccentric exercise

We compared the effects of low intensity concentric(CON)and eccentric(ECC)exercise on the force and neural responses of the dominant(exercised)elbow flexors(EFs),and studied if these conditions could induce cross-over effects to the contralateral(non-exercised)EFs.Fifteen subjects(8 males)completed all conditions(CON and ECC:6 sets of low intensity exercise to failure;control:rest)in separate visits with a randomized order.Maximal isometric force and electromyography(EMG)of the dominant and contralateral EFs were assessed at pre,immediate-,24-,and 48-h-post.Two-factor(condition and time)linear mixed-model analyses were performed to examine the force and EMG responses.Immediately post CON,contralateral EFs force was significantly(p=0.026)higher(12.41%)than control,but no cross-over effects regarding the neural responses were observed.Immediately post ECC,dominant EFs force was significantly lower in ECC,compared to CON(p=0.003)and control(p<0.001).This force remained depressed at 24-and 48-h post ECC,when compared to CON(p<0.001)and control(p<0.001).Our data suggests that submaximal unilateral exercises are not likely to impair contralateral muscle strength performance.Instead,concentric exercises may acutely improve muscle strength for the contralateral limb.However,this effect is not explained by changes in muscle excitation.