Pre-Post Practice Changes in Countermovement Vertical Jump Force-Time Metrics in Professional Male Basketball Players.

ABSTRACT: Cabarkapa, D, Cabarkapa, DV, Philipp, NM, Knezevic, OM, Mirkov, DM, and Fry, AC. Pre-post practice changes in countermovement vertical jump force-time metrics in professional male basketball players. J Strength Cond Res 37(11): e609-e612, 2023-Despite the countermovement vertical jump (CVJ) being one of the most popular noninvasive and time-efficient methods for monitoring neuromuscular status, there is a lack of scientific literature focused on examining fatigue-induced alterations in performance in elite athletes. Thus, the purpose of this study was to examine changes in force-time metrics pre-post practice in professional male basketball players. Seventeen athletes competing in first-tier and second-tier national basketball leagues in Europe participated in this study. While standing on a uniaxial force plate sampling at 1,000 Hz, each athlete completed 3 CVJs pre-practice and post-practice. The practice consisted of individual and team shooting drills, position-specific player development drills, 5-on-0 offensive actions, and 5-on-5 play, including full-court transition (∼2 hours). The findings reveal that pre-post practice changes in force-time metrics seem to be phase specific. Despite a trivial increase in eccentric mean force (920.4 ± 100.2, 929.4 ± 100.0 N), most changes were observed within the concentric phase of the CVJ. The concentric phase duration increased pre-post practice (0.233 ± 0.027, 0.242 ± 0.033 seconds), whereas concentric impulse (262.9 ± 18.8, 258.6 ± 21.6 N·s), peak velocity (2.93 ± 0.22, 2.86 ± 0.22 m·s-1), mean force (2052.4 ± 179.2, 2002.7 ± 188.2 N), mean power (3,165.5 ± 269.5, 3,030.9 ± 326.8 W), and peak power (5,523.4 ± 607.3, 5,246.6 ± 663.7 W) experienced a significant decrease. Moreover, alongside longer contraction time (0.663 ± 0.065, 0.686 ± 0.074 seconds), lower vertical jump height (41.0 ± 6.8, 38.9 ± 6.6 cm) and reactive strength index-modified (0.634 ± 0.113, 0.579 ± 0.111 m·s-1) values were observed post-practice. Overall, these findings may allow practitioners to detect fatigue-induced changes in CVJ force-time metrics in professional male basketball players that can ultimately improve the acute and longitudinal training-adaptation monitoring process.

Reliability of Sprint Force-Velocity-Power Profiles Obtained with KiSprint System.

This study aimed to assess the within- and between-session reliability of the KiSprint system for determining force-velocity-power (FVP) profiling during sprint running. Thirty (23 males, 7 females; 18.7 ± 2.6 years;) young high-level sprinters performed maximal effort sprints in two sessions separated by one week. Split times (5, 10, 20 and 30 m), which were recorded with a laser distance meter (a component of the KiSprint system), were used to determine the horizontal FVP profile using the Samozino's field-based method. This method assesses the FVP relationships through estimates of the step-averaged ground reaction forces in sagittal plane during sprint acceleration using only anthropometric and spatiotemporal (split times) data. We also calculated the maximal theoretical power, force and velocity capabilities and the slope of the FV relationship, the maximal ratio of horizontal-to-resultant force (RF), and the decrease in the RF (DRF). Overall, the results showed moderate or good to excellent within- and between-session reliability for all variables (ICC > 0.75; CV < 10 %), with the exception of FV slope and DRF that showed low relative reliability (ICC = 0.47-0.48 within session, 0.31-0.33 between-session) and unacceptable between-session absolute reliability values (CV = 10.9-11.1 %). Future studies are needed to optimize the protocol in order to maximize the reliability of the FVP variables, especially when practitioners are interested in the FV slope and DRF. In summary, our results question the utility of the sprint-based FVP profiling for individualized training prescription, since the reliability of the FV slope and D RF variables is highly questionable.

Feasibility of the two-point method for assessing the force-velocity relationship during lower-body and upper-body isokinetic tests.

This study aimed to (1) evaluate the shape of the force-velocity (F-V) relationship obtained from different muscles, (2) explore the concurrent validity of the two-point method with respect to the multiple-point method, (3) evaluate whether the F-V relationship can discriminate between muscle groups and genders, and (4) explore the generalisability of the same F-V relationship parameters (maximal force [F0], maximal velocity [V0]), and maximal power [P0]) between different tasks. The F-V relationship of 22 physically active participants (12 women) were tested during knee extension, knee flexion, elbow extension and elbow flexion through the multiple- (eight velocities: 30-60-90-120-150-180-210-240º/s) and two-point (two velocities: 60-180º/s) methods. The findings revealed (1) highly linear F-V relationships (r ≥ 0.893), (2) high concurrent validity of the two-point method for F0, but lower for V0 and P0, (3) the outcomes of both methods were sensitive to the muscle groups (higher for knee muscles) and gender (higher for men), and (4) the magnitude of the same F-V parameters were poorly correlated between different tasks (median r < 0.1). These results support the two-point method as a valid and sensitive procedure for determining the maximal capacities of the muscles to produce F, but not V, during isokinetic tests.

Self-Preferred Initial Position Could Be a Viable Alternative to the Standard Squat Jump Testing Procedure.

Petronijevic, MS, Garcia Ramos, A, Mirkov, DM, Jaric, S, Valdevit, Z, and Knezevic, OM. Self-preferred initial position could be a viable alternative to the standard squat jump testing procedure. J Strength Cond Res 32(11): 3267-3275, 2018-The purpose of this study was to compare both the magnitude and reliability of different variables (knee angle, squat depth, jump height [Hmax], maximum force [Fmax], and maximum power [Pmax]) between the standardized squat jump (SJ) and the SJ performed from the self-preferred position. Eleven team handball players (age: 19.5 ± 1.1 years; height: 1.88 ± 0.06 m; and body mass: 82.1 ± 8.7 kg) and 13 physically active students (age: 20.5 ± 0.9 years; height: 1.81 ± 0.06 m; and body mass: 76.6 ± 6.6 kg) were evaluated on 2 sessions during the standardized SJ (knee angle fixed at 90°) and the self-preferred SJ (self-selected knee angle to maximize Hmax). Two blocks of both 3 standardized SJ and 3 self-preferred SJ were performed on the first session, whereas only 1 block was performed in the second session. The squat depth was smaller for the self-preferred SJ, whereas the knee angle, Fmax, and Pmax were higher for the self-preferred SJ (p < 0.025). The magnitude of Hmax did not significantly differ between both jump types. Most importantly, the reliability of the mechanical outputs (Hmax, Fmax, and Pmax) was generally higher for the self-preferred SJ (9 of 12 comparisons), whereas only in 2 of 12 comparisons the reliability was meaningfully higher for the standardized SJ. No differences were observed between presumably more (handball players) and less skilled individuals (physically active subjects). These results suggest that the self-preferred SJ should be recommended over the standardized SJ (90° knee angle) because it is not only quicker and more ecologically valid, but could also provide the performance variables with higher reliability.

A Novel Two-Velocity Method for Elaborate Isokinetic Testing of Knee Extensors.

Single outcomes of standard isokinetic dynamometry tests do not discern between various muscle mechanical capacities. In this study, we aimed to (1) evaluate the shape and strength of the force-velocity relationship of knee extensors, as observed in isokinetic tests conducted at a wide range of angular velocities, and (2) explore the concurrent validity of a simple 2-velocity method. Thirteen physically active females were tested for both the peak and averaged knee extensor concentric force exerted at the angular velocities of 30°-240°/s recorded in the 90°-170° range of knee extension. The results revealed strong (0.960

Contralateral limb deficit after ACL-reconstruction: an analysis of early and late phase of rate of force development.

The aim of this study was to assess the effect of a unilateral anterior cruciate ligament reconstruction (ACLR) on maximum voluntary contraction (MVC) and explosive strength of both the involved limb and the uninvolved limb. Nineteen male athletes completed a standard isometric testing protocol 4 months post-ACLR, while 16 healthy participants served as a control group (CG). The explosive strength of the knee extensors and flexors was assessed as RFD obtained from the slope of the force-time curves over various time intervals. Both muscle groups of the involved limb had significantly lower MVC compared to the uninvolved. The involved limb also had significantly lower RFD in the late phase of contraction (140-250 ms) for both knee extensors and flexors (P < 0.05). There was no difference in MVC between the uninvolved limb and the CG. However, RFD of the uninvolved limb was lower compared to CG for both knee extensors (0-180 ms; P < 0.01) and flexors (0-150 ms; P < 0.05). ACLR leads to lower MVC and explosive strength of the involved limb. As a consequence of potential crossover (presumably neural-mediated) effects, explosive strength deficits could be bilateral, particularly in the early phase of the contraction (<100 ms).

Generalization of Muscle Strength Capacities as Assessed From Different Variables, Tests, and Muscle Groups.

Cuk, I, Prebeg, G, Sreckovic, S, Mirkov, DM, and Jaric, S. Generalization of muscle strength capacities as assessed from different variables, tests, and muscle groups. J Strength Cond Res 31(2): 305-312, 2017-The muscle strength capacities to exert force under various movement conditions have been indiscriminately assessed from various strength tests and variables applied on different muscles. We tested the hypotheses that the distinctive strength capacities would be revealed (H1) through different strength tests, and (H2) through different strength variables. Alternatively, (H3) all strength variables independent of the selected test could depict the same strength capacity of the tested muscle. Sixty subjects performed both the standard strength test and the test of alternating contractions of 6 pairs of antagonistic muscles acting in different leg and arm joints. The dependent variables obtained from each test and muscle were the maximum isometric force and the rate of force development. A confirmatory principle component analysis set to 2 factors explained 31.9% of the total variance. The factor loadings discerned between the tested arm and leg muscles, but not between the strength tests and variables. An exploratory analysis applied on the same data revealed 6 factors that explained 60.1% of the total variance. Again, the individual factors were mainly loaded by different tests and variables obtained from the same pair of antagonistic muscles. Therefore, a comprehensive assessment of the muscle strength capacity of the tested individual should be based on a single strength test and variable obtained from a number of different muscles, than on a single muscle tested through different tests and variables. The selected muscles should act in different limbs and joints, while the maximum isometric force should be the variable of choice.

Evaluation of force-velocity and power-velocity relationship of arm muscles.

PURPOSE: A number of recent studies have revealed an approximately linear force-velocity (F-V) and, consequently, a parabolic power-velocity (P-V) relationship of multi-joint tasks. However, the measurement characteristics of their parameters have been neglected, particularly those regarding arm muscles, which could be a problem for using the linear F-V model in both research and routine testing. Therefore, the aims of the present study were to evaluate the strength, shape, reliability, and concurrent validity of the F-V relationship of arm muscles. METHODS: Twelve healthy participants performed maximum bench press throws against loads ranging from 20 to 70 % of their maximum strength, and linear regression model was applied on the obtained range of F and V data. One-repetition maximum bench press and medicine ball throw tests were also conducted. RESULTS: The observed individual F-V relationships were exceptionally strong (r = 0.96-0.99; all P < 0.05) and fairly linear, although it remains unresolved whether a polynomial fit could provide even stronger relationships. The reliability of parameters obtained from the linear F-V regressions proved to be mainly high (ICC > 0.80), while their concurrent validity regarding directly measured F, P, and V ranged from high (for maximum F) to medium-to-low (for maximum P and V). CONCLUSIONS: The findings add to the evidence that the linear F-V and, consequently, parabolic P-V models could be used to study the mechanical properties of muscular systems, as well as to design a relatively simple, reliable, and ecologically valid routine test of the muscle ability of force, power, and velocity production.

Evaluation of isokinetic and isometric strength measures for monitoring muscle function recovery after anterior cruciate ligament reconstruction.

Although various strength tests and their outcome measures have been proposed for anterior cruciate ligament (ACL) reconstruction (ACLR), their measurement properties still remain relatively underexplored. The aim of this study was to investigate the longitudinal construct validity of the standard isokinetic (IKT) and isometric test (IMT), and of the IMT of alternating consecutive maximal contractions (ACMC). In addition, the concurrent validity of ACMC was assessed and compared with the validity of IMT. The strength of quadriceps and hamstrings in 20 male athletes with an anterior cruciate ligament (ACL) injury were assessed before ACLR, 4 and 6 months after ACLR, by means of IMT, ACMC, and IKT performed at 60 and 180° · s(-1). Significant between-session differences in muscle strength variables were found in the involved quadriceps (F > 6.5; p ≤ 0.05), but not in the uninvolved leg (F < 2.5; p > 0.05). Coefficients of variations in the uninvolved leg (all below 13.5%) were lower than the involved leg (11.7-22.1%). Intraclass correlation coefficients were moderate-to-high for the uninvolved leg and low-to-high for quadriceps of the involved leg. The concurrent validity of ACMC with respect to the IKT (r = 0.57-0.92; p ≤ 0.05) was comparable with the validity of IMT (r = 0.52-0.87; p ≤ 0.05). We conclude that the explored longitudinal construct validity of most of the evaluated variables could be sufficiently sensitive to detect the effects of the applied rehabilitation procedures. In addition, the obtained sensitivity and concurrent validity and the potential advantages of ACMC over IMT, all suggest that ACMC could be a particularly promising method for routine testing of neuromuscular function after ACLR.

Effects of Spinal Decompression and Segmental Spinal Instrumentation on Lower Limb Functionality in Patients with Spinal Osteoarthritis.

Spinal osteoarthritis, a degenerative condition of the spine, significantly impairs quality of life, causing pain, stiffness, and limited mobility. Treatment primarily aims to improve functionality and quality of life. This study investigated the effects of lower back surgery (e.g., spinal decompression with vertebra stabilization) on upper-limb maximum strength, lower limb functionality, and quality of life in osteoarthritis patients. A total of 16 patients (♀ = 10 and ♂ = 6) patients from the orthopedic clinic who were diagnosed with osteoarthritis based on MRI and clinical symptoms underwent the surgery. Their handgrip strength, lower limb functionality (6 min walk test), and quality of life (Flanagan quality of life scale) were assessed before and after the surgery. A repeated measures ANOVA was conducted to assess differences in these metrics pre- and post-surgery across the entire patient cohort, as well as within gender-specific subgroups. A large positive effect was seen in the 6 min walk (p = 0.02, d = -0.83) test but not in the handgrip test or quality of life. However, female patients showed a trend towards better quality of life. The results of this study provide evidence for the effectiveness of surgery in the treatment of spinal osteoarthritis and underscore the need for personalized treatment approaches in the treatment of spinal osteoarthritis.

Vertical jump neuromuscular performance of professional female handball players-starters vs. non-starters comparison.

Given the complex nature of the handball as a game, players are required to possess a distinct set of physical and physiological attributes to attain peak performance. With the countermovement vertical jump (CVJ) being widely implemented as a non-invasive and time-efficient testing modality in sports settings, the purpose of the present study was twofold: (a) to establish a CVJ profile of professional female handball players and (b) to examine differences in force-time metrics between starters and non-starters. Forty-two professional female handball players (e.g., SuperLeague) volunteered to participate in this study. Each athlete performed three maximum-effort CVJs with no arm swing while standing on a uni-axial force plate system sampling at 1,000 Hz. Independent t-tests were used to examine differences in each variable between starters and non-starters. The results revealed that starters attained superior performance within the eccentric phase of the CVJ when compared to non-starters, particularly in terms of eccentric peak velocity (-0.957 ± 0.242 vs. -0.794 ± 0.177 m·s-1), eccentric mean power (320.0 ± 77.7 vs. 267.1 ± 75.2 W), and eccentric peak power (929.0 ± 388.1 vs. 684.4 ± 214.2 W). While not reaching the level of statistical significance, moderate-to-large effect sizes were observed for concentric impulse, peak velocity, and mean and peak force and power, all in favor of players included in the starting lineup (g = 0.439-0.655). Overall, these findings suggest that at the top-tier level of handball competition, the ability to secure a spot in a starting lineup may be possibly influenced by the athlete's eccentric performance capabilities. Thus, the development of lower-body eccentric strength and power may positively impact on-court athlete performance and ultimately help the team secure the desired game outcome.

Jump training with different loads: effects on jumping performance and power output.

PURPOSE: To investigate the selective effects of different types of external loads applied in vertical jump training on both the performance and muscle power output of the squat (SJ) and countermovement jump (CMJ). METHODS: Physically active males practiced maximum unconstrained vertical jumps over an 8-week period with no load, with either a negative or positive load exerted by a nearly constant external force that altered their body weight, and with a loaded vest that increased both the body weight and inertia. The magnitude of all applied loads corresponded to 30 % of body weight. RESULTS: A similar training-associated increase in jump height was observed in all experimental groups in both CMJ (7.4-11.8 %) and SJ (6.4-14.1 %). The relative increase in power output was comparable to the increase in jump height in SJ (7.4-11.5 %), while the power increase in CMJ was relatively small and load-specific (0.5-9.5 %). The observed differences could originate from the changes in the CMJ pattern, reflected through the depth of the counter movement that particularly increased after the training with negative load (42 %) and no load (21 %). The same participants also revealed increased CMJ duration, reduced ground reaction forces, as well as reduced maximum and average power output when compared with other training groups. CONCLUSION: Jump training with the applied loads could lead to a comparable improvement in jumping performance. However, the observed load-specific adaptations of CMJ pattern could decouple the training-associated increase in jump height from the increase in muscle power output.

Gender-specific influences of balance, speed, and power on agility performance.

The quick change of direction (i.e., agility) is an important athletic ability in numerous sports. Because of the diverse and therefore hardly predictable manifestations of agility in sports, studies noted that the improvement in speed, power, and balance should result in an improvement of agility. However, there is evident lack of data regarding the influence of potential predictors on different agility manifestations. The aim of this study was to determine the gender-specific influence of speed, power, and balance on different agility tests. A total of 32 college-aged male athletes and 31 college-aged female athletes (age 20.02 ± 1.89 years) participated in this study. The subjects were mostly involved in team sports (soccer, team handball, basketball, and volleyball; 80% of men, and 75% of women), martial arts, gymnastics, and dance. Anthropometric variables consisted of body height, body weight, and the body mass index. Five agility tests were used: a t-test (T-TEST), zig-zag test, 20-yard shuttle test, agility test with a 180-degree turn, and forward-backward running agility test (FWDBWD). Other tests included 1 jumping ability power test (squat jump, SQJ), 2 balance tests to determine the overall stability index and an overall limit of stability score (both measured by Biodex Balance System), and 2 running speed tests using a straight sprint for 10 and 20 m (S10 and S20, respectively). A reliability analysis showed that all the agility tests were reliable. Multiple regression and correlation analysis found speed and power (among women), and balance (among men), as most significant predictors of agility. The highest Pearson's correlation in both genders is found between the results of the FWDBWD and S10M tests (0.77 and 0.81 for men and women, respectively; p < 0.05). Power, measured using the SQJ, is significantly (p < 0.05) related to FWDBWD and T-TEST results but only for women (-0.44; -0.41). The balance measures were significantly related to the agility performance for men but not for women. In addition to demonstrating a known relationship between speed and agility in both genders, and a small but statistically significant relationship between power and agility in women, these results indicate that balance should be considered as a potential predictor of agility in trained adult men.

Assessment of Countermovement Jump: What Should We Report?

The purpose of the present study was (i) to explore the reliability of the most commonly used countermovement jump (CMJ) metrics, and (ii) to reduce a large pool of metrics with acceptable levels of reliability via principal component analysis to the significant factors capable of providing distinctive aspects of CMJ performance. Seventy-nine physically active participants (thirty-seven females and forty-two males) performed three maximal CMJs while standing on a force platform. Each participant visited the laboratory on two occasions, separated by 24-48 h. The most reliable variables were performance variables (CV = 4.2-11.1%), followed by kinetic variables (CV = 1.6-93.4%), and finally kinematic variables (CV = 1.9-37.4%). From the 45 CMJ computed metrics, only 24 demonstrated acceptable levels of reliability (CV ≤ 10%). These variables were included in the principal component analysis and loaded a total of four factors, explaining 91% of the CMJ variance: performance component (variables responsible for overall jump performance), eccentric component (variables related to the breaking phase), concentric component (variables related to the upward phase), and jump strategy component (variables influencing the jumping style). Overall, the findings revealed important implications for sports scientists and practitioners regarding the CMJ-derived metrics that should be considered to gain a comprehensive insight into the biomechanical parameters related to CMJ performance.

Evaluation of a composite test of kicking performance.

The aim of this study was to evaluate the reliability and sensitivity of variables for the direct assessment of kicking performance in young soccer players. One hundred and six elite young soccer players were divided into 4 age groups (12-15 years). Absolute error (AE), variable error (VE), and constant error (CE) were evaluated as the variables of kicking accuracy, whereas the kicking velocity variables involved the maximum ball velocity (BVmax) and the ball velocity during accurate kicks (BVacc). Results suggested low-to-moderate reliability of the kicking accuracy (intraclass correlation coefficient [ICC] = 0.00-0.67) and high reliability of the kicking velocity variables (ICC = 0.87-0.94). Regarding the sensitivity, most of the variables detected the differences both between the dominant and nondominant legs and among the age groups. Because the evaluated variables should have a property of face validity, the findings obtained generally suggest that AE (and perhaps VE, as the measures of kicking accuracy) and both BVmax and BVacc (as the measures of kicking velocity) could be used within a routine composite test of kicking performance in young elite soccer players. Further development of the evaluated composite test of kicking performance could be based on the involvement of other kicking techniques and on testing the athletes of different ages, levels of skill, or sport specialization.

The Validity of the 2-Point Method for Assessing the Force-Velocity Relationship of the Knee Flexors and Knee Extensors: The Relevance of Distant Force-Velocity Testing.

Over the past decade, force-velocity (F-v) profiling has emerged as a promising tool for assessing neuromuscular capacity to design individually tailored interventions in diverse populations. To date, a limited number of studies have addressed the optimization of the linear method for measuring F-v profiles of single-joint isokinetic movements. We aimed to simplify the measurement of knee extension (KE) and knee flexion (KF) isokinetic tasks by evaluating the most appropriate combination of two velocities (i.e., the 2-point method). Twenty-two healthy participants (11 males and 11 females) were included in the study. Isokinetic peak torque was measured at nine angular velocities (30-60-90-120-150-180-210-240-300°/s) and under isometric conditions (at 150° and 120° of KF for KE, and KF, respectively). Maximal theoretical force (F0), maximal theoretical velocity (v0), slope of the relationship (Sfv) and maximal theoretical power (Pmax) were derived from the linear F-v profiles of KE and KF and compared between the 9-point method and all possible combinations (36 in total) of the 2-point methods. The F-v profiles obtained from nine points were linear for KE (R2 = 0.95; 95% CI = 0.94-0.96) and KF (R2 = 0.93; 95% CI = 0.90-0.95), with F0 underestimating isometric force. Further analyses revealed great to excellent validity (range: ICCs = 0.89-0.99; CV = 2.54%-4.34%) and trivial systematic error (range: ES = -0.11-0.24) of the KE 2-point method when force from distant velocities (30°/s, 60°/s or 90°/s combined with 210°/s, 240°/s or 300°/s) was used. Similarly, great to excellent validity and trivial systematic error of the KF 2-point method for F0 and Pmax (range: ICC = 0.90-0.96; CV = 2.94%-6.38%; ES = -0.07-0.14) were observed when using the previously described combinations of velocities. These results suggest that practitioners should consider using more distant velocities when performing simplified isokinetic 2-point single-joint F-v profiling. Furthermore, the F-v profile has the potential to differentiate between the mechanical properties of knee extensors and flexors and could therefore serve as a potential descriptor of performance.

Methodological considerations for assessing whole-body strength capacity through isometric dynamometry.

The present study aimed to explore the possibility of comprehensively assessing whole-body muscle strength by testing as few muscle groups as possible, using a single testing method (isometric or isokinetic dynamometry) and a single variable (maximal force or rate of force development). Knee, hip, shoulder and elbow extensors and flexors were evaluated in males with high (n = 26) and low strength levels (n = 32). The principal component analysis revealed three factors that explained 62.5% of the total variance, while the main factors were loaded by the different testing methods and strength variables for the muscles acting on the knee (first component), hip (second component) and arm joints (third component). These results were confirmed by a three-way ANOVA which revealed a significant factor of group (P < 0.001) and the interaction test type × group (P = 0.002), but not of test type (P = 0.644), muscle group (P = 0.999), or their interactions (P > 0.205). The correlations of strength outcomes across the muscles ranged from trivial to very large (r range = -0.17, 0.84), being generally higher for the antagonistic muscles. Overall, a comprehensive assessment of whole-body muscle strength can be obtained using isometric dynamometry and maximal force, but it should consider at least one muscle group from the antagonistic pair.

Is the physical fitness of schoolchildren dependent on their physical activity levels and nutritional status? The experience from Serbia.

Introduction: Introduction: based on the presumed relationship between physical activity and physical fitness, it seems reasonable to expect that being active would result in sufficient fitness levels. However, the relationship between physical activity and fitness is not so plausible and needs deeper exploration. Objectives: we conducted a study aimed at exploring: a) the potential variation in physical activity level by gender and school grade; and b) the effect of physical activity level, gender, and nutritional status on physical fitness test results. Methods: a total of 2795 males and 2614 females participated in the study (11-14 years). Their physical fitness was assessed by measuring anthropometric status, muscular fitness, endurance, flexibility, agility, and cardiorespiratory fitness, while their physical activity was assessed using the Physical Activity Questionnaire (PAQ-A). Results: a higher percentage of boys were highly active, while a higher percentage of girls were scarcely active (χ2 [2, n = 5017] = 151.4, p < 0.01). Decline in physical activity with age was higher in girls than in boys (χ2 [6, n = 5409] = 90.4, p < 0.01). MANOVA revealed a significant effect of gender (V = 0.009, F [6, 3669] = 5.29, p < 0.01), nutritional status (V = 0.193, F [12, 7340] = 65.16, p < 0.01) and physical activity levels (V = 0.043, F [12, 7340] = 13.60, p < 0.01) on tests outcomes. Conclusions: our study has shown that activity level in Serbian schoolchildren declines with age and is affected by nutritional status. Nutritional status represents a significant factor confounding physical fitness scores, regardless of activity levels.

Introducción: Introducción: basado en la presumible relación entre la actividad física y la condición física, parece razonable esperar que ser activo físicamente resulte en niveles suficientes de condición física. Sin embargo, la relación entre la actividad física y la forma física no es tan plausible y necesita una exploración más profunda. Objetivos: realizamos un estudio con el objetivo de explorar: a) la variación potencial en el nivel de actividad física en función del género y el grado escolar; y b) el efecto del nivel de actividad física, el género y el estado nutricional en el resultado de las pruebas de condición física. Métodos: un total de 2795 hombres y 2614 mujeres participaron en el estudio (11-14 años). Su condición física se evaluó midiendo el estado antropométrico, la aptitud muscular, la resistencia, la flexibilidad, la agilidad y la aptitud cardiorrespiratoria, mientras que la actividad física se evaluó mediante el Cuestionario de Actividad Física (PAQ-A). Resultados: un mayor porcentaje de niños eran muy activos, mientras que un mayor porcentaje de niñas eran poco activas (χ2 [2, n = 5017] = 151,4, p < 0,01). La disminución de la actividad física con el incremento de la edad fue mayor en las niñas que en los niños (χ2 [6, n = 5409] = 90,4, p < 0,01). El MANOVA reveló un efecto significativo del género (V = 0.009, F [6, 3669] = 5.29, p < 0.01), el estado nutricional (V = 0.193, F [12, 7340] = 65.16, p < 0.01) y los niveles de actividad física (V = 0.043, F [12, 7340] = 13.60, p < 0.01) en los resultados de las pruebas. Conclusiones: nuestro estudio ha demostrado que el nivel de actividad física en los escolares serbios disminuye con la edad y se ve afectado por el estado nutricional. El estado nutricional representa un factor significativo que confunde las puntuaciones de condición física, independientemente de los niveles de actividad.

Evaluation of the Limb Symmetry Index: The Side Hop Test.

The main objective of present study was to evaluate inter-rater reliability and concurrent validity of Side Hop Test stopwatch vs. force plates timing, and to determine the number of sessions and trials required to minimize the effects of learning on Side Hop Test total time and limb symmetry index. Fifteen healthy male physical education students (mean ± SD: age, 23 ± 3 years; height, 181 ± 9 cm; and weight 72 ± 6 kg) participated. Side Hop Test total time (stopwatch and force plates) of left and right leg, and limb symmetry index (force plates) were obtained over seven sessions conducted 5-7 days apart. Time recordings of two raters were similar (t = -0.56, p > 0.05) with high reliability (all ICC >0.99 and CV% <0.1) and no systematic bias when compared to force plate data (p > 0.05; for rater 1 and 2, respectively). Total time improved across the Sessions (F = 25.87, p < 0.01, ω 2 = 0.18) and Trials (F = 68.15, p < 0.01, ω 2 = 0.10), with no significant interaction between factors. No between-leg differences were detected (F = 0.52, p > 0.05, ω 2 = 0.001). Limb symmetry index ranged from 0.999 to 1.055 across all sessions and trials (all p > 0.05 and ω 2 < 0.00). Due to low coefficient of correlation, high interclass correlation coefficient, and the lack in heteroscedasticity, stopwatch measurements are valid to measure total time in the Side Hop Test. Moreover, stopwatch measurements could be reliably used to measure total time in the Side Hop Test, while the test could be administrated with only one experienced rater. Unlike total times, findings on limb symmetry index suggest it could be reliably assessed after seven familiarization sessions.

The force-velocity relationship obtained during the squat jump exercise is meaningfully influenced by the initial knee angle.

This study aimed to compare the magnitude of the force-velocity (F-V) relationship parameters (maximum force [F0], maximum velocity [V0], F-V slope, and maximum power [Pmax]) between the squat jumps (SJ) performed from different knee angles. The F-V relationships of 12 men were assessed in 3 sessions during the SJ performed from a knee angle of 80° (SJ80), 90° (SJ90) and 100° (SJ100). The SJ100 provided likely to very likely higher values of F0 and Pmax compared to SJ80 (86% and 98%, respectively) and SJ90 (73% and 94%, respectively), while unclear and trivial differences were observed for the remaining comparisons. The magnitude of the correlations between the 3 SJ types was very large to nearly perfect for Pmax (r range = 0.864 to 0.940), moderate to very large for F0 (r range = 0.438 to 0.778), and small to large for V0 (r range = 0.361 to 0.642) and the F-V slope (r range = 0.178 to 0.645). These results suggest that the F-V relationship assessed during the SJ exercise is affected by the initial knee angle with the increase of the knee angle from 80° to 100° being associated with higher values of F0 and Pmax, while V0 remains unchanged.