Differentiation of the Strength of Back Muscle Contraction Under Fatigue: Does Force Feedback Play a Role?

OBJECTIVE: This study investigates the ability of subjects to differentiate the strength of back muscle contraction with and without feedback information on force produced under fatigue and nonfatigue conditions. DESIGN: Controlled laboratory study. SETTING: Research laboratory environment. PARTICIPANTS: A group of 52 healthy young men participated in the study. INTERVENTION: Subjects self-estimated 50% of the maximal voluntary isometric contraction of back muscles either on their own volition or on the basis of information about the actual force, before and after the Sørensen fatigue test. MAIN OUTCOME MEASURES: The force was measured by means of the FiTRO Back Dynamometer. RESULTS: The self-estimated 50% maximal voluntary isometric contraction was significantly higher than the one calculated from maximal voluntary isometric contraction during 10 trials in 2 repeated sessions (8.3% and 10.0%, P < .05). However, when feedback on the force produced was provided, significantly higher values were observed during an initial trial in both sessions (8.5%, P = .04 and 12.1%, P = .01). Subjects were able to estimate the target force during the following trials. Fatigue induced a decrease in peak force (7.7%, P = .04), whereas the ability to regulate the prescribed force was not compromised. Constant error was lower with than without force feedback during both measurements (2.15% and 6.85%; 3.06% and 8.56%). However, constant and variable errors were greater under fatigue than nonfatigue conditions (8.43% and 5.55%; 0.41% and 0.37%). Similarly, root mean square error decreased with force feedback (from 6.88% to 3.48% and from 8.74% to 5.09%) and increased under fatigue (from 5.87% to 8.67%). CONCLUSIONS: These findings indicate that force feedback plays a role in the differentiation of the strength of back muscle contraction, regardless of fatigue. It contributes to a more precise regulation of force produced during voluntary isometric contraction of back muscles. This promising method awaits further experimentation to be applied for individuals with low back pain.

Association of Trunk Rotational Velocity with Spine Mobility and Curvatures in Para Table Tennis Players.

This study investigates the relationship of trunk rotational velocity with mobility and curvature of the spine in wheelchair table tennis players. Eleven para table tennis players and 13 able-bodied athletes performed 5 seated trunk rotations to each side with 1 kg barbell placed on the shoulders. Trunk rotational velocity and respective angular displacement were significantly lower in para table tennis players compared to able-bodied athletes. Both groups showed similar values of thoracic kyphosis. However, para table tennis players exhibited lower lumbar inversion and pelvic retroversion compared to able-bodied athletes. Peak and mean velocity in the acceleration phase of trunk rotation correlated with angular displacement in both para table tennis players (r=0.912, p=0.001; r=0.819, p=0.013) and able-bodied athletes (r=0.790, p=0.026; r=0.673, p=0.032). These velocity values were also associated with lumbar curvature (r=-0.787, p=0.003; r=- 0.713, p=0.009) and pelvic tilt angle (r=0.694, p=0.014; r=0.746, p=0.007) in para table tennis players. Findings indicate that slower velocity of trunk rotations in para table tennis players might be due to their limited range of trunk rotational motion. Decreased posterior concavity could also contribute to these lower values. However, other biomechanical factors may have an impact on the association between these variables and have yet to be documented.

Agility Index as a Measurement Tool Based on Stimuli Number and Traveling Distances.

Zemková, E. Agility Index as a measurement tool based on stimuli number and traveling distances. J Strength Cond Res 31(8): 2141-2146, 2017-The purpose of this study was to develop an Agility Index (AI), plus related methodology, for representing and quantifying the data variability of agility performance, incorporating varied stimuli number and traveling distances. A group of 84 physically fit subjects performed the agility test. Their task was to touch, with either the left or the right foot, 1 of 4 mats according to the location of a stimulus in one of the corners of the screen. Results showed no significant changes in agility time during 60 responses, when subjects traveled a distance of 0.4 m (6.3%, p = 0.326). However, agility time increased significantly after 40 responses when traveling 0.8 m (12.5%, p = 0.044), after 20 responses when traveling 1.6 m (18.6%, p = 0.028), and after 10 responses when traveling 3.2 m (18.3%, p = 0.029). To estimate the contribution of movement time to the agility time, the AI was proposed. This was defined as a ratio of reaction time and agility time, divided by the previously determined coefficient for each distance traveled. No significant differences between the 2 test occasions in the AI and the ICC values in range from 0.88 to 0.93 and SEM from 7.6 to 8.8% signify that it is of sufficient stability and reliability to be used in practice. The AI is crucial for the sport-specific assessment of agility performance that differs in the number of stimuli and traveling distances. Also, it can be useful for comparing individuals with varied performance levels and the evaluation of agility training efficiency.

A Novel Method for Assessing Muscle Power During the Standing Cable Wood Chop Exercise.

Zemková, E, Cepková, A, Uvacek, M, and Soos, L. A novel method for assessing muscle power during the standing cable wood chop exercise. J Strength Cond Res 31(8): 2246-2254, 2017-The study estimates the repeatability and sensitivity of a novel method for assessing (a) the maximal power during the standing cable wood chop exercise with different weights and (b) the endurance of the core muscles. A group of 23 fit men performed (a) maximal effort single repetitions of the standing cable wood chop exercise with weights increasing stepwise up to 1 repetition maximum (1RM) and (b) a set of 20 repetitions at a previously established weight at which maximal power was achieved. Results showed that mean power during the standing cable wood chop exercise is a reliable parameter, with intraclass correlation coefficient values above 0.90 for all weights tested. It was also shown to be a sensitive parameter able to discriminate within-group differences in the maximal power and endurance of core muscles. Substantial individual differences were found in mean power, especially at higher weights, and in the maximal power achieved at about 75% of 1RM (462.2 ± 57.4 W, n = 11), 67% of 1RM (327.2 ± 49.7 W, n = 7), and 83% of 1RM (524.0 ± 63.2 W, n = 5). At these weights, there were also significant differences between the initial and the final repetitions of the wood chop exercise (13.9%, p = 0.025; 10.2%, p = 0.036; and 13.8%, p = 0.028, respectively). These findings indicate that evaluation of the maximal power and endurance of the core muscles during the standing cable wood chop exercise on a weight stack machine is a reliable method and sensitive to differences among physically active individuals.

Jumping From a Chair is a More Sensitive Measure of Power Performance In Older Adults Than Chair Rising.

Background/Study Context: The study estimates the reliability of peak velocity and peak power during chair rising and chair jumping tests and their ability to discriminate between different age and physical activity level groups. METHODS: Physically active and sedentary individuals (N = 262) of different ages (young: 22.9 ± 2.0 years, range: 21-25 years; older: 63.1 ± 1.8 years, range: 61-65 years) performed, in random order, chair rising and chair jumping tests on a force plate. Randomly selected young subjects performed both tests repeatedly on two different occasions separated by 1 week. From the sitting position with the arms crossed on the chest, they either stand up completely (chair rising test), or jump as high as possible (chair jumping test). RESULTS: The test-retest reliability of peak power and peak velocity during chair rising as well as chair jumping was excellent, with high intraclass correlation coefficients (ICCs; .90-.98) and low standard error of measurement (SEM; 7.0-9.1%). Post hoc analysis revealed significant differences in peak power and peak velocity between the sedentary and physically active young and older subjects. However, greater coefficients of variation for both parameters were found for chair jumping than chair rising (21.1-40.2% vs. 11.0-15.2%). Additionaly, there were moderate correlations of peak power and peak velocity between chair rising and chair jumping (r = .42-.49). There were greater within- and between-group differences in peak force and peak power and a steeper increase in their values during the initial phase of chair jumping than chair rising. CONCLUSION: Both chair rising and chair jumping tests provide reliable data and are valid indicators of lower body power in young and older adults. However, jumping from a chair is a more sensitive measure of strength and power performance than chair rising.

The role of lateral preference of lower limbs in a postural stabilization task.

OBJECTIVE: Postural control is a complex skill based on the interaction of dynamic sensorimotor processes. This study assessed the effect of lateral perturbations on postural re-stabilization regarding lower limb preference. METHODS: A group of 14 physically active individuals (9 male, 5 female) randomly underwent postural perturbations in lateral-left and lateral-right directions at a velocity of 0.2 m.s-1 and a platform shift of 6 cm. Perturbation to the preferred limb side (PS) was noted when the contralateral body movement was primarily controlled by the preferred limb and perturbation to the non-preferred limb side (NS) was noted when the contralateral body movement was primarily controlled by the non-preferred (stabilizing) limb. Prior to, during and after the perturbation centre of pressure (CoP) was registered using a computerized motor driven FiTRO Dynamic Posturography System based on force plate (Fitro Sway Check) with a sampling rate of 100 Hz. The basic stabilographic parameters of peak displacement (Peak 1), peak-to-peak displacement (Peak 2), time to peak displacement (Time 1), time to peak-to-peak displacement (Time 2) and re-stabilization time (Time 3) were analyzed. RESULTS: Results showed significantly larger Time 3 on PS than on NS (2.81 ± 1.32 s and 1.73 ± 1.10 s; p=0.02). However, there were no significant differences in other parameters between PS and NS. CONCLUSION: It may be concluded that the observed shorter re-stabilization time at NS was due to the stabilization role of the non-preferred limb.

Effects of obesity, diabetes and exercise on Fndc5 gene expression and irisin release in human skeletal muscle and adipose tissue: in vivo and in vitro studies.

Irisin was identified as a myokine secreted by contracting skeletal muscle, possibly mediating some exercise health benefits via 'browning' of white adipose tissue. However, a controversy exists concerning irisin origin, regulation and function in humans. Thus, we have explored Fndc5 gene and irisin protein in two clinical studies: (i) a cross-sectional study (effects of type 2 diabetes (T2D) in drug-naive men) and (ii) an intervention study (exercise effects in sedentary, overweight/obese individuals). Glucose tolerance and insulin sensitivity were assessed. Maximal aerobic capacity and muscle strength were measured before and after training. Body composition (magnetic resonance imaging), muscle and liver fat content (1H-magnetic resonance spectroscopy (MRS)) and in vivo muscle metabolism (32P-MRS) were determined. Skeletal muscle and subcutaneous abdominal adipose tissue samples were taken in the fasted state and during euglycaemic hyperinsulinaemia (adipose tissue) and before/after exercise training (muscle). We found that muscle Fndc5 mRNA was increased in prediabetes but not T2D. Fndc5 in adipose tissue and irisin in plasma were reduced in T2D by 40% and 50%, respectively. In contrast, T2D-derived myotubes expressed/secreted the highest levels of Fndc5/irisin. Neither hyperinsulinaemia (adipose tissue/plasma) nor exercise (muscle/plasma) affected Fndc5/irisin in vivo. Circulating irisin was positively associated with muscle mass, strength and metabolism and negatively with fasting glycaemia. Glucose and palmitate decreased Fndc5 mRNA in myotubes in vitro. We conclude that distinct patterns of Fndc5/irisin in muscle, adipose tissue and circulation, and concordant in vivo down-regulation in T2D, indicate that irisin might distinguish metabolic health and disease. Moreover, Fndc5/irisin was discordantly regulated in diabetic muscle and myotubes in vitro, suggesting that whole body factors, such as glucose and fatty acids, might be important for irisin regulation. Exercise did not affect Fndc5/irisin. However, irisin was positively linked to muscle mass, strength and metabolism, pointing to common regulatory factors and/or the potential for irisin to modify muscle phenotype.

Enhancement of peak and mean power in concentric phase of resistance exercises.

The study compares the differences in peak and mean power of concentric-only and countermovement resistance exercises (ΔP) with different weights. A group of 27 fit men randomly performed 3 repetitions of either barbell bench presses or barbell squats on different days. The initial weight of 20 kg was increased by 10 or 5 kg (at higher loads) up to at least 85% of a previously established 1 repetition maximum (1RM). A computer-based system FiTRO Dyne Premium was used to monitor force and velocity and to calculate power. The peak values and mean values of power during the entire concentric phase of lifting and during the acceleration phase were analyzed. Results showed that maximal ΔP calculated from the peak and mean values in the acceleration phase of bench presses was achieved at lower weights (118.4 ± 19.0 W at 47% of 1RM and 116.2 ± 15.3 W at 48% of 1RM, respectively) than the one calculated from mean values in the entire concentric phase of lifting (114.8 ± 14.8 W at 57% 1RM). Likewise, maximal ΔP calculated from the peak and mean values in the acceleration phase of squats was achieved at lower weights (127.7 ± 20.4 W at 67% of 1RM and 124.3 ± 22.1 W at 69% of 1RM, respectively) than the one calculated from the mean values in the entire concentric phase of lifting (125.0 ± 19.2 W at 77% of 1RM). This fact has to be taken into account when training efficiency is evaluated, namely, in sports requiring the production of maximal force in a short time.

Weight lifted and countermovement potentiation of power in concentric phase of unstable and traditional resistance exercise.

The study evaluates the effect of weight lifted on power in the concentric phase of resistance exercises on stable and unstable surfaces. A group of 19 fit men performed randomly on different days 3 reps of (a) barbell chest presses on the bench and Swiss ball, and (b) barbell squats on stable base and BOSU ball. Exercises were performed without and with countermovement (CM) using maximal effort in concentric phase. Initial weight of 20 kg was increased by 10 kg or 5 kg (at higher loads) up to at least 85% of previously established 1RM under stable conditions. Results showed no significant differences in mean power in the concentric phase of stable and unstable CM chest presses at lower weights lifted (from 20 to 50 kg). However, its values were significantly higher during chest presses on the bench than on Swiss ball while lifting higher weights (from 60 to 90 kg). Similarly, mean power in the concentric phase of squats was significantly higher on stable base than on BOSU ball at higher weights lifted (from 60 to 90 kg). Though a set of data showed significant differences, the effect sizes≤0.7 suggest no practically meaningful differences. It may be concluded that unstable base compromises the power in the concentric phase of resistance exercises, however, only at higher weights lifted.

The importance of core strength for change of direction speed.

Change of direction speed (CODS) is determined by several physical aspects, such as linear sprint speed, reactive strength and power of leg muscles. It appears that core strength may also play a role in CODS, however, its relationship to CODS remains unclear. The aim of this narrative review was to analyze the literature addressing a) the relationship between core strength and CODS and b) the effect of core strength training on CODS. This analysis revealed a significant relationship between the parameters of core strength and stability (the pressure of the activated core muscles during lower limb movement and the greatest mean force output of maximum volunteered contraction) and the time in the Agility T-Test. However, this parameter was not significantly related to the strength endurance of core muscles (total time in the plank test). Core training provides a sufficient stimulus for the development of CODS in less-skilled middle-adolescent athletes, while its effectiveness decreases in higher-skilled adult athletes. These findings indicate that core muscle strength contributes significantly to the change of direction speed. Core training is therefore useful for improving CODS.

The effects of aerobic gymnastics training on performance-related variables in an elite athlete: a 2-year follow-up study.

This study investigates individual performance adaptations on 2 years of training between European Aerobics Championships. An elite, 22-year-old aerobic gymnast performed postural coordination test, Y-Balance test, squat and countermovement jumps, 60 s test of repeated jumps, an isokinetic leg muscle strength test, and the Wingate test. Postural stability and flexibility improved in terms of increased distance achieved in the Y-Balance test in the anterior (by 6.3%), posteromedial (by 2%), and posterolateral (by 4.8%) directions. Lower limb muscular endurance also increased, which can be corroborated by a reduced fatigue index in the 60 s test of repeated jumps (from 42% to 27% after the 1st and to 22% after the 2nd year of training). In addition, mean power increased during dominant (by 23.2% at 60°/s and by 18.5% at 180°/s) and non-dominant leg extension (by 4.9% at 180°/s and by 15.5% at 300°/s), plus dominant leg flexion (by 2.0% at 60°/s and by 6.9% at 300°/s). Similarly, peak torque/body weight ratio increased during dominant (by 24.9% at 60°/s, by 11.5% at 180°/s, and by 2.1% at 300°/s) and non-dominant leg extension (by 0.5% at 60°/s and by 6.4% at 300°/s), plus dominant leg flexion (by 1.7% at 60°/s and by 5.4% at 300°/s). However, 2 years of training failed to show any significant improvements in the explosive power of lower limbs and anaerobic performance. These findings indicate that general aerobic gymnastics training without any specific inputs leads to performance adaptation, namely, in abilities closely related to competition routine (dynamic balance and strength endurance of lower limbs).

Potential neurophysiological and biomechanical risk factors for sport-related back problems: A scoping review.

This scoping review aims (1) to map the literature dealing with neurophysiological and biomechanical aspects of back problems in athletes in order to identify valid risk-factors for their prevention, plus (2) to identify gaps in the existing research and propose suggestions for future studies. A literature search conducted with Scopus, Web of Science, MEDLINE and Cochrane Library was completed by Elsevier, SpringerLink and Google Scholar. The main neurophysiological risk factors identified leading to back problems in athletes are neuromuscular imbalance, increased muscle fatigability, muscle dysfunction and impaired motor control, whilst biomechanical risk factors include maladaptive spinal, spinopelvic and lower limb kinematics, side-to-side imbalances in axial strength and hip rotation range of motion, spinal overloading and deficits in movement pattern. However, most studies focused on back pain in the lumbar region, whereas less attention has been paid to thoracic and cervical spine problems. The range of sports where this topic has been studied is relatively small. There is a lack of research in sports in which the core muscles are highly involved in specific movements such as lifting weights or trunk rotations. A limited number of studies include female athletes and master athletes of both genders. In addition to chronic back pain patients, it is equally important to conduct research on healthy athletes with a predisposition to spine problems. Investigators should focus their empirical work on identifying modifiable risk factors, predict which athletes are at risk for back problems, and develop personalized sport-specific assessment tools and targeted prevention strategies for them. This review was registered using the Open Science Framework Registries (https://osf.io/ha5n7).

Interrelation of 31P-MRS metabolism measurements in resting and exercised quadriceps muscle of overweight-to-obese sedentary individuals.

Phosphorus magnetic resonance spectroscopy ((31)P-MRS) enables the non-invasive evaluation of muscle metabolism. Resting Pi-to-ATP flux can be assessed through magnetization transfer (MT) techniques, and maximal oxidative flux (Q(max)) can be calculated by monitoring of phosphocreatine (PCr) recovery after exercise. In this study, the muscle metabolism parameters of 13 overweight-to-obese sedentary individuals were measured with both MT and dynamic PCr recovery measurements, and the interrelation between these measurements was investigated. In the dynamic experiments, knee extensions were performed at a workload of 30% of maximal voluntary capacity, and the consecutive PCr recovery was measured in a quadriceps muscle with a time resolution of 2 s with non-localized (31)P-MRS at 3 T. Resting skeletal muscle metabolism was assessed through MT measurements of the same muscle group at 7 T. Significant linear correlations between the Q(max) and the MT parameters k(ATP) (r = 0.77, P = 0.002) and F(ATP) (r = 0.62, P = 0.023) were found in the study population. This would imply that the MT technique can possibly be used as an alternative method to assess muscle metabolism when necessary (e.g. in individuals after stroke or in uncooperative patients).

Reaction time in the agility test under simulated competitive and noncompetitive conditions.

The study evaluates a reaction time in the Agility Test under simulated competitive and noncompetitive conditions. A group of 16 fit men performed, in random order, 2 versions of the Agility Test: non-competitive Agility Single and Agility Dual in form of simulated competition. In both cases, subjects had to touch, as fast as possible, with either the left or the right foot 1 of 4 mats located in 4 corners outside of an 80 cm square. Mats had to be touched in accordance with the location of the stimulus in one of the corners of the screen. The test consisted of 20 visual stimuli with random generation of their location on the screen and time generation from 500 to 2,500 milliseconds. The result was total reaction time (RT) for all 20 reactions measured by a PC-based system FiTRO Agility Check. Results showed significantly (p < 0.01) better RT in the Agility Dual than in the Agility Single Test (690.6 ± 83.8 milliseconds and 805.8 ± 101.1 milliseconds, respectively). Further comparisons of RT under noncompetitive and simulated competitive conditions for the best 8 subjects proceeded in the second match showed a decrease from 781.3 ± 111.2 milliseconds to 693.6 ± 97.8 milliseconds in the first match and to 637.0 ± 53.0 milliseconds in the second match. It may be concluded that RT is better when the Agility Test is performed in simulated competitive than noncompetitive conditions. The Agility Test in form of competition may be used for children and young athletes to enhance their attention level and motivation.

Diaphragmatic breathing exercises in recovery from fatigue-induced changes in spinal mobility and postural stability: a study protocol.

Prolonged periods of sitting at work can increase trunk muscle fatigue from the continuous contraction of deep trunk muscles. Insufficient activity of these muscles can decrease muscular support to the spine and increases stress on its passive structures. This can lead to reduced spinal mobility and impaired postural stability. It may also stimulate nociceptor activity leading to pain. However, frequently used recovery modalities such as muscle strengthening and stretching exercises, can be time-consuming, impractical, and difficult to implement in the workplace. Diaphragmatic breathing exercises, which increase the activity of the deep trunk muscles by raising intra-abdominal pressure, seem to be a suitable alternative. However, little is known as to what extent diaphragmatic breathing exercises contribute to the reduction of fatigue induced by prolonged sitting. This paper presents a study protocol that aims to investigate the acute effect of diaphragmatic breathing exercises on recovery of fatigue-induced changes in spinal mobility and postural stability in sedentary middle-aged adults at risk of developing non-specific low back pain. Twenty sedentary adults aged between 25 and 44 years will perform Abt's fatigue protocol, followed by 1) active recovery using diaphragmatic breathing exercises and 2) passive recovery in the form of lying on the bed, respectively. There will be 1 week of rest in-between. Pre-fatigue, post-fatigue, and after the active and passive recovery, spinal mobility and postural stability will be evaluated using the spinal mouse device and a posturography system, respectively. The electromyography will be used to determine the muscle-fatigue conditions. We hypothesize that active recovery in a form of diaphragmatic breathing exercises would be more effective in restoring spinal mobility and postural stability followed by the fatigue of back and hamstring muscles compared to passive recovery in sedentary adults. Increasing core and respiratory muscle strength via these exercises could be beneficial for overall mobility and stability of the spine. Reducing compressive stress on the passive structures of the spine may be also beneficial for lowering low back pain. Therefore, we believe that diaphragmatic breathing exercises have the possibility to be incorporated into the workplace and contribute to better back health in sedentary middle-aged adults. Clinical Trial Registration: [https://www.irct.ir/trial/67015], identifier [IRCT20221126056606N1].

Performance in the Yo-Yo Intermittent Recovery Test May Improve with Repeated Trials: Does Practice Matter?

The Yo-Yo Intermittent Test is frequently used to monitor changes in athletes' performance in response to different interventions. However, the question remains as to whether, and to what extent, retakes of this test would contribute to these changes. This case study sought to determine the magnitude of practice effects, involving test repetition, on performance in the Yo-Yo Intermittent Recovery Test. A recreational soccer player performed four attempts of the Yo-Yo Intermittent Recovery Test-Level 1 (YYIR1) with a week's rest in between. The same participant repeated this test protocol (four attempts of the YYIR1) again after six months. Changes in distance covered, level achieved, maximal oxygen uptake, and heart rate between the first and last attempt were assessed. The smallest worthwhile change (SWC), the coefficient of variation (CV), and the 2CV were calculated to identify a trivial, a possibly meaningful, and a certainly meaningful change in YYIR1 performance. The distance covered in the first set of measurements increased from 1320 m to 1560 m (15.4%), which corresponds to a 4.6% increase in the level achieved (from 16.6 to 17.4). Similarly, the distance covered in the second set of measurements increased from 1280 m to 1560 m (17.9%), which corresponds to a 5.5% increase in the level achieved (from 16.5 to 17.4). The participant's performance changes fell outside of the SWC and the CV, but not the 2CV during both sets of measurements. These improvements in YYIR1 performance may be ascribed to practice with repeated attempts of the test by improving running technique at the turning point and/or by simply increasing the linear speed. This fact should always be kept in mind when interpreting the effects of training. Practitioners should differentiate between practice effects associated with repeated test execution and adaptation induced by conducting sport-specific training.

Neuromuscular and perceptual-cognitive response to 4v4 small-sided game in youth soccer players.

The physical and psychological load of small-sided games (SSGs) can affect players' neuromuscular and cognitive functions. Yet, little is known about the acute performance changes after such a specific exercise in young soccer players and their association with exercise load applied. This study investigates i) the neuromuscular and perceptual-cognitive response to the SSG exercise load, and ii) the relationship between pre- and post-SSG changes in variables of performance and the respective exercise load in youth soccer players. Sixteen participants (13.6 ± 0.5 years) underwent a 30-min SSG 4v4 + GK protocol. Prior to and after the SSG they performed countermovement jump (CMJ), planned and reactive Y-shaped agility tests (PA, RA), and go/no-go task (GNG). Their subjective perception of fatigue was evaluated by visual analog scale. Fatigue induced by SSG (perception of fatigue increased by 41.56%, p = .001, g = 4.15) increased PA time (4.04%, p = .002, g = .97), RA time (6.45%, p = .003, g = 1.16), and number of errors in the response inhibition task (87.1%, p = .023, rc = .57), whilst decreased CMJ height (-6.65%, p = .014, g = .56). These performance deteriorations were not significantly associated with neither internal nor external load variables. However, a less pronounced drop in performance was related to external load variables, i.e., ∆CMJ height and ∆RA time correlated with very high-speed running (rs = .66, p = .006; rs = -.50, p = .022; respectively) and maximal speed (r = .54, p = .032; r = -.52, p = .037; respectively), whilst ∆PA time was associated with high-intensity accelerations (rs = -.76, p = .002). These findings indicate that fatigue induced by SSG affects both planned and reactive agility, decision-making in response inhibition task, and explosive strength in youth soccer players regardless of significant contribution of any robust internal or external load variables. Nonetheless, high-intensity actions within SSG partially compensate for the decrements in their agility performance and explosive strength. The load variables encountered during SSG do not fully reflect youth players' neuromuscular and perceptual-cognitive responses to sport-specific exercise.

The Effects of Biological Age on Speed-Explosive Properties in Young Tennis Players.

Biological maturity can affect performance on motor tests, thus young players can have advantages or disadvantages during testing by being more or less mature than their peers of the same chronological age. The aim of this study was to investigate the effects of biological age on speed, agility, and explosive power in young tennis players. Fifty tennis players (age 12.3 ± 1.2 years, height 156.7 ± 12.8 cm, body mass 45.9 ± 8.9 kg) who were ranked up to 50th place in the ranking of the National Tennis Association, as well as up to 300th place on the international "Tennis Europe" ranking, participated in the study. They were divided into three groups according to their maturation status, that is, the peak height velocity (PHV) maturity offset: pre-PHV [<0] (n = 10); circa-PHV [0 to 1] (n = 10); and post-PHV [>1.5] (n = 25). They performed tests of speed (5, 10, and 20 m sprints), agility (20 yards, 4 × 10 yards, T-test, TENCODS, and TENRAG), and explosive power (countermovement jump, one-leg countermovement jump, squat jump, long jump, and one-leg triple jump). Results showed significantly higher height of the vertical jump in the post-PHV group compared to the pre-PHV group, in the range of ~16% to ~27%. Moreover, linear and change of direction speed was significantly faster in the post-PHV group compared to the pre-PHV group, in the range of ~5% to ~8%. Height of the squat jump and speed in the T-test were significantly better in the post-PHV group compared to the circa-PHV group, in the range of ~7% to ~15%, while height of the single-leg triple jump was significantly higher in the circa-PHV group compared to the pre-PHV group by ~7%. This study showed that tennis players of older biological age achieve better results in almost all variables of speed, agility, and explosive power compared to players of younger biological age. Coaches should be aware of the differences found in physical performance and consider the practical implications that maturation can have in the long-term development of young tennis players.

Advancing age is associated with more impaired mediolateral balance control after step down task.

BACKGROUND: To prevent the risk of stair descent falls and associated injuries in the older adults, it is important to understand the factors that affect this frequent locomotion of daily living. The fact that falls are in most cases the result of the interaction between intrinsic and extrinsic factors is very often underestimated when designing test protocols. RESEARCH QUESTION: This study aimed at evaluating balance control during and immediately after step down onto an unstable surface. METHODS: Physically active men and women aged 60-69 years (n = 28) and 70-79 years (n = 18) were asked to perform a step down onto a foam pad and stand still for 30 s (restabilization phase). Centre of pressure (CoP) velocity and standard deviation of CoP sway in anteroposterior and mediolateral direction were evaluated during the step down (CoP VAP, CoP VML, CoP SDAP, CoP SDML) and in the first 5 s of restabilization (CoP VAP5, CoP VML5, CoP SDAP5, CoP SDML5). In addition, time to complete step was investigated. RESULTS: Participants aged 70-79 years presented worse ML balance control after step down onto an unstable surface than their younger counterparts. This was represented by the significantly higher values of CoP SDML5 and CoP VML5 (p = 0.022 and p = 0.017). No other significant differences were detected. SIGNIFICANCE: Age is associated with a more significant ML center of pressure velocity and sway after step down onto a foam pad in physically active older adults. Exercises aimed at improving ML balance control in unstable conditions should be the subject of physical interventions even in older adults with overall good state of health and physical fitness.

Exercise-Induced Fatigue Impairs Change of Direction Performance and Serve Precision among Young Male Tennis Players.

This study investigates the effect of exercise-induced fatigue on change of direction performance and serve precision among young tennis players. A group of 21 players (age 12.90 ± 0.76 years), ranked among the top 50 players on the national tennis federation scale and the top 300 on the "Tennis Europe" scale, participated in the study. They underwent a standardized physiological load protocol using the "300-m running test" which consists of consecutive runs for 15 shares of 20 m (15 × 20). Its intensity was determined using the Borg Rating of Perceived Exertion (RPE) scale where subjects evaluated their level of experienced load on a scale from 0 to 10. Prior to and after the protocol, they performed a pre-planned change of direction T-test and serve precision test. Results showed significant increase of time in the T-test (from 11.75 ± 0.45 s to 12.99 ± 0.4 s, p = 0.00) and decrease in serve precision parameter from (6.00 ± 1.04 to 4.00 ± 1.26, p = 0.00) after the fatigue test protocol. The RPE increased from 5 to 9, after the fatigue protocol, indicating the desired fatigue effect. These findings indicate that exercise-induced fatigue impairs change of direction performance and serve precision among young tennis players.