Muscle Strength Asymmetries of the Lower Limbs and Trunk in Ballet Dancers.

BACKGROUND: Ballet dancers are expected to use their bodies symmetrically during training, because dance movements are performed on both sides. However, there is a general belief that ballet training encourages the use of one side of the body more than the other. Frequent repetition of a particular exercise can lead to body asymmetries and musculoskeletal injuries. The aim of this cross-sectional study was to investigate the presence of lower limbs and trunk muscle strength asymmetries in ballet dancers and secondly to assess whether there is a difference between professional dancers and ballet students. METHODS: Ballet students (n=19) and professional ballet dancers (n=23) performed maximal voluntary isometric contractions of the trunk (flexion, extension, lateral flexion), hip (flexion, extension, adduction, abduction, external and internal rotation), knee (flexion, extension) and ankle (flexion, extension) on isometric dynamometer. RESULTS: The results showed that the percentage of ballet dancers with contralateral muscle strength asymmetries >10% ranged from 22.5% (ballet students) to 31.6% (professional dancers). The percentage of ballet dancers deviating by >10% from the normative maximum torque agonist/antagonist ratio ranged from 56.5% to 100%. A statistically significant difference between ballet students and professional ballet dancers was found in the trunk flexion/extension ratio (t(40) = -3 .55; p = 0.001; d = 0.55). CONCLUSION: This study revealed strength asymmetries in the lower limbs and trunk in ballet dancers, both professionals and students. Further research is needed to develop appropriate complementary exercise to address and eliminate asymmetries in muscle strength in ballet dancers.

The relationship between elastography-based muscle properties and vertical jump performance, countermovement utilization ratio, and rate of force development.

This study explored the relationships between passive muscle stiffness (shear modulus) and vertical jumping performance, countermovement utilization ratio (CUR) and rate of force development (RFD) in an attempt to unravel the mechanism that may explain the association between shear modulus and performance. 32 recreationally active participants (16 males, 16 females; age: 22.4 ± 5.1 years) participated. Shear modulus was assessed for the lateral and medial gastrocnemius (GL and GM), and vastus medialis (VM) and lateralis (VL) muscles using shear wave elastography. Squat jump (SJ) and countermovement (CMJ) jump were determined, with CUR being expressed as the ratio between the two. RFD in ankle and knee extension tasks was measured using isometric dynamometers. Our results suggest that within a heterogeneous group of recreational athletes, passive muscle stiffness is not related to RFD and jump performance, but positively related to CUR. In males, shear modulus of the GL was positively related to SJ height (r = 0.55). We also found inverse moderate correlations between VL and VM shear modulus and RFD in females only (r = -0.50 to -0.51), but this relationship was possibly affected by age and body fat content. Different mechanisms may underpin the association between shear modulus and performance depending on the muscle, task and population investigated.

Influence of unilateral and bilateral knee extension fatigue protocols on inter-limb asymmetries assessed through countermovement jumps.

This study explores the sensitivity of jump type (unilateral and bilateral) and output variable (mean force, propulsive impulse, and jump height) to detect the changes in inter-limb asymmetries induced by unilateral and bilateral fatigue protocols. Thirty-eight individuals performed two testing sessions that consisted of (I) nine "pre-fatigued" countermovement jumps (CMJs; three bilateral and six unilateral [three with each leg]), (II) fatigue protocol and (III) nine "post-fatigued" CMJs. The testing sessions only differed in the fatigue protocol (five sets to failure against the 15-repetition maximum load using either the unilateral or bilateral knee extension exercise). The magnitude of all CMJ-derived variables (mean force, impulse, and jump height) decreased following both unilateral (p ≤ 0.002) and bilateral fatigue protocols (p ≤ 0.018). However, only unilateral protocol accentuated inter-limb asymmetries, which was detected for all variables during the unilateral CMJ (from -4.33% to -2.04%; all p < 0.05) but not during the bilateral CMJ (from -0.64% to 0.54%; all p > 0.05). The changes in inter-limb asymmetries following the unilateral and bilateral fatigue protocols were not significantly correlated between the unilateral and bilateral CMJs (rs ≤ 0.172). The unilateral CMJ should be recommended for the testing purposes over the bilateral CMJ due to its greater sensitivity to detect the selective effects of fatigue.

Validity of the German Version of Daily Activity Behaviours Questionnaire Among Older Adults.

Time spent in physical activity, sedentary behavior, and sleep collectively impact health of older adults. There is a need for valid self-reported methods for the assessment of movement behaviors across the entire 24-hr day. The aim of this study was to explore the validity of the German version of Daily Activity Behaviours Questionnaire (DABQ), the "Schlaf- und Aktivitätsfragebogen (SAF)," among older adults. Participants were asked to wear activity monitor (activPAL) for a period of 8 days and to complete the German version of DABQ. Seventy-seven participants (45 females; 68 ± 5 years of age) completed the protocol. Spearman's correlation coefficients between DABQ and activPAL estimates for time spent in sleep, sedentary behavior, light physical activity, and moderate to vigorous physical activity were .69, .35, .24, and .52, respectively. The German version of the DABQ showed satisfactory validity to be used in epidemiological research and population surveillance among older adults.

The Effects of Workload Difference Between Limbs in Plyometric and Strength Exercises in Reducing Asymmetry in Change of Direction Ability During Basketball Season.

ABSTRACT: Ujakovic, F, and Sarabon, N. The effects of workload difference between limbs in plyometric and strength exercises in reducing asymmetry in change of direction ability during basketball season. J Strength Cond Res 37(11): 2282-2288, 2023-The topic of interlimb asymmetries has recently been extensively investigated; despite that, intervention for purpose of reducing them have been narrowly examined. The present study investigated the effects of maximal strength and plyometric resistance training program with 3:1 workload ratio (nondominant to dominant side) on change of direction deficit (CoDdef) asymmetry during the basketball season. Thirty-six highly trained male basketball players (19.3 ± 4.9 years) from different age categories (U-17, U-19, and senior) were divided into 2 groups: the control group ( n = 18) and the intervention group ( n = 18). The intervention group performed once-a-week training that consisted of strength (Bulgarian split squats) and jumping (single-leg countermovement jumps [SLCMJs]) part in which nondominant-to-dominant workload ratio was 3:1 during the 8-week period. Preintervention and postintervention testing included single-leg isometric squat, SLCMJ, 10-m sprint, and 505 test. Analysis of covariance results showed a statistically significant difference with large effect size ( p < 0.05; η2 = 0.26) when preintervention asymmetry was included as covariate. Post hoc analysis showed a statistically significant decrease in mean CoDdef asymmetry between preintervention (11.88%) and postintervention (4.47%) testing ( p < 0.01, effect size [ES] = -1.37). Isometric squat strength increased on nondominant side in the intervention group ( p < 0.05; ES = 0.77). No other performance or asymmetry variable showed significant change. In conclusion, a 3:1 workload of nonspecific movement task in favor of nondominant limb is an effective training strategy to reduce CoDdef asymmetry during basketball season.

Comparison of Vertical and Horizontal Reactive Strength Index Variants and Association With Change of Direction Performance.

ABSTRACT: Sarabon, N, Kozinc, Z, and Bishop, C. A comparison of vertical and horizontal reactive strength index variants and association with change of direction performance. J Strength Cond Res 37(1): 84-90, 2023-This study sought to investigate the interrelationship between different vertical and horizontal variants of reactive strength index (RSI) and change of direction (CoD) performance. Thirty-one male volleyball players (age: 22.4 ± 3.9 years) performed bilateral drop jumps (DJs), bilateral and unilateral countermovement jumps (CMJs), and triple hops for distance. The RSI was calculated as the ratio of jump height and contact time (DJ), jump height and time to takeoff (CMJ), flight time or hop distance and contact time (triple hop), and 505 CoD test. Reactive strength index obtained from DJ and CMJ tasks exhibited excellent trial-to-trial reliability (intraclass correlation coefficient [ICC] = 0.91-0.94), whereas triple hop-based RSI had only moderate reliability (ICC = 0.67-0.74). The relationships among different RSI variants were moderate to high (i.e., DJ to CMJ: r = 0.57-0.69, p ≤ 0.004; DJ to triple hop: r = 0.54-0.66, p ≤ 0.021; and CMJ to triple hop: r = 0.42-0.63, p ≤ 0.037). For the triple hop, the associations between RSI based on hop flight time and RSI based on hop distance were high for hop 1-2 (r = 0.77-0.83; p < 0.001) and very high for hop 2-3 (r = 0.91-0.92; p < 0.001). All RSI variants were in small to moderate negative correlation with 505 test performance (r = -0.38 to -0.45; p ≤ 0.042). The agreement in interlimb asymmetry direction between RSI from unilateral CMJ and triple hop RSI variables was slight to moderate (kappa coefficient = 0.06-0.36). In conclusion, although interrelationships between RSI variants were moderate to high, the direction of interlimb asymmetry was inconsistent, highlighting the notion of movement variability in limb dominance.

Effect of a High-Intensity Handball-Specific Fatigue Protocol Focused on the Leg Contralateral to the Throwing Arm on Interlimb Asymmetries.

ABSTRACT: Janicijevic, D, Pérez-Castilla, A, Miras-Moreno, S, Ortega-Becerra, M, Morenas-Aguilar, MD, Smajla, D, Sarabon, N, and García-Ramos, A. Effect of a high-intensity handball-specific fatigue protocol focused on the leg contralateral to the throwing arm on interlimb asymmetries. J Strength Cond Res 37(7): 1382-1389, 2023-This study aimed to elucidate which countermovement jump (CMJ) variant (unilateral or bilateral) is more sensitive to detect the decrement in kinetic CMJ-derived variables of the leg more actively involved in a handball-specific fatigue protocol. Seventeen female handball players (age: 20.6 ± 2.5 years) performed a fatigue protocol consisting of 8 repetitions of the following circuit separated by 10 seconds: 10 m sprint, 180° change of direction with the leg contralateral to the throwing arm, 10 m sprint with ball reception at 7 m, and handball throw preceded by a jump over a 40 cm hurdle with the leg contralateral to the throwing arm. Before and after the fatigue protocol, 6 unilateral CMJs (3 with each leg) and 3 bilateral CMJs were performed on a dual Kistler force platform (model 9260AA6). Bilateral CMJ height was reduced by 5.4% after the fatigue protocol (p < 0.001). However, the fatigue protocol did not promote any significant change in peak force, mean force, and propulsive impulse or in their asymmetry values (17 of 18 comparisons). The decrement of 6.1% during the unilateral CMJ in the propulsive impulse developed by the leg ipsilateral to the throwing arm (less involved in the fatigue protocol) was the only variable that reached statistical significance (p = 0.038). The pre-post fatigue changes in asymmetry values presented negligible correlations between both CMJ variants (rs = 0.01 to -0.19). These results suggest that neither unilateral nor bilateral CMJs are able to detect changes in interlimb asymmetries after a high-intensity handball-specific fatigue protocol.

The rate of force development scaling factor: a review of underlying factors, assessment methods and potential for practical applications.

When humans perform isometric ballistic contractions of different intensities, a strong linear relationship is observed between peak force and peak rate of force development. The slope of this relationship, termed the rate of force development scaling factor (RFD-SF), has been proposed as a measure of neuromuscular quickness of submaximal contractions. The RFD-SF is largely dependent on motor unit firing rate and its modulation with contraction intensity, and possibly by agonist-antagonist coordination. The purpose of this review was to critically evaluate the available literature on RFD-SF, including the underlying physiological mechanisms, assessment protocols and methodological considerations, practical application to clinical and sports practice and additional factors such as sex, muscle group, task and fatigue. The main findings of the review are: (a) RFD-SF is very sensitive to impairments associated with aging and neurological diseases, (b) the literature lacks studies on the importance of RFD-SF for athletes, with some evidence that RFD-SF may affect the accuracy of submaximal movements, (c) RFD-SF is not largely influenced by fatigue and sex, (d) the RFD-SF values have been very consistent across muscle groups, with an exception of ankle muscles, (e) the methodology for RFD-SF assessment appears to be well established and standardized. Future research is needed to determine if RFD-SF provides any independent information that cannot be captured with peak force/torque and RFD.

Associations of meeting 24-h movement guidelines with stress and self-rated health among adults: is meeting more guidelines associated with greater benefits?

BACKGROUND: Several countries have recently issued 24-h movement guidelines that include quantitative recommendations for moderate-to-vigorous physical activity (MVPA), sedentary behaviour (SB), and sleep. This study explored the associations of meeting the 24-h movement guidelines with stress and self-rated health among adults, and whether the likelihood of favourable outcomes increases with the number of guidelines met. METHODS: A total of 2476 adults aged 18 years and over completed a questionnaire on their time spent in MVPA, SB and sleep, frequency of stress (never, very rarely, occasionally, often, every day), self-rated health (very good, good, fair, bad, very bad), sociodemographic characteristics, and lifestyle variables. RESULTS: In an ordinal logistic regression analysis adjusted for age, sex, body mass index, education, socio-economic status, employment, place of residence, living with or without partner, and smoking, lower odds of higher frequency of stress were found for those meeting the combined 24-h movement guidelines (adjusted odds ratio [OR] = 0.45; 95% confidence interval [CI]: 0.32, 0.63; p <  0.001), any combination of two guidelines (OR range: 0.48-0.63; p <  0.05 for all), and sleep guideline only (OR = 0.51; 95% CI: 0.35, 0.75; p = 0.001). Higher odds of better self-rated health were found for those meeting the combined 24-h movement guidelines (OR = 2.94; 95% CI: 2.07, 4.19; p <  0.001), combination of MVPA and SB guidelines (OR = 2.33; 95% CI: 1.57, 3.44; p <  0.001), combination of MVPA and sleep guidelines (OR = 1.78; 95% CI: 1.23, 2.59; p = 0.002), and MVPA guideline only (OR = 2.24; 95% CI: 1.50, 3.36; p <  0.001). Meeting more guidelines was associated with greater odds of favourable outcomes (p for linear trend < 0.001). CONCLUSION: Adults who meet the sleep guideline, any combination of two guidelines, or all three guidelines experience stress less frequently. Meeting the MVPA guideline alone or in combination with any other movement behaviour guideline was associated with better self-rated health. The likelihood of less frequent stress and better self-rated health increases with the number of guidelines met. Adults should be encouraged to meet as many movement behaviour guidelines as possible.

Relationship between force-velocity-power profiles and inter-limb asymmetries obtained during unilateral vertical jumping and singe-joint isokinetic tasks.

We explored the associations between force-velocity-power (FVP) profiles and inter-limb asymmetries in FVP profiles obtained during unilateral jumping and single-leg isokinetic knee extension and flexion tasks. Thirty (13 senior and 17 junior) basketball players performed unilateral jumps with different levels of unloading (0-40% body mass) and isokinetic knee extension and flexion at different angular velocities (60-300°/s) with each leg. Senior and junior basketball players exhibited similar jump performance, normalized knee extensor and flexor strength, and relatively similar jumping and isokinetic FVP profiles. No significant associations between unilateral jump and isokinetic knee extension and flexion FVP parameters were found (all r ≤ 0.26; p > 0.05). Inter-limb asymmetries had no or very weak associations with unilateral jump height (-0.2 < r < 0.2; all p > 0.1); also, inter-limb asymmetries in jump and isokinetic FVP parameters were generally weakly associated (median r < 0.1). Finally, unilateral jump height was weakly-to-moderately associated with isokinetic F0, jumping Pmax, and normalized knee extension isokinetic peak torque at 60°/s and 240°/s (r = 0.35-0.57). These results suggest that the generalizability of the FVP profile of isolated muscle groups to complex ballistic tasks is weak.

Effects of Nordic hamstring exercise combined with glider exercise on hip flexion flexibility and hamstring passive stiffness.

Eccentric training proved to be effective in hamstring injury prevention; however, little is known about effects of eccentric hamstring training at long muscle length on hamstring flexibility. Hence, the aim was to evaluate the effect of eccentric training at long muscle lengths on flexibility and passive properties of the hamstring muscles. 34 physically active young adults were randomized to either the control or intervention group (6 weeks of eccentric hamstring training at long muscle length; control group resumed with their usual activities). Maximal passive hip flexion range of motion (ROM), passive hamstring stiffness, shear modulus and tendon length of the biceps femoris long head (BFlh) were measured pre- and post-intervention. A significant time × group effect was observed for maximal passive hip ROM. Post-hoc testing revealed a significant increase in the intervention group (+11.2%; p < 0.001; d = 1.55). Additionally, a significant time effect was shown for shear modulus in a relaxed position (p < 0.001). No significant interaction was shown for other parameters. Results indicate that eccentric hamstring training at long muscle length elicits large gains in hamstring flexibility, which are most likely not related to changes in passive hamstring stiffness or BFlh distal tendon length.

Short-Term Effects of a Passive Spinal Exoskeleton on Functional Performance, Discomfort and User Satisfaction in Patients with Low Back Pain.

Purpose Low back pain (LBP) remains a major worldwide healthcare issue. Recently, spinal exoskeletons were proposed as a potentially useful solution for LBP prevention and vocational reintegration for people who perform heavy load lifting, repetitive movements or work in prolonged static postures. The purpose of this study was to investigate how patients with LBP respond to the novel passive SPEXOR exoskeleton regarding functional performance, discomfort and general user impression. Methods Fourteen patients, with low to moderate LBP (2-7 on a 0-10 scale), performed 12 functional tasks with and without the exoskeleton. In addition to objective performance measures, participants subjectively assessed the level of local low back discomfort, task difficulty and general discomfort on a 0-10 visual analogue scales. Results The SPEXOR exoskeleton had favourable effects on performance and local discomfort during prolonged static forward bending. Minor reductions in performance were observed for sit-stand and ladder climbing tasks. The discomfort associated with the exoskeleton was generally low to moderate (median < 4), except for the 6-min walk test (median = 4.5), which is likely due to the weight of the device and obstruction of upper limb movement. The general impressions were mostly positive, with good adjustability, low interference with the movement and moderate support reported by the participants. Conclusion The SPEXOR exoskeleton is potentially useful for LBP prevention or management, however, further improvements are needed to provide higher levels support during heavy load lifting.

Validity, Reliability and Sensitivity to Change of Three Consumer-Grade Activity Trackers in Controlled and Free-Living Conditions among Older Adults.

Wrist-worn consumer-grade activity trackers are popular devices, developed mainly for personal use. This study aimed to explore the validity, reliability and sensitivity to change of movement behaviors metrics from three activity trackers (Polar Vantage M, Garmin Vivoactive 4s and Garmin Vivosport) in controlled and free-living conditions when worn by older adults. Participants (n = 28; 74 ± 5 years) underwent a videotaped laboratory protocol while wearing all three trackers. On a separate occasion, participants (n = 17 for each of the trackers) wore one (randomly assigned) tracker and a research-grade activity monitor ActiGraph wGT3X-BT simultaneously for six consecutive days. Both Garmin trackers showed excellent performance for step counts, with a mean absolute percentage error (MAPE) below 20% and intraclass correlation coefficient (ICC2,1) above 0.90 (p < 0.05). The MAPE for sleep time was within 10% for all the trackers tested, while it was far beyond 20% for all other movement behaviors metrics. The results suggested that all three trackers could be used for measuring sleep time with a high level of accuracy, and both Garmin trackers could also be used for step counts. All other output metrics should be used with caution. The results provided in this study could be used to guide choice on activity trackers aiming for different purposes-individual use, longitudinal monitoring or in clinical trial setting.

Factors Underlying Bench Press Performance in Elite Competitive Powerlifters.

ABSTRACT: Reya, M, Skarabot, J, Cveticanin, B, and Sarabon, N. Factors underlying bench press performance in elite competitive powerlifters. J Strength Cond Res 35(8): 2179-2186, 2021-Previous investigations of 1 repetition maximum bench press (1RM BP) performance have been either descriptive or have explored a limited number of contributing variables. The purpose of this study was to investigate the interplay between structural, technical, and neuromuscular factors in relation to 1RM BP in competitive powerlifters. Thirteen national and international level male powerlifters (26 ± 9 years, 178 ± 6 cm, and 93.8 ± 9.9 kg) visited the laboratory twice. Anthropometric and ultrasound measures were taken on the first visit, whereas performance measures (voluntary activation level, isokinetic strength, and kinetic, kinematic, and electromyographic measurements during 1RM BP) were recorded on the second visit. Correlation and multiple regression were used to investigate the contribution of structural, technical, and neuromuscular variables to 1RM BP corrected for body mass using the Wilks coefficient. The highest degree of association was shown for structural (lean and bone mass, brachial index, arm circumference, and agonist cross-sectional area [CSA]; r = 0.58-0.74) followed by neuromuscular factors (elbow and shoulder flexion strength; r = 0.57-0.71), whereas technical factors did not correlate with 1RM BP performance (r ≤ 0.49). The multiple regression showed that lean body mass, brachial index, and isometric shoulder flexion torque predicted 59% of the common variance in 1RM BP. These data suggest that in a sample of elite competitive powerlifters, multiple factors contribute to 1RM BP with variables such as lean body mass, the agonist CSA, brachial index, and strength of the elbow and shoulder flexors being the greatest predictors of performance.

Association between trunk muscle strength and static balance in older women.

The aim of this study was to determine the relationship between trunk muscle strength and balance ability in both simple and dual-task conditions in older women. Sixty-seven older women (age: 67.2 ± 1.1 years) volunteered in this study. Each participant performed isometric trunk muscle strength testing in standing position (extension, flexion, and lateral flexion) and balance testing in semitandem stance for 30 seconds in two conditions: with and without an additional cognitive task (counting down by 3 from 300). Balance was quantified via force plate as a mean velocity of center of pressure (CoP) displacement over 30 seconds. We found a statistically significant correlation between the trunk extensor strength and mean velocity of CoP displacement, both without (r = 0.5, p < .05) and with an additional cognitive task (r = 0.4, p < .05). There were no statistically significant correlations between trunk flexor and lateral flexors strength and balance measures (r < 0.2; p > .05). These results indicate that trunk extensor muscle strength is related to balance ability of healthy older women. Although additional studies are needed, our findings suggest that trunk strength training could be of importance in prevention of falls and fall-related injuries in seniors.

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.

Effects of eccentric training at long-muscle length on architectural and functional characteristics of the hamstrings.

Hamstring strain injuries during sprinting or stretching frequently occur at long-muscle length. Yet, previous research has mainly focused on studying the effectiveness of eccentric hamstring strengthening at shorter muscle length on hamstring performance, morphology, and hamstring strain injury risk factors. Here, we evaluated the effects of 6-week eccentric hamstring training at long-muscle length on functional and architectural characteristics of the hamstrings. Healthy and injury-free participants (n = 40; age 23.7 ± 2.5 years) were randomly assigned to control or intervention group. Training intervention consisted of 12 sessions with two eccentric hamstring exercises in a lengthened position. Outcome measures included isokinetic and isometric knee flexion peak torque, Nordic hamstring exercise peak torque, voluntary activation level, and countermovement jump performance. Ultrasonography was used to determine muscle thickness, pennation angle, and fascicle length of biceps femoris long head (BFlh). A significant time × group interaction effect was observed for all measured parameters except countermovement jump performance and muscle thickness. The training intervention resulted in increased concentric and eccentric knee flexion peak torque at 60°/s (d = 0.55-0.62, P = .02 and .03) and concentric peak torque at 180°/s (d = 0.99, P = .001), increased isometric knee flexion peak torque (d = 0.73, P = .008) and Nordic hamstring exercise peak torque (d = 1.19, P < .001), increased voluntary activation level (d = 1.29, P < .001), decreased pennation angle (d = 1.31, P < .001), and increased fascicle length (d = 1.12, P < .001) of BFlh. These results provide evidence that short-term eccentric hamstring strengthening at long-muscle length can have significant favorable effects on various architectural and functional characteristics of the hamstrings.

The Effect of Unicycle Riding Course on Trunk Strength and Trunk Stability Functions in Children.

Kocjan, A and Sarabon, N. The effect of unicycle riding course on trunk strength and trunk stability functions in children. J Strength Cond Res 34(12): 3560-3568, 2020-The aim of the study was to assess the effect of unicycling on trunk strength and timing of automatic stability actions of the selected trunk muscles (multifidus, obliquus externus, and erector spine). Twenty healthy 12-year-old children (12 boys, 8 girls; age 12.1 ± 0.2 years; body height 1.57 ± 0.05 m; body mass 52.8 ± 10.6 kg) were assigned to experimental and control group. Experimental group performed a supervised 12-session course of unicycling. Trunk strength was measured with a multipurpose diagnostic machine in frontal and sagittal planes in standing position. Trunk reflex responses and anticipatory actions were assessed through unexpected loading over the hands and rapid shoulder flexion, respectively. After the intervention, strength increased significantly (p < 0.01) in the experimental group in all analyzed positions. A significant interaction effect was observed during trunk extension (p < 0.01) and lateral flexion exertions (p < 0.03). Postural reflex latency improved significantly (p < 0.001) in the experimental group with a significant interaction effect in all analyzed muscles (p < 0.001). Anticipatory postural adaptations improved significantly (p ≤ 0.05) in multifidus and obliquus externus of the experimental group only. Unicycling proved to be an effective and funny tool to develop proximal stability and strength, which prevents low back pain and improves the efficiency of energy transfer between body segments. To improve the efficiency of physical education classes, unicycling should be considered a useful tool to increase trunk strength and stability among prepubertal children.

Nordic Hamstring Strength of Highly Trained Youth Football Players and Its Relation to Sprint Performance.

Markovic, G, Sarabon, N, Boban, F, Zoric, I, Jelcic, M, Sos, K, and Scappaticci, M. Nordic hamstring strength of highly trained youth football players and its relation to sprint performance. J Strength Cond Res 34(3): 800-807, 2020-We aimed to characterize Nordic hamstring (NH) strength and bilateral NH strength asymmetry in highly trained youth footballers and to investigate the relationship between NH strength and sprint performance. Twenty-two adult and 133 highly trained youth footballers in the age groups U12-U18 participated in this study. Eccentric hamstring strength was assessed using the NH device. Youth footballers (n = 119) also performed 20-m sprint test. Age-related changes in absolute and relative NH strength, and bilateral NH strength asymmetry were analyzed using 1-way analysis of variance. The linear relationship between relative NH strength and sprint performance was established using a Pearson correlation analysis. Significant age-related increases (F = 3.6-18.9; all p < 0.01) in NH strength were reported for all units except N·kg (F = 1.9; p = 0.08). The largest differences in absolute NH strength were seen between U15 and U16 groups. Bilateral NH strength asymmetry varied from 8 to 16% (F = 1.8; p = 0.09) across all age groups. A large correlation between NH strength and sprint performance was observed (r = -0.52; p < 0.01). Our results indicate that NH strength increases nonlinearly with players' age, with the highest values observed in U16 group. Furthermore, bilateral NH strength asymmetry varied nonsignificantly between 8 and 16%. Finally, 27% of variance of sprint performance of youth footballers could be explained by relative NH strength. The reported NH strength data could be used as normative standards during testing and training of youth football players. Present results also suggest that coaches should pay close attention to eccentric hamstring function in youth footballers.

Impairments of Postural Balance in Surgically Treated Lumbar Disc Herniation Patients.

Reports on body sway control following microdiscectomy lack reports on side-specific balance deficits as well as the effects of trunk balance control deficits on body sway during upright stances. About 3 weeks post microdiscectomy, the body sway of 27 patients and 25 controls was measured while standing in an upright quiet stance with feet positioned parallel on an unstable support surface, a tandem stance with the involved leg positioned in front or at the back, a single-leg stance with both legs, and sitting on an unstable surface. Velocity, average amplitude, and frequency-direction-specific parameters were analyzed from the center of pressure movement, measured by the force plate. Statistically significant differences between the 2 groups were observed for the medial-lateral body sway frequency in parallel stance on a stable and unstable support surface and for the sitting balance task in medial-lateral body sway parameters. Medium to high correlations were observed between body sway during sitting and the parallel stance, as well as between the tandem and single-legged stances. Following microdiscectomy, deficits in postural balance were side specific, as expected by the nature of the pathology. In addition, the results of this study confirmed the connection between proximal balance control deficits and balance during upright quiet balance tasks.