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

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

Dissipative control for singular Markovian jump systems with time delay

This paper focuses on the problem of dissipative control for continuous‐time singular time‐delayed systems with Markovian jump parameters. Different from usually mode‐dependent or mode‐independent controller design methods, a partially mode‐dependent dissipative controller is firstly proposed via using a mode‐dependent Lyapunov function. The stochastic property of the mode available to a controller operation is taken into consideration in the corresponding controller design. Moreover, the existence of the established controller is given in terms of strict linear matrix inequalities. Finally, numerical examples are used to demonstrate the effectiveness of the given theoretical results. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance progression of elite jumpers: Early performances do not predict later success

This study aimed (a) to estimate the transition rate for top 50 ranked track and field jumpers, (b) to compare the performance progression of top 50 ranked senior jumpers (top50 senior) to those who failed to be top 50 ranked in the senior category despite being top 50 ranked in the under 18 category (only U18), and (c) to verify whether relative age effect may at least partially explain the differences in the two above‐mentioned subgroups. The career performance trajectories of 5981 athletes (2837 females) competing in jump events from 2000 to 2019 were extracted from the World Athletics database. The all‐time top 50 ranked athletes for each age from 16 years to senior category were identified. Performance progression characteristics were compared using linear mixed‐effects model. Only 8% of males and 16% of females top 50 ranked at the age of 16 years managed to be included among the top50 senior. Only U18subgroup made the first appearance in the database (at 15‐16 years) and reached the peak performance (at 20 years) earlier than top50 senior (17‐18 and 26‐27 years, respectively). The relative age effect was largely present in Only U18 but not in top50 senior subgroups. Most of the early‐successful U18 world‐class jumpers did not manage to maintain the same level of competitiveness in adulthood since they experienced a plateau in performance from 20 years of age. Conversely, top 50 ranked senior jumpers continued to produce consistent performance improvement up to 26‐27 years of age.     

Back squat velocity to assess neuromuscular status of rugby league players following a match

ObjectivesBack squat mean concentric velocity (MV) and countermovement jump (CMJ) performance were examined in sub-elite rugby league players post-match to monitor changes in neuromuscular status (NMS) from baseline. Relationships between changes in back squat MV and CMJ performance variables were used to compare back squat MV to an established method to monitor NMS.DesignLongitudinal observational design.Methods18 male sub-elite rugby league players (mean ± SD, 20.5 ± 2.4 yr; 180.0 ± 6.7 cm; 93.3 ± 11.2 kg) performed 3 repetitions of CMJ and back squat with an individualised, pre-determined load at ?2 h (baseline), +30 min, +24 h, and +48 h in relation to a match. Back squat MV, CMJ height, CMJ peak power, and CMJ peak velocity were measured with a linear position transducer.ResultsSignificant (p < 0.05), small to large decreases (ES = 0.52–1.24) were observed in back squat MV up to +48 h post-match. Significant (p < 0.05), small to moderate decreases (ES = 0.52–0.70) in CMJ height were also observed up to +24 h post-match, returning to baseline at +48 h. CMJ peak power and peak velocity post-match changes were not significant compared to baseline (p > 0.05). Significant positive correlations were found between changes in back squat MV and CMJ height at +30 min (r = 0.59; p = 0.009) and +48 h (r = 0.51; p = 0.03).ConclusionsThese findings suggest back squat MV may be a suitable alternative or addition to CMJ testing for monitoring NMS in rugby league players.     

Lower preseason reactive strength index scores are associated with injury in female collegiate volleyball players but not male collegiate basketball players

ObjectivesFunctional tests are used by sports medicine professionals to discriminate injury risk in athletes. One test that has shown promise is the drop vertical jump (DVJ); however, it is primarily used to evaluate measures associated with anterior cruciate ligament injury. The DVJ test can also be used to calculate the reactive strength index (RSI); a measure used to assess an athlete’s power. The ability of the RSI to discriminate injury risk is unknown. The purpose of this study was to prospectively evaluate the ability of preseason RSI scores to identify athletes at risk for a noncontact time-loss injury to the low back or lower extremities.DesignProspective cohort.MethodsOne hundred and fifty-five male collegiate basketball (BB) players and 117 female collegiate volleyball (VB) players were recruited for this study. DVJ tests were performed in a motion capture lab.ResultsFemale VB players with a RSI 0.9125 m/s or less (30.48 cm box) were 4 times (relative risk = 4.2 [95% CI: 1.0, 17.7]; p-value = 0.024) more likely to be injured. There was no association between preseason scores and injury in the male BB athletes.ConclusionRSI scores should be collected for female collegiate VB players as part of a preseason screen.     

The Concerted Activity in Parallel Proprioceptive Pathways Controls the Initiation of Co-activation in the Locust Kick Motor Programme

To jump and kick the locust uses a catapult mechanism implemented by a three-stage motor programme: initial flexion of the hind tibiae, co-activation of the antagonist flexor and extensor tibiae motor neurons and trigger inhibition of the flexor motorneurons. The transition from stage 1 to stage 2 thus involves a switch from the normal alternate activation to co-activation of the antagonist tibiae motorneurons. However, co-activation has never been observed when the central nervous system has been isolated from the leg. This led us to investigate the possibility that the transition from stage 1 to stage 2 is controlled by a proprioceptive signal. In the first set of experiments intracellular recordings were made in the flexor and extensor motorneurons while the position of the tendon of the femoral chordotonal organ (FCO), which signals tibial position and movement, was experimentally controlled. In these heavily dissected preparations, stretch of the FCO tendon (signalling tibial flexion) was a necessary condition for co-activation. However, in minimally dissected preparations (in which merely EMG recordings were made), we found that co-activation occurred even when the FCO was signalling tibial extension, suggesting the involvement of other proprioceptors. A series of experiments were then conducted on minimally dissected preparations to determine the relative contributions of each of the three main hind leg proprioceptors which might signal tibial flexion: the FCO, the lump receptor and Brünners organ. When all three proprioceptors were intact the chance of evoking co-activation was largest, when all three were eliminated co-activation could no longer be evoked, irrespective of the level of arousal. Various combinations of partial de-afferentation showed that the FCO plays the major role, with the lump receptor and Brünners organ playing significant, but progressively less important, roles. We conclude that the three receptors act together as a permissive proprioceptive gate for the kick and jump motor programme, but with a hierarchy of the strengths of their effectiveness.     

H∞ control for discrete-time Markovian jump linear systems with partially uncertain transition probabilities

This work is concerned with the H_∞ state-feedback control for the discrete-time Markovian jump systems with incomplete knowledge of transition probabilities. A less conservative criterion is proposed via linear matrix inequalities (LMIs) such that the considered systems are stochastically stable and have a prescribed H_∞ disturbance attention level. Furthermore, based on the obtained results, a state-feedback controller is presented via LMIs to guarantee the stochastic stability of the resulted closed-loop system with a prescribed H_∞ performance level γ . The number of the LMIs to be solved in the developed methods is much less than that in many existing methods. A numerical example and a practical example are provided to verify the effectiveness of the presented methods.     

Partially observed time‐inconsistent stochastic linear‐quadratic control with random jumps

This paper studies a partially observed time‐inconsistent stochastic linear‐quadratic control system, in which the state follows a stochastic differential equation driven by a Brownian motion and an independent Poisson random measure. The cost functional contains a state‐dependent term and a quadratic term of the conditional expected state process, which will cause the time inconsistency in dynamic systems. By virtue of a classical spike variation approach, we define an equilibrium and derive a sufficient condition for the equilibrium in the fully observed system with stochastic coefficients. Then, we obtain the equilibrium with an explicit feedback form in deterministic coefficients case and discuss the existence and uniqueness of the solution of corresponding Riccati equations. Furthermore, we get filtering equations of the partially observed system and get the explicit equilibrium in some special case.