Changes in power and force generation during coupled eccentric–concentric versus concentric muscle contraction with training and aging

Age-related decline in maximal concentric muscle power is associated with frailty and functional impairments in the elderly. Compared to concentric contraction, mechanical muscle output is generally enhanced when muscles are rapidly pre-stretched (eccentric contraction), albeit less pronounced with increasing age. Exercise has been recommended to prevent loss of muscle power and function and recent guidelines indicate training program for increasing muscle power highly relevant for elderly subjects. This study examined the differences in muscle power, force and movement pattern during concentric-alone and coupled eccentric–concentric contraction and selected functional motor performances before and after 36-week multicomponent training including aerobic, strength, balance, flexibility and coordination components in elderly males. Vertical force, excursion, velocity, power and acceleration of the body center of mass were measured in two standardised vertical jumps (squatting jump, SQJ; countermovement jump, CMJ). Pre-stretch enhancement during CMJ did not improve performance [i.e., no enhanced maximal muscle power (P _peak) and jump height (JH)] compared to concentric-alone muscle contraction (SQJ). Nevertheless, pre-stretch enhancement occurred as for similar SQJ and CMJ maximal performance, elderly people employed lower mechanical work, higher mean muscle power (P _mean), shorter concentric phase duration and shorter body center of mass displacement during CMJ. Post training, CMJ P_peak, P _mean and JH increased in training group (P < 0.05) while P _peak and JH decreased in control group during the CMJ and SQJ (P < 0.05). In conclusion, long-term training counteracted the age-related decline in muscle power and functional performance observed in the control subjects, while substantial gains in muscular performance were observed in the trained elderly.     

A fully Bayesian mixture model approach for identifying noncompliance in a regulatory tobacco clinical trial

Identifying noncompliance in a randomized trial is challenging, but could be improved by leveraging biomarker data to identify participants that did not comply with their assigned treatment. For randomized trials of very low nicotine content (VLNC) cigarettes, the biomarker of total nicotine equivalents (TNE) could be used to identify noncompliance. Compliant participants should have lower levels of TNEs than participants that did not comply and smoked normal nicotine content cigarettes, resulting in a mixture of compliant and noncompliant participants at each dose level. Thresholds of TNE could then be identified from the compliant groups at each dose level and used to determine which study participants were compliant. Furthermore, proposed biological relationships of TNE with nicotine dose could be incorporated into improve the efficiency of estimation, but may introduce bias if misspecified. To account for multiple modeling assumptions across dose levels, we explore model averaging via reversible jump markov chain monte carlo (MCMC) within each dose level to take advantage of improvements in efficiency when the proposed relationship is true and to downweight the biological model when it is misspecified. In simulation studies, we demonstrate that model averaging in the presence of a correct biological relationship results in a decrease in the mean square error (MSE) of up to 85%, but downweights the model in dose levels where the relationship is not appropriate. We apply our approach to data from a randomized trial of VLNC cigarettes to estimate TNE thresholds and probability of compliance curves as a function of TNEs for each nicotine dose used in the trial.     

The impact of strength level on adaptations to combined weightlifting,plyometric, and ballistic training

The purpose of this investigation was to determine whether the magnitude of adaptation to integrated ballistic training is influenced by initial strength level. Such information is needed to inform resistance training guidelines for both higher‐ and lower‐level athlete populations. To this end, two groups of distinctly different strength levels (stronger: one‐repetition‐maximum (1RM ) squat = 2.01 ± 0.15 kg·BM⁻¹; weaker: 1.20 ± 0.20 kg·BM⁻¹) completed 10 weeks of resistance training incorporating weightlifting derivatives, plyometric actions, and ballistic exercises. Testing occurred at pre‐, mid‐, and post‐training. Measures included variables derived from the incremental‐load jump squat and the 1RM squat, alongside muscle activity (electromyography), and jump mechanics (force‐time comparisons throughout the entire movement). The primary outcome variable was peak velocity derived from the unloaded jump squat. It was revealed that the stronger group displayed a greater (P  =  .05) change in peak velocity at mid‐test (baseline: 2.65 ± 0.10 m/s, mid‐test: 2.80 ± 0.17 m/s) but not post‐test (2.85 ± 0.18 m/s) when compared to the weaker participants (baseline 2.43 ± 0.09, mid‐test. 2.47 ± 0.11, post‐test: 2.61 ± 0.10 m/s). Different changes occurred between groups in the force‐velocity relationship (P  =  .001‐.04) and jump mechanics (P  ≤  .05), while only the stronger group displayed increases in muscle activation (P  =  .05). In conclusion, the magnitude of improvement in peak velocity was significantly influenced by pre‐existing strength level in the early stage of training. Changes in the mechanisms underpinning performance were less distinct.     

A valid and reliable method to measure jump‐specific training and competition load in elite volleyball players

Use of a commercially available wearable device to monitor jump load with elite volleyball players has become common practice. The purpose of this study was to evaluate the validity and reliability of this device, the Vert, to count jumps and measure jump height with professional volleyball players. Jump count accuracy was determined by comparing jumps recorded by the device to jumps observed through systematic video analysis of three practice sessions and two league matches performed by a men's professional volleyball team. Jumps performed by 14 players were each coded for time and jump type and individually matched to device recorded jumps. Jump height validity of the device was examined against reference standards as participants performed countermovement jumps on a force plate and volleyball‐specific jumps with a Vertec. The Vert device accurately counted 99.3% of the 3637 jumps performed during practice and match play. The device showed excellent jump height interdevice reliability for two devices placed in the same pouch during volleyball jumps (r  = .99, 95% CI 0.98‐0.99). The device had a minimum detectable change (MDC ) of 9.7 cm and overestimated jump height by an average of 5.5 cm (95% CI 4.5‐6.5) across all volleyball jumps. The Vert device demonstrates excellent accuracy counting volleyball‐specific jumps during training and competition. While the device is not recommended to measure maximal jumping ability when precision is needed, it provides an acceptable measure of on‐court jump height that can be used to monitor athlete jump load.     

Potentiation of GABA-Activated Currents by Imidazobenzoimidazole Derivative RU-353 in Isolated Cerebellum Purkinje Cells

Voltage clamp and concentration jump experiments performed on Purkinje cells isolated from rat cerebellum showed that novel imidazobenzoimidazole derivative RU-353 increased the amplitude of GABA-activated chlorine current in a dose-dependent manner (EC₅₀=15 μM for the currents activated by 1 μM GABA). RU-353 shifted the GABA dose-response curve to the left, but produced no effect on the maximum response (EC₅₀ in control and in the presence of 30 μM RU-353 were 6.9 and 2.0 μM, respectively). __________ Translated from Byulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 140, No. 9, pp. 311–315, September, 2005     

Change‐point detection of cognitive states across multiple trials in functional neuroimaging

Many functional neuroimaging‐based studies involve repetitions of a task that may require several phases, or states, of mental activity. An appealing idea is to use relevant brain regions to identify the states. We developed a novel change‐point methodology that adapts to the repeated trial structure of such experiments by assuming the number of states stays fixed across similar trials while allowing the timing of change‐points to change across trials. Model fitting is based on reversible‐jump MCMC. Simulation studies verified its ability to identify change‐points successfully. We applied this technique to data collected via functional magnetic resonance imaging (fMRI) while each of 20 subjects solved unfamiliar arithmetic problems. Our methodology supplies both a summary of state dimensionality and uncertainty assessments about number of states and the timing of state transitions. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of fatigue and surface instability on neuromuscular performance during jumping

It has previously been shown that fatigue and unstable surfaces affect jump performance. However, the combination thereof is unresolved. Thus, the purpose of this study was to examine the effects of fatigue and surface instability on jump performance and leg muscle activity. Twenty elite volleyball players (18 ± 2 years) performed repetitive vertical double‐leg box jumps until failure. Before and after a fatigue protocol, jump performance (i.e., jump height) and electromyographic activity of selected lower limb muscles were recorded during drop jumps (DJs) and countermovement jumps (CMJs) on a force plate on stable and unstable surfaces (i.e., balance pad on top of force plate). Jump performance (3–7%; P < 0.05; 1.14 ≤ d ≤ 2.82), and muscle activity (2–27%; P < 0.05; 0.59 ≤ d ≤ 3.13) were lower following fatigue during DJs and CMJs, and on unstable compared with stable surfaces during DJs only (jump performance: 8%; P < 0.01; d = 1.90; muscle activity: 9–25%; P < 0.05; 1.08 ≤ d ≤ 2.54). No statistically significant interactions of fatigue by surface condition were observed. Our findings revealed that fatigue impairs neuromuscular performance during DJs and CMJs in elite volleyball players, whereas surface instability affects neuromuscular DJ performance only. Absent fatigue × surface interactions indicate that fatigue‐induced changes in jump performance are similar on stable and unstable surfaces in jump‐trained athletes.     

Choice of jumping strategy in two standard jumps,squat and countermovement jump-effect of training background or inherited preference?

Six male subjects, three professional ballet dancers and three elite volleyball players, performed maximal vertical jumps from 1) a static preparatory position (squat jump), 2) starting with a countermovement (countermovement jump) and 3) a specific jump for ballet and for volleyball, respectively. The jumps were recorded on highspeed film (500 Hz) combined with registration of ground reaction forces, and net joint moments were calculated by inverse dynamics. The purpose was to investigate the choice of strategy in two standard jumps, squat jump and countermovement jump. The volleyball jump was performed with a sequential strategy and the ballet jump was performed with a simultaneous strategy. In the two standard jumps, the choice of strategy was individual and not related to training background. This was additionally confirmed in a test of seven ballet dancers and seven volleyball players.     

Robust inventory‐production control problem with stochastic demand

This paper deals with the inventory‐production control problem where the produced items are supposed to be deteriorating with a rate that depends on the stochastic demand rate. The inventory‐production control problem is formulated as a jump linear quadratic control problem. The optimal policy that solves the optimal control problem is obtained in terms of a set of coupled Riccati equations. The guaranteed cost control problem is also investigated. Copyright © 1999 John Wiley & Sons, Ltd.