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.     

针刺配合扳法治疗腰椎间盘突出症临床观察

针刺夹脊穴、环跳、踝三针和扳法治疗腰椎间盘突出症90例,并与针刺配合牵引理疗等对照组90例对照观察,结果：观察组痊愈率61％,总有效率92％;对照组痊愈率40％,总有效率78％。提示针刺配合扳法治疗腰间盘突出症效果好。     

Mechanographic characteristics of adolescents and young adults with congenital heart disease

The present study comprised 29 adolescents and young adults (15 females, 14 males; aged 14.1–23.9 years) with congenital heart disease (CHD) and focused on the interaction between the biomechanical system and CHD. Individuals were characterized by auxological (height, weight), dynamometric (MIGF, maximal isometric grip force) and mechanograpic parameters (V_max, maximal velocity; PJF, peak jump force; PJP, peak jump power; time of five stand-ups in chair-rising test). PJF, PJP and MIGF were transformed into height-related SD-scores. MIGF-SDS and PJP-SDS were lower in the CHD patients than in reference individuals. PJP-SDS was lower than PJF-SDS. PJP-SDS was correlated to V_max (r = 0.62) and to the time of five-stand-ups in chair-rising (r = −0.62). Transcutaneous oxygen saturation and NYHA classes were correlated to V_max (r = 0.42 and r = −0.57, respectively) and to chair-rising performance (r = −0.60 and r = 0.50, respectively). To conclude, individuals with CHD are characterized by an impaired inter- and intramuscular coordination, which is characterized by a greater decrease in muscular power than muscle force. Conflict of interest: a possible conflict of interest is disclosed.     

空降兵跳伞心理应激对心率变异性的影响

目的观察空降兵跳伞心理应激对心率变异性(HRV)的影响,为评价跳伞心理应激强度提供方法和生理学依据。方法参加跳伞训练的空降兵30名,跳伞当天佩戴飞行员飞行生理参数记录检测仪,连续动态记录跳伞前安静状态下30 min的心电信号,采用与记录检测仪配套的数据分析系统对心率变异性指标进行时域和频域分析,并与跳伞后1个月平时状态下的测量结果对照。结果与对照相比,跳伞前心率变异性时域指标SDNN、RMSSD、HRVI、SDNNindex、SDANN分别降低了32.1%、41.2%、15.0%、14.2%、46.2%,差异有显著性(P0.05);频域指标HF、HFNU、TP分别降低了66.7%、60.1%、25.8%,差异有显著性(P0.05);而LFNU、LF/HF则分别升高了40.3%、335.9%,差异有显著性(P0.01)。结论心率变异性是反映空降兵跳伞心理应激的一个较灵敏指标,可用于评价跳伞应激强度。     

Failed jump landing trials: deficits in neuromuscular control

The purpose of this investigation was to compare neuromuscular control variables during successful and failed jump landings in multiple directions (sagittal, diagonal, and lateral). All data were collected during a single leg hop stabilization maneuver, which required subjects to stand 70 cm from the center of a force plate, jump off both legs, touch a designated marker placed at a height equivalent to 50% of their maximum vertical jump, and land on a single leg for all directions. Twenty-six subjects [10 males (22+/-3.9 years of age, 70.9+/-7.6 kg, and 176.8+/-0.5 cm) and 16 females (20.6+/-0.5 years of age, 65.6+/-9.1 kg, and 166.4+/-5.9 cm)] volunteered to participate in this investigation. Muscle activation times, average preparatory, and reactive electromyographic (EMG) amplitudes were calculated for the vastus medialis, semi-membranosis, lateral gastrocnemius, and tibialis anterior. EMG data revealed that successful jump landing trials had earlier activation times and higher preparatory and reactive EMG amplitudes. There was no difference for EMG activation times or amplitudes among directions. The results indicate neuromuscular control differences between successful and failed trials because of earlier muscle onset and greater amplitude. The results also suggest that in a healthy population, the direction of the jump protocol will not affect lower extremity EMG characteristics.     

Muscular strength, functional performances and injury risk in professional and junior elite soccer players

Muscle strength and anaerobic power of the lower extremities are neuromuscular variables that influence performance in many sports activities, including soccer. Despite frequent contradictions in the literature, it may be assumed that muscle strength and balance play a key role in targeted acute muscle injuries. The purpose of the present study was to provide and compare pre-season muscular strength and power profiles in professional and junior elite soccer players throughout the developmental years of 15-21. One original aspect of our study was that isokinetic data were considered alongside the past history of injury in these players. Fifty-seven elite and junior elite male soccer players were assigned to three groups: PRO, n=19; U-21, n=20 and U-17, n=18. Players benefited from knee flexor and extensor isokinetic testing consisting of concentric and eccentric exercises. A context of lingering muscle disorder was defined using statistically selected cut-offs. Functional performance was evaluated throughout a squat jump and 10 m sprint. The PRO group ran faster and jumped higher than the U-17 group (P<0.05). No significant difference in isokinetic muscle strength performance was observed between the three groups when considering normalized body mass parameters. Individual isokinetic profiles enabled the identification of 32/57 (56%) subjects presenting lower limb muscular imbalance. Thirty-six out of 57 players were identified as having sustained a previous major lower limb injury. Of these 36 players, 23 still showed significant muscular imbalance (64%). New trends in rational training could focus more on the risk of imbalance and implement antagonist strengthening aimed at injury prevention. Such an intervention would benefit not only athletes recovering from injury, but also uninjured players. An interdisciplinary approach involving trainers, a physical coach, and medical staff would be of interest to consider in implementing a prevention programme.     

Mechanical efficiency during repetitive vertical jumping

The purpose of this study was to compare mechanical efficiency between repeated static jumps (SJ), countermovement jumps (CMJ), drop jumps from 75% of maximum CMJ jump height (75DJ) and drop jumps from 125% of maximum CMJ height (125DJ). Subjects included eight jump-trained males. All subjects completed 30 continuous repetitions in the SJ, CMJ, 75DJ, and 125DJ. Oxygen consumption, peak force and center of mass displacement for each repetition during the four jumping patterns were measured. ME was calculated from a combination of force-time curves, displacement-time curves and lactate-corrected oxygen consumption values. In addition, muscle activity was recorded from the vastus medialis, vastus lateralis and biceps femoris using surface electromyography (EMG). 125DJ and 75DJ resulted in significantly (P ≤ 0.05) greater ME in comparison to CMJ and SJ. CMJ resulted in significantly greater ME in comparison to SJ. In addition, braking phase muscle activity was significantly greater in 125DJ and 75DJ in comparison to CMJ. Negative work was significantly different between 125DJ, 75DJ and CMJ (125DJ > 75DJ > CMJ). There was a significant positive correlation (r = 0.68) between ME and negative work performed across 125DJ, 75DJ and CMJ. These findings suggest that stretch-shortening cycle movements, which include a strenuous braking phase combined with simultaneous high muscle activity, increase ME. This may be due to optimal muscle-tendon unit kinetics and usage of stored elastic energy.     

Preseason functional tests discriminate injury risk in female collegiate volleyball players

ObjectivePreseason functional tests have been previously associated with noncontact time-loss lower extremity injuries in a variety of athletic populations. This study aimed to determine the effectiveness of one or more functional tests to discriminate injury risk in a cohort of female collegiate volleyball players in the United States.DesignProspective cohort study.SettingNational Collegiate Athletic Association Division II & III, National Association of Intercollegiate Athletics female volleyball players;Participants130 female volleyball players (mean age: 19.31 ± 1.1 years).Main outcome measureLower extremity functional test, single limb hop for distance, standing long jump, previous injury history, lower quarter Y-balance test, and limb symmetry index were measured prior to the season beginning. Noncontact time-loss lower quarter injuries were tracked during the season.ResultsAthletes with suboptimal scores on the single limb hop test (<70% of height) and standing long jump (<80% of height), combined with a previous history of injury, were three times more likely to sustain an injury during the season (OR = 3.0; 95% CI: 1.09 to 8.30). Individual functional tests did not discriminate injury risk.ConclusionsA battery of preseason functional tests and injury history discriminates injury risk in female collegiate volleyball players.