Effect of chainring ovality on joint power during cycling at different workloads and cadences

Non-circular chainrings theoretically enhance cycling performance by increasing effective chainring diameter and varying crank velocity, but research has failed to consistently reproduce the benefits in cycling trials. The aim of this study was (1) to investigate the effect of different chainring shapes on sagittal knee joint moment and sagittal lower limb joint powers and (2) to investigate whether alterations are affected by cadence and workload. Fourteen elite cyclists cycled in six conditions (70, 90 and 110 rpm, each at 180 and 300 W), for 2 min each, using three chainrings of different ovalities (1.0–1.215). Kinematic data and pedal forces were collected. For most conditions, only the chainring with the highest ovality (1.215) was characterised by smaller sagittal knee joint moments, smaller relative sagittal knee joint power contribution and larger relative sagittal hip joint power contribution, which suggests a change from maximising efficiency to maximising power production. Effect sizes increased with higher cadences, but not with higher workload. This study has application for athletes, clinicians and sports equipment industry as a non-circular chainring can change joint-specific power generation and decrease knee joint moment, but certain ovality seems to be necessary to provoke this effect.     

Comparison of support leg kinetics between side-step and cross-step cutting techniques

The purpose of this study was to investigate differences in the support leg joint moment and moment power between side-step (SS) and cross-step (CS) cutting techniques with a prescribed 90° cutting angle. Ground reaction forces (1,000 Hz) and three-dimensional kinematics (250 Hz) of SS and CS cutting techniques were collected from 20 male college athletes. Normalised peak knee extension moment was larger in the SS technique than in the CS technique (0.40 ± 0.10 in SS; 0.26 ± 0.08 in CS). In the SS technique, the knee extensors ( ? 0.10 ± 0.06 in SS; ? 0.02 ± 0.04 in CS) and ankle plantarflexors ( ? 0.12 ± 0.05 in SS; ? 0.07 ± 0.03 in CS) did significantly more negative work (normalised). The direction change angle (40.5 ± 8.7° in SS; 33.0 ± 6.8° in CS) and the decrease in horizontal velocity of the centre of mass ( ? 0.63 ± 0.23 m/s in SS; ? 0.31 ± 0.23 m/s in CS) were significantly larger in the SS technique. These results suggest that the SS technique is an effective means of changing running direction at the expense of velocity of the centre of mass and that the CS technique is better for minimising the reduction in horizontal velocity of the centre of mass.     

The differential effects of PNF versus passive stretch conditioning on neuromuscular performance

Abstract

The effects of flexibility conditioning on neuromuscular and sensorimotor performance were assessed near to full knee extension (25°). Eighteen males who were randomly assigned into two groups underwent eight weeks (three-times per week) of flexibility conditioning (hip region/knee flexor musculature; dominant limb) involving either proprioceptive neuromuscular facilitation (PNF) (n=9) or passive stretching (PASS) (n=9). Both modes of flexibility conditioning are popular within contemporary exercise and clinical settings and have demonstrated efficacy in improving range of motion. The contralateral limb and a prior ‘no exercise’ condition were used as controls. The PNF and PASS modes of conditioning improved passive hip flexibility to a similar extent (mean 19.3% vs. baseline, intervention limb, p<0.01) but did not alter knee flexor strength (overall mean 309.6±81 N) or sensorimotor performance (force and positional errors: 2.3±8.2% and 0.48±7.1%). Voluntary and magnetically evoked electromechanical delays (EMD_V and EMD_E, respectively) were increased but to a greater extent following PASS compared to PNF (PASS: 10.8% and 16.9% lengthening of EMD_V and EMD_E, respectively vs. PNF: 3.2% and 6.2%, p<0.01).The attenuated change to electromechanical delay (EMD) performance during PNF conditioning suggests a preserved capability for rapid muscle activation, which is important in the maintenance of dynamic joint stability. That PNF was also equally efficacious in flexibility conditioning would suggest that this mode of flexibility training should be used over passive to help preserve dynamic joint stability capabilities at this extended and vulnerable joint position.     

The influence of variable resistance moment arm on knee extensor performance

Abstract

To enhance muscular strength, resistance training machines with a cam, incorporating a variable resistance moment arm, are widely used. However, little information is available about the influence of the variable resistance moment arm on torque, velocity, and power during muscle contraction. To address this, a knee extensor machine was equipped with a cam or with a semi-circular pulley that imposed a variable or a constant resistance moment arm, respectively. Fourteen physically active men performed two full knee extensions against loads of 40–80 kg in both conditions. Participants developed significantly higher torque with the pulley than with the cam (P < 0.001). The relative differences between pulley and cam conditions across all loads ranged from 8.72% to 19.87% (P < 0.001). Average knee extension velocity was significantly higher in the cam condition than in the pulley condition. No differences were observed in average and peak power, except at 50 and 55 kg. Torque–velocity and power–velocity relationships were modified when the resistance moment arm was changed. In conclusion, whatever the link, namely cam or pulley, the participants produced similar power at each load. However, the torque–velocity and power–velocity relationships were different in the cam and pulley conditions. The results further suggest that the influence of the machine's mechanism on muscular performance has to be known when prescribing resistance exercises.     

优秀散打运动员膝/肘关节等速测试与站位攻防技术分析

采用ISOMED2000等速系统检测优秀散打运动员膝、肘关节肌群的生物力学特征,分析优秀散打运动员站位攻防动作技术.在进攻训练中伸膝关节速度达到300°/s,能充分发挥运动员小腿伸肌群的爆发能力;采用300°/s的速度可以快速屈小腿,有利于运动员快速回防,为再次打击对方在时间上创造了条件;在站位时,膝关节角度保持157.3°～160.1°,小腿屈肌群离心爆发能力强,有利于运动员调整自身的身体姿势,避开对方的快速进攻.在进攻训练中应使肘关节速度达到300°/s,能充分发挥运动员前臂伸肌群的爆发能力.肘关节角度在81°～86°的范围,有利于运动员快速出拳,打击对方.以240°/s的速度屈肘,能充分发挥运动员前臂屈肌群的爆发能力,有利于运动员快速回防.在防守训练中,肘关节角度在78°～85°的范围内,对运动员快速回防,抵御时方快速、重拳的进攻是有利的;肘关节角度在128°～137°的范围内,可远距离隔挡对方,阻击对方的快速进攻.     

终点裁判员判定名次方法的研究

径赛长距离跑非常引人注目,由于运动员参加人数多,运动水平各异,加上裁判员工作环境喧闹,其名次判定也容易出错,是裁判工作中的一个重点和难点。文章对终点裁判的记圈方法和判定名次进行了研究,结论认为:在大型比赛中,新终点磁性电子记圈牌更能得到观众、运动员、裁判员对比赛结果的公认,对裁判工作的名次判定和工作质量、效率是极大的提高,使终点裁判工作有条不紊,迅速准确。     

Influence of Magnetic Field on the Rectification Process of Binary Heterogeneous Azeotrope

To improve separate effect of binary heterogeneous azeotrope in the magnetic field with different magnetic induction intensity, the influence of magnetic field on the rectification process of binary heterogeneous azeotrope was investigated with l-butanol-water system. The results show that the composition of liquid-liquid phase equilibrium of l-butanol-water system has definitely changed, the composition of l-butanol in light phase （l-butanol layer） increases by 1. 17%-1.63% and the composition of water in heavy phase （water layer） increases by 1.21%-1.58% under the influence of magnetic field. By separation of magnetization, the composition of l-butanol increases by 0.8%-1.2% and the recovery ratio of 1 -butanol increases by 1.6%-2.5%. Magnetic field has positive effect, however, the magnetized effect is not in proportion to magnetic induction intensity and has an optimum condition, in the range of 0.25 T-0. 3 T.     

刚体转动惯量测量的研究

在刚体转动惯量测量实验中,利用数字毫秒计时器计时,改进测量角加速度的原理和方法,大大提高实验的精确度和效率.     

磁性SO2-4/ZrO2固体超强酸的制备及催化性能的研究

利用磁性对固体超强酸组装,制备出磁性SO42-/ZrO2固体超强酸催化剂,应用于合成乙酸异戊酯的反应中,最佳反应条件为:乙酸0.2mol,异戊醇0.4mol,磁性催化剂1.2 g,反应时间2.0h,酯化率可达93.7％.利用催化剂的磁性可将催化剂迅速分离,回收率达84.3％,并能重复使用.     

低纯度Gd制成的Gd5Si1.8Ge2.2的磁热效应

实验中使用国产2N级金属Gd粉为原料，用电弧熔炼法在氩气气氛中制备了Gd5Si1.8Ge22合金，并通过X射线衍射仪和超导量子磁强计分析手段对合金的性能进行测试和分析．发现合金没有重现一级磁相变和巨磁热效应，这是由于国产2N级Gd中杂质等因素时磁相变、磁热性能产生了重要影响，导致一级磁相变的破坏及磁热效应的大幅度下降．