A kinematic and dynamic comparison of surface and underwater displacement in high level monofin swimming

International audienceFin-swimming performance can be divided into underwater and surface water races. World records are about 10% faster for underwater swimming vs. surface swimming, but little is known about the advantage of underwater swimming for monofin swimming. Some authors reported that the air-water interface influences the kinematics and leads to a narrow vertical amplitude of the fin. On the one hand, surface swimming is expected to affect drag parameters (cross-sectional area (S) and active drag (AD)) when compared to underwater swimming. On the other hand, the surface swimming technique may also affect efficiency (η). The aim of this study is therefore to evaluate and compare drag parameters and efficiency during underwater and surface swimming. To this end, 12 international level monofin swimmers were measured during both underwater and surface swimming. Kinematic parameters (both dimensional and non-dimensional), η (calculated according to the Elongated-Body Theory), and AD (computed with Velocity Perturbation Method) were calculated for an underwater and a surface fin-swimming trial, performed at maximal speed. As expected, results showed significantly lower velocities during surface swimming vs. underwater ( = 2.5 m.s vs. = 2.36 m.s, < .01). Velocities during underwater and surface swimming were strongly correlated ( = .97, < .01). Underwater swimming was also associated with higher vertical amplitudes of the fin compared to surface swimming ( = 0.55 m vs. = 0.46 m, < .01). Length-specific amplitudes (A/L) were in the order of 20% during underwater swimming as for undulating fish, and significantly higher than during surface swimming (A/L = 17%, < .01). Efficiency for surface swimming was about 6% lower than for underwater swimming ( = 0.79 vs. = 0.74, < .01). This decrease could be associated with an increase in swimming frequency for surface swimming ( = 2.15 Hz vs. = 2.08 Hz, < .01). Active drag during surface swimming was about 7% higher than for underwater swimming ( = 78.9 N vs. = 84.7 N, < .01). A significantly smaller cross-sectional area for surface swimming ( = 0.053 m vs. = 0.044 m, < .01) and higher drag coefficient for surface swimming ( = 0.47 vs. = 0.69, < .01) were measured. Finally, correlation between cross-sectional area and vertical amplitude of the fin was reported for both underwater and surface swimming. These results suggest that the performance improvement during underwater swimming is not only linked to a wave drag reduction effect but also to a specific swimming technique due to the free surface

JOINT KINEMATIC ASYMMETRY BASED ON SPATIOTEMPORAL PATTERN DEVIATION DURING AN INCREMENTAL TEST IN HIGH LEVEL CYCLING

The aim of this study was to use two asymmetry indexes from a kinematical point of view in professional cyclists during an incremental test to exhaustion. Twelve professional cyclists were evaluated during the French Cycling Federation’s protocol. Based on motion capture, asymmetry analysis was addressed by means of cross-correlation technique and a normalized symmetry index (NSI). Results pointed out that NSI could vary up to 18% throughout the pedaling cycle, with different behavior between upward and downward pedaling phases. Both methods exhibited low values of asymmetry especially for flexion/extension, but higher asymmetry values for other DOF. This study shows the complementarity of both NSI and cross-correlation methods. It enables to continuously evaluate changes during the crank cycle associated to skeletal movement

Advantages and limitations of virtual reality for balance assessment and rehabilitation

International audienceVirtual reality (VR) is now commonly used in many domains because of its ability to provide a standardized, reproducible and controllable environment. In balance assessment, it can be used to control stimuli presented to patients and thus accurately evaluate their progression or compare them to different populations in standardized situations. In balance rehabilitation, VR allows the creation of new generation tools and at the same time the means to assess the efficiency of each parameter of these tools in order to optimize them. Moreover, with the development of low-cost devices, this rehabilitation can be continued at home, making access to these tools much easier, in addition to their entertaining and thus motivating properties. Nevertheless, and even more with low-cost systems, VR has limits that can alter the results of the studies that use it: the latency of the system (the delay cumulated on each step of the process from data acquisition on the patients to multimodal outputs); and distance perception, which tends to be underestimated in VR. After having described why VR is an essential tool for balance assessment and rehabilitation and illustrated this statement with a case study, this review discusses the previous works in the domain with regards to the technological limits of V

INFLUENCE OF A PROLONGED TENNIS MATCH PLAY ON TENNIS SERVE BIOMECHANICS

The aim of this study was to quantify biomechanical changes that occur in the serve throughout a prolonged tennis match. Serves of tennis players were recorded with a motion capture system before (T0) during (T90), and after (T180) a 3-hour match. Before and after each match, EMG data of upper limb muscles were analyzed to determine the presence of muscular fatigue. RPE and ball velocity and biomechanical variables were analyzed. Decreases in ball velocity; maximal angular velocities and increase in RPE were observed. The majority of the upper limb joint kinetics decreases between T0 and T180. No change in timing of maximal angular velocities was observed. A prolonged tennis match induces fatigue in upper limb muscles, which decreases performance and modifies serve biomechanics

Detecting Deception in Movement: The Case of the Side-Step in Rugby

Although coordinated patterns of body movement can be used to communicate action intention, they can also be used to deceive. Often known as deceptive movements, these unpredictable patterns of body movement can give a competitive advantage to an attacker when trying to outwit a defender. In this particular study, we immersed novice and expert rugby players in an interactive virtual rugby environment to understand how the dynamics of deceptive body movement influence a defending player’s decisions about how and when to act. When asked to judge final running direction, expert players who were found to tune into prospective tau-based information specified in the dynamics of ‘honest’ movement signals (Centre of Mass), performed significantly better than novices who tuned into the dynamics of ‘deceptive’ movement signals (upper trunk yaw and out-foot placement) (p<.001). These findings were further corroborated in a second experiment where players were able to move as if to intercept or ‘tackle’ the virtual attacker. An analysis of action responses showed that experts waited significantly longer before initiating movement (p<.001). By waiting longer and picking up more information that would inform about future running direction these experts made significantly fewer errors (p<.05). In this paper we not only present a mathematical model that describes how deception in body-based movement is detected, but we also show how perceptual expertise is manifested in action expertise. We conclude that being able to tune into the ‘honest’ information specifying true running action intention gives a strong competitive advantage

High-impedence NbSi TES sensors for studying the cosmic microwave background radiation

Precise measurements of the cosmic microwave background (CMB) are crucial in cosmology, because any proposed model of the universe must account for the features of this radiation. Of all CMB measurements that the scientific community has not yet been able to perform, the CMB B-mode polarization is probably the most challenging from the instrumental point of view. The signature of primordial gravitational waves, which give rise to a B-type polarization, is one of the goals in cosmology today and amongst the first objectives in the field. For this purpose, high-performance low-temperature bolometric cameras, made of thousands of pixels, are currently being developed by many groups, which will improve the sensitivity to B-mode CMB polarization by one or two orders of magnitude compared to the Planck satellite HFI detectors. We present here a new bolometer structure that is able to increase the pixel sensitivities and to simplify the fabrication procedure. This innovative device replaces delicate membrane-based structures and eliminates the mediation of phonons: the incoming energy is directly captured and measured in the electron bath of an appropriate sensor and the thermal decoupling is achieved via the intrinsic electron-phonon decoupling of the sensor at very low temperature. Reported results come from a 204-pixel array of Nb$_{x}$Si$_{1-x}$ transition edge sensors with a meander structure fabricated on a 2-inch silicon wafer using electron-beam co-evaporation and a cleanroom lithography process. To validate the application of this device to CMB measurements, we have performed an optical calibration of our sample in the focal plane of a dilution cryostat test bench. We have demonstrated a light absorption close to 20% and an NEP of about 7$\times10^{-16}$ W/$\sqrt{Hz}$, which is highly encouraging given the scope for improvement in this type of detectors.Comment: 6 pages, 10 figures. arXiv admin note: text overlap with arXiv:1005.0555 by other author

BIOMECHANICAL ANALYSIS OF STARTING PREFERENCE FOR EXPERT SWIMMERS

The purpose of this study was to compare kinetics, body angles and angular momenta during the swimming start for preferred and non-preferred technique of expert grab starters. Results showed that in preferred technique, starts were executed with less global angular momentum around the transverse axis. By searching further, less loss of angular momentum in the other dimensions was found for grab start as preferred technique, inducing a less efficiency in non-preferred technique (twisting effect in track start as non-preferred technique). Body angles showed that legs in non-preferred technique permit to increase quantity of body rotation during aerial phase. Finally, subject effect was found for arms movements (confirming that expert swimmers can organize themselves differently to achieve to an optimal performance

Effects of a six-week period of congested match play on plasma volume variations, hematological parameters, training workload and physical fitness in elite soccer players

[EN] Objectives The aims of this study were to investigate the effects of a six-week in-season period of soc- cer training and games (congested period) on plasma volume variations (PV), hematological parameters, and physical fitness in elite players. In addition, we analyzed relationships between training load, hematological parameters and players’ physical fitness. Methods Eighteen elite players were evaluated before (T1) and after (T2) a six-week in-season period interspersed with 10 soccer matches. At T1 and T2, players performed the Yo-Yo inter- mittent recovery test level 1 (YYIR1), the repeated shuttle sprint ability test (RSSA), the countermovement jump test (CMJ), and the squat jump test (SJ). In addition, PV and hema- tological parameters (erythrocytes [M/mm3], hematocrit [%], hemoglobin [g/dl], mean cor- puscular volume [fl], mean corpuscular hemoglobin content [pg], and mean hemoglobin concentration [%]) were assessed. Daily ratings of perceived exertion (RPE) were moni- tored in order to quantify the internal training load. Results From T1 to T2, significant performance declines were found for the YYIR1 (p<0.001, effect size [ES] = 0.5), RSSA (p<0.01, ES = 0.6) and SJ tests (p< 0.046, ES = 0.7). However, no significant changes were found for the CMJ (p = 0.86, ES = 0.1). Post-exercise, RSSA blood lactate (p<0.012, ES = 0.2) and PV (p<0.01, ES = 0.7) increased significantly from T1 to T2. A significant decrease was found from T1 to T2 for the erythrocyte value (p<0.002, ES = 0.5) and the hemoglobin concentration (p<0.018, ES = 0.8). The hematocrit percentage rate was also significantly lower (p<0.001, ES = 0.6) at T2. The mean corpuscular volume, mean corpuscular hemoglobin content and the mean hemoglobin content values were not statisti- cally different from T1 to T2. No significant relationships were detected between training load parameters and percentage changes of hematological parameters. However, a signifi- cant relationship was observed between training load and changes in RSSA performance (r = -0.60; p<0.003). Conclusions An intensive period of “congested match play” over 6 weeks significantly compromised play- ers’ physical fitness. These changes were not related to hematological parameters, even though significant alterations were detected for selected measures

Impact of osteopathic manipulative treatment on range of motion of the pelvis during the one-sided tilt test: a pilot study

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