1,721,046 research outputs found
The energy cost of swimming and its determinants
No full textThe energy expended to transport the body over a given distance (C, the energy cost) increases with speed both on land and in water. At any given speed, C is lower on land (e.g., running or cycling) than in water (e.g., swimming or kayaking) and this difference can be easily understood when one considers that energy should be expended (among the others) to overcome resistive forces since these, at any given speed, are far larger in water (hydrodynamic resistance, drag) than on land (aerodynamic resistance). Another reason for the differences in C between water and land locomotion is the lower capability to exert useful forces in water than on land (e.g., a lower propelling efficiency in the former case). These two parameters (drag and efficiency) not only can explain the differences in C between land and water locomotion but can also explain the differences in C within a given form of locomotion (swimming at the surface, which is the topic of this review): e.g., differences between strokes or between swimmers of different age, sex, and technical level. In this review, the determinants of C (drag and efficiency, as well as energy expenditure in its aerobic and anaerobic components) will, thus, be described and discussed. In aquatic locomotion it is difficult to obtain quantitative measures of drag and efficiency and only a comprehensive (biophysical) approach could allow to understand which estimates are "reasonable" and which are not. Examples of these calculations are also reported and discussed
Effects of intra-cyclic velocity variations on the drag exerted by different swimming parachutes
No full textSwimming parachutes are often used as a tool for resisted swimming training. However, little is known on their behavior in terms of exerted drag as a consequence of intra-cyclic velocity fluctuations. This study aimed to assess the drag provided by two swimming parachutes of different shape, also characterized by different volumes and cross-sectional areas, under conditions of velocity variations in the range of those occurring in swimming. A flat square shaped parachute (FLAT, cross-sectional area and volume: 400 cm; 0.12 l) and a truncated cone shaped parachute (CONE, 380 cm; 7.15 l) were passively towed: 1) at constant velocities ranging from 1.0 to 2.2 m/s, and 2) with velocity fluctuations from 10 to 40 % around a mean of 1.6 m/s. At constant velocities, FLAT showed 0.1 N (at 1.0 m/s) to 10.8 N (at 2.2 m/s) higher drag than CONE. For both parachutes, the average drag showed trivial differences between constant and any fluctuating velocity. Conversely, the maximum drag values were higher under conditions of velocity fluctuations than the respective values estimated under stationary instantaneous velocity, although this was observed in CONE only. These findings suggest that swimmers and coaches can select the parachute characteristics based on whether the focus is on increasing/decreasing the average drag or regulating the maximum resistance provided
The effect of swim-cap surface roughness on passive drag.
No full textIn the last decade, great attention has been given to the improvements in swimming performance that can be obtained by wearing "technical swimsuits"; the technological evolution of these materials only marginally involved swim caps production, even if several studies have pointed out the important role of the head (as main impact point with the fluid) on hydrodynamics. The aim of this study was to compare the effects on passive drag (Dp) of 3 swim cap models: a smooth silicon helmet cap (usually used during swimming competitions), a silicon helmet cap with "dimples," and a silicon helmet cap with "wrinkles." Experiments were performed on 10 swimmers who were towed underwater (at a depth of 60 cm) at 3 speeds (1.5, 1.7, and 1.9 m·s) and in 2 body positions: LA (arms above the swimmer's head) and SA (arms alongside the body). The Dp values obtained in each trial were divided by the square of the corresponding speed to obtain the speed-specific drag (the k coefficient = Dp/v). No differences in k were observed among swim caps in the LA position. No differences in k were observed between the smooth and dimpled helmets also in the SA position; however, the wrinkled swim cap helmet showed a significant larger k (4.4%) in comparison with the model with dimples, when the swimmers kept their arms alongside the body (in the SA position). These data suggest that wearing a wrinkled swim cap helmet can be detrimental to performance at least in this specific position
Estimating active drag based on full and semi-tethered swimming tests
No full textDuring full tethered swimming no hydrodynamic resistance is generated (since v = 0) and all the swimmer's propulsive force (F-P) is utilized to exert force on the tether (F-T = F-P). During semitethered swimming FP can be made useful to one of two ends: exerting force on the tether (F-ST) or overcoming drag in the water (active drag: Da). At constant stroke rate, the mean propulsive force (F-P) is constant and the quantity F-P - F-ST (the "residual thrust") corresponds to Da. In this study we explored the possibility to estimate Da based on this method ("residual thrust method") and we compared these values with passive drag values (Dp) and with values of active drag estimated by means of the "planimetric method". Based on data obtained from resisted swimming (full and semi-tethered tests at 100% and 35, 50, 60, 75, 85% of the individual F-T), active drag was calculated as: Da(ST) = kaST .v(ST)(2 )= F-P - F-ST ("residual thrust method"). Passive drag (Dp) was calculated based on data obtained from passive towing tests and active drag ("planimetric method") was estimated as: D-aPL = Dp.1.5. Speed-specific drag (k = D/v(2)) in passive conditions (kp) was approximate to 25 kg.m(-1) and in active conditions (ka) approximate to 38 kg.m(1) (with either method); thus, D-aST > D-p and D-aST approximate to D-aPL. In human swimming active drag is, thus, about 1.5 times larger than cal setting (in the swimming pool) by using basic instrumentation and a simple set of calculations
Recovery time profiling after short-, middle- and long-distance swimming performance
No full textWe investigated cardiac autonomic responses and hemodynamic parameters on recovery time following short-, middle- and long-swimming performance. Ten male regional-level swimmers were tested to estimate time and frequency domains of arterial baroreflex sensitivity and heart rate variability after 100-, 200-, and 400-m of front crawl. We found a BRS reduction for 90 min after a maximal 100- and 200-m front crawl event, meanwhile the reflex was restored back to the baseline value about 70 min after 400-m. The vagally mediated HF power of R-R intervals was significantly reduced for 30 min after 400-m, and more than 90 min after 100- and 200-m, with a concomitant increase of sympathetic modulation. After 400-m athletes have reduced their stroke volume for 50 min, which remained at the baseline level following 100- and 200-m. HR was restored back after 90 min in all conditions, whereas TPR was significantly reduced for 50 min after 200- and 400-m, with a persistent reduction after 100-m. Time course of autonomic recovery after 3 different swimming performances is influenced by exercise intensity and duration, showing a rapid recovery after 400-m, an intermediate recovery after 200-m, and a significantly delayed recovery after a more strictly anaerobic performance like 100-m of front crawl. These results could encourage coaches to consider that athlete might be affected by the specific recovery time of the previous exercise performed, suggesting that the management of the exercise intensity, and appropriate monitoring of cardiac autonomic parameters might be helpful to know the physical condition of each athlete
The Relationship between Power Generated by Thrust and Power to Overcome Drag in Elite Short Distance Swimmers.
Full text linkAt constant average speed (v), a balance between thrust force (Ft) and drag force (Fd) should occur: Ft-Fd = 0; hence the power generated by thrust forces (Pt = Ft·v) should be equal to the power needed to overcome drag forces at that speed (Pd = Fd·v); the aim of this study was to measure Pt (tethered swims), to estimate Pd in active conditions (at sprint speed) and to compare these values. 10 front crawl male elite swimmers (expertise: 93.1 ± 2.4% of 50 m world record) participated to the study; their sprint speed was measured during a 30 m maximal trial. Ft was assessed during a 15 s tethered effort; passive towing measurement were performed to determine speed specific drag in passive conditions (kP = passive drag force/v2); drag force in active conditions (Fd = kA·v2) was calculated assuming that kA = 1.5·kP. Average sprint speed was 2.20 ± 0.07 m·s-1; kA, at this speed, was 37.2 ± 2.7 N·s2·m-2. No significant differences (paired t-test: p > 0.8) were observed between Pt (399 ± 56 W) and Pd (400 ± 57 W) and a strong correlation (R = 0.95, p < 0.001) was observed between these two parameters. The Bland-Altman plot indicated a good agreement and a small, acceptable, error (bias: -0.89 W, limits of agreement: -25.5 and 23.7 W). Power thrust experiments can thus be suggested as a valid tool for estimating a swimmer's power propulsion
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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