BIOMECHANICS OF PROPULSION AND DRAG IN FRONT CRAWL SWIMMING

Peak performances in sport require the full deployment of all powers an athlete possesses. How factors like force, technique, and endurance each on itself, but also in concert determine swimming performance is subject of inquiry. The accent in this overview of swimming biomechanics is with 2 performance factors: (i) drag encountered by the body during swimming, and (ii) the generation of propulsion in water

AN ALTERNATIVE FLUID DYNAMIC EXPLANATION FOR PROPULSION IN FRONT CRAWL SWIMMING

Calculation of propulsion in front crawl swimming relies on the quasi-steady assumption that the fluid dynamic behaviour of a hand model in a flow channel (constant velocity and orientation) is similar to that of a hand of a real swimmer swimming the front crawl. It has been shown that quasi-steady calculations cannot account for the observed propulsive forces during front crawl swimming. Using woollen tufts the flow pattern around the arm and hand during the front crawl stroke was visualised. The flow direction varied strongly throughout the stroke and a strong, accelerating axial flow, not in the direction of the arm movement, was observed. These observations discredit the quasi-steady analysis of front crawl swimming. Instead, it is proposed that rotation of the arm leads to a proximo-distal pressure gradient, inducing axial flow. Such axial flow along the trailing side of the arm could greatly enhance the pressure difference over the hand, thus assisting propulsion by paddling

WAVE DRAG IN FRONT CRAWL SWIMMING

When swimming at the surface, swimmers will experience wave drag. It is observed that above a certain speed, wave height grows rapidly with further speed increase, suggesting that wave drag cannot be neglected at high speed. Therefore, the magnitude of this component of total drag was estimated. Total drag was decomposed in 2 parts; wave and pressure drag. Assuming wave drag to be negligible below 1.6 m/s, the velocity dependence of pressure drag was assessed by drag determinations at speeds below 1.6 m/s. By subtracting the estimated pressure drag from total drag values measured at higher speeds, wave drag was estimated. At a mean speed of 1.89 m/s, mean wave drag was 11.5 N, amounting to 12.1 % of total drag. These results underline the importance of reducing wave drag by diving under the surface after start and turns

Swimming obstructed by dead-water

In nautical literature, 'dead-water' refers to the obstructive effect encountered by ships moving in stratified water due to the ship generating waves on an interface that separates different water masses. To investigate the hypothesis that open water swimming may also be obstructed by an encounter of dead-water, possibly causing drowning, we performed two experiments that assess the impact of stratified water on swimming. In the first experiment, subjects made a single front-crawl stroke while lying on a carriage that was rolling just above the water surface. The gain in kinetic energy, as a result of the stroke, was far less in stratified than in homogeneous water. In the second experiment, four subjects swam a short distance (5 m) in homogeneous and in two different settings of stratified water. At the same stroke frequency, swimming in stratified conditions was slower by 15%, implying a loss in propulsive power by 40%. Although in nature stratification will be less strong, extrapolation of the results suggests that dead-water might indeed obstruct swimming in open water as well. This effect will be most pronounced during fair weather, when stratification of a shallow surface layer is most easily established. Our findings indicate that swimmers' anecdotal evidence on 'water behaving strangely' may have to be taken more seriously than previously thought. © 2008 Springer-Verlag

Concurrent validity and discriminative ability of Dutch performance-based motor tests in 5 to 6 years old children

AIM: To assess the concurrent validity and discriminative ability of total, gross and fine motor (TM, GM and FM) scores of Dutch performance-based motor tests, the Baecke-Fassaert Motor Test (BFMT) and the 8- and 4-Skills Scan (SkSc) with the Movement Assessment Battery (MABC) for children at age 5. METHOD: 116 Dutch children (40.3% boys) were included. Spearman's rho correlations and area under the curves (AUC) were assessed. RESULTS: Correlations between the TM scores of the tests were strong (absolute values from 0.58 to .65); the correlations between the GM scores and the FM scores between and within tests were weaker (absolute values from 0.30 to 0.45). Related to the cut-off (15th percentile) of the MABC, the AUC of the BFMT, 8- and 4-SkSc, the AUC was 0.853 (95% CI: 0.757-0.949), 0.905 (95% CI: 0.837-0.972) and 0.844 (95% CI: 0.730-0.957), respectively. At optimal cut-offs, the sensitivity and specificity of the BFMT, the 8- and 4-SkSc were 78.6 and 78.4%, 92.2 and 73.2%, 78.6 and 76.3%, respectively. CONCLUSION: All tests had a reasonably high discriminative ability, but validation with the MABC-2 and adaptations are needed to meet the requirements for screening (i.e. sensitivity ≥80% and specificity ≥90%). The relatively weak correlation between GM and FM scores implies that tests should be normalized and validated for GM and FM ability, separately

PLAYgrounds: Effect of a PE playground program in primary schools on PA levels during recess in 6 to 12 year old children. Design of a prospective controlled trial

Background The relative number of children meeting the minimal required dose of daily physical activity remains execrably low. It has been estimated that in 2015 one out of five children will be overweight. Therefore, low levels of physical activity during early childhood may compromise the current and future health and well-being of the population, and promoting physical activity in younger children is a major public health priority. This study is to gain insight into effects of a Physical Education based playground program on the PA levels during recess in primary school children aged 6-12. Methods/design The effectiveness of the intervention program will be evaluated using a prospective controlled trial design in which schools will be matched, with a follow-up of one school year. The research population will consist of 6-12 year old primary school children. The intervention program will be aimed at improving physical activity levels and will consist of a multi-component alteration of the schools' playground. In addition, playground usage will be increased through altered time management of recess times, as well as a modification of the Physical Education content. Discussion The effects of the intervention on physical activity levels during recess (primary outcome measure), overall daily physical activity and changes in physical fitness (secondary outcome measures) will be assessed. Results of this study could possibly lead to changes in the current playground system of primary schools and provide structured health promotion for future public health. Trial registration Netherlands Trial Register (NTR): NTR238

Biomechanics of competitive front crawl swimming

Essential performance-determining factors in front crawl swimming can be analysed within a biomechanical framework, in reference to the physiological basis of performance. These factors include: active drag forces, effective propulsive forces, propelling efficiency and power output. The success of a swimmer is determined by the ability to generate propulsive force, while reducing the resistance to forward motion. Although for a given competitive stroke a range of optimal stroking styles may be expected across a sample of swimmers, a common element of technique related to a high performance level is the use of complex sculling motions of the hands to generate especially lift forces. By changing the orientation of the hand the propulsive force acting on the hand is aimed successfully in the direction of motion. Furthermore, the swimming velocity (v) is related to drag (A), power input (Pi, the rate of energy liberation via the aerobic/ anaerobic metabolism), the gross efficiency (eg), propelling efficiency (ep), and power output (Po) according to: [Figure not available: see fulltext.] Based on the research available at present it is concluded that: (a) drag in groups of elite swimmers homogeneous with respect to swimming technique is determined by anthropometric dimensions; (b) total mechanical power output (Po) is important since improvement in performance is related to increased Po. Furthermore, it shows dramatic changes with training and possibly reflects the size of the ‘swimming engine’; (c) propelling efficiency seems to be important since it is much higher in elite swimmers (61%) than in triathletes (44%); and (d) distance per stroke gives a fairly good indication of propelling efficiency and may be used to evaluate individual progress in technical ability

Scaling of lifting forces in relation to object size in whole body lifting

Subjects prepare for a whole body lifting movement by adjusting their posture and scaling their lifting forces to the expected object weight. The expectancy is based on visual and haptic size cues. This study aimed to find out whether lifting force overshoots related to object size cues disappear or persist over a number of repeated lifts. In addition, the influence of the degree of alternation between load sizes, and the influence of knowledge of actual object weights prior to the lifts, were investigated with regard to their effect on force overshoots. Four experiments were performed using a large and a small box, each of 8.4 kg weight, and varying degrees of alternation between boxes. In two of the experiments, subjects were informed about the weight of the objects, while in the other two experiments they were not informed about the weight of the objects. When boxes were lifted 15 times before switching to the other box, rapid diminishing of force scaling errors was observed. However, when boxes were alternated each lift or after three lifts, persisting force scaling overshoots were found in lifting the large box compared to the small one. When participants were given information regarding the actual object weight, force overshoots in the first pair of large and small box lifts were not different from overshoots in experiments where subjects were not informed about the weight of the objects. This shows that, for occupational lifting, risks related to force overshoots in lifting large objects can persist despite experience in lifting the objects and despite the use of labels indicating the weight of the objects. © 2005 Taylor & Francis Group Ltd