Bioenergetics and biomechanics of front crawl swimming.

"Underwater torque" (T') is one of the main factors determining the energy cost of front crawl swimming per unit distance (Cs). In turn, T' is defined as the product of the force with which the swimmer's feet tend to sink times the distance between the feet and the center of volume of the lungs. The dependency of Cs on T' was further investigated by determining Cs in a group of 10 recreational swimmers (G1: 4 women and 6 men) and in a group of 8 male elite swimmers (G2) after T' was experimentally modified. This was achieved by securing around the swimmers' waist a plastic tube filled, on different occasions, with air, water, or 1 or 2 kg of lead. Thus, T' was either decreased, unchanged, or increased compared with the natural condition (tube filled with water). Cs was determined, for each T' configuration, at 0.7 m/s for G1 and at 1.0 and 1.2 m/s for G2. For T' equal to the natural value, Cs (in kJ.m-1.m body surface area-2) was 0.36 +/- 0.09 and 0.53 +/- 0.13 for G1 in women and men, respectively, and 0.45 +/- 0.05 and 0.53 +/- 0.06 for G2 at 1.0 and 1.2 m/s, respectively. In a given subject at a given speed, Cs and T' were linearly correlated. To compare different subjects and different speeds, the single values of Cs and T' were normalized by dividing them by the corresponding individual averages. These were calculated from all single values (of Cs or T') obtained from that subject at that speed.(ABSTRACT TRUNCATED AT 250 WORDS)

Energetics of swimming at maximal speeds in humans.

The energy cost per unit of distance (Cs, kilojoules per metre) of the front-crawl, back, breast and butterfly strokes was assessed in 20 elite swimmers. At sub-maximal speeds (v), Cs was measured dividing steady-state oxygen consumption (VO2) by the speed (v, metres per second). At supra-maximal v, Cs was calculated by dividing the total metabolic energy (E, kilojoules) spent in covering 45.7, 91.4 and 182.9 m by the distance. E was obtained as: E = Ean + alphaVO2maxtp - alphaVO2maxtau(1 - e(-(tp/tau))), where Ean was the amount of energy (kilojoules) derived from anaerobic sources, VO2max litres per second was the maximal oxygen uptake, alpha( = 20.9 kJ x 1 O2(-1)) was the energy equivalent of O2, tau (24 s) was the time constant assumed for the attainment of VO2max at muscle level at the onset of exercise, and tp (seconds) was the performance time. The lactic acid component was assumed to increase exponentially with tp to an asymptotic value of 0.418 kJ x kg(-1) of body mass for tp> or =120 s. The lactic acid component of Ean was obtained from the net increase of lactate concentration after exercise (delta[La]b) assuming that, when delta[La]b = 1 mmol x 1(-1) the net amount of metabolic energy released by lactate formation was 0.069 kJ x kg(-1). Over the entire range of v, front crawl was the least costly stroke. For example at 1 m x s(-1), Cs amounted, on average, to 0.70, 0.84, 0.82 and 0.124 kJ x m(-1) in front crawl, backstroke, butterfly and breaststroke, respectively; at 1.5 m x s(-1), Cs was 1.23, 1.47, 1.55 and 1.87 kJ x m(-1) in the four strokes, respectively. The Cs was a continuous function of the speed in all of the four strokes. It increased exponentially in crawl and backstroke, whereas in butterfly Cs attained a minimum at the two lowest v to increase exponentially at higher v. The Cs in breaststroke was a linear function of the v, probably because of the considerable amount of energy spent in this stroke for accelerating the body during the pushing phase so as to compensate for the loss of v occurring in the non-propulsive phase.

Effect of the underwater torque on the energy cost, drag and efficiency of front crawl swimming.

Underwater torque (T') is defined as the product of the force with which the swimmer's feet tend to sink times the distance between the feet and the centre of volume of the lungs. It has previously been shown that experimental changes of T', obtained by securing around the swimmer's waist a plastic tube filled, on different occasions, with air, water or 2-kg lead, were accompanied by changes in the energy cost of swimming per unit of distance (Cs) at any given speed. The aim of this study was to investigate whether the observed increases of Cs with T' during front crawl swimming were due to an increase of active body drag (Db), a decrease of drag efficiency (eta /d) or both. The effect of experimental changes of T' on Cs, Db and eta /d were therefore studied on a group of eight male elite swimmers at two submaximal speeds (1.00 and 1.23 m.s-1). To compare different subjects and different speeds, the individual data for Cs, Db, eta /d and T' were normalized dividing them by the corresponding individual averages. These were calculated from all individual data (of Cs, Db, eta /d and T') obtained from that subject at that speed. It was found that, between the two extremes of this study (tube filled with air and with 2-kg lead), T' increased by 73% and that Cs, Db and eta /d increased linearly with T'. The increase of Cs between the two extremes was intermediate (approximately 20%) between that of Db (approximately 35%) and of eta /d (approximately 16%). Thus, the actual strategy implemented by the swimmers to counteract T', was to tolerate a large increase of Db. This led also to a substantial (albeit smaller) increase of r/d, the effect of which was to reduce the increase of Cs that would otherwise have occurred.

How fins affect the economy and efficiency of human swimming.

The aim of the present study was to quantify the improvements in the economy and efficiency of surface swimming brought about by the use of fins over a range of speeds (v) that could be sustained aerobically. At comparable speeds, the energy cost (C) when swimming with fins was about 40 % lower than when swimming without them; when compared at the same metabolic power, the decrease in C allowed an increase in v of about 0.2 ms(-1). Fins only slightly decrease the amplitude of the kick (by about 10 %) but cause a large reduction (about 40 %) in the kick frequency. The decrease in kick frequency leads to a parallel decrease of the internal work rate ((int), about 75 % at comparable speeds) and of the power wasted to impart kinetic energy to the water ((k), about 40 %). These two components of total power expenditure were calculated from video analysis ((int)) and from measurements of Froude efficiency ((k)). Froude efficiency (eta(F)) was calculated by computing the speed of the bending waves moving along the body in a caudal direction (as proposed for the undulating movements of slender fish); eta(F) was found to be 0.70 when swimming with fins and 0.61 when swimming without them. No difference in the power to overcome frictional forces ((d)) was observed between the two conditions at comparable speeds. Mechanical efficiency [(tot)/(Cv), where (tot)=(k)+(int)+(d)] was found to be about 10 % larger when swimming with fins, i.e. 0.13+/-0.02 with and 0.11+/-0.02 without fins (average for all subjects at comparable speeds).