Rotational effect of buoyancy in frontcrawl: Does it really cause the legs to sink?

The purposes of this study were to quantify the rotational effect of buoyant force (buoyant torque) during the performance of front crawl and to reexamine the mechanics of horizontal alignment of the swimmers. Three-dimensional videography was used to measure the position and orientation of the body segments of 11 competitive swimmers performing front crawl stroke at a sub-maximum sprinting speed. The dimensions of each body segment were defined mathematically to match the body segment parameters (mass, density, and centroid position) reported in the literature. The buoyant force and torque were computed for every video-field (60fields/s), assuming that the water surface followed a sine curve along the length of the swimmer. The average buoyant torque over the stroke cycle (mean=22Nm) was directed to raise the legs and lower the head, primarily because the recovery arm and a part of the head were lifted out of the water and the center of buoyancy shifted toward the feet. This finding contradicts the prevailing speculation that buoyancy only causes the legs to sink throughout the stroke cycle. On the basis of a theoretical analysis of the results, it is postulated that the buoyant torque, and perhaps the forces generated by kicks, function to counteract the torque generated by the hydrodynamic forces acting on the hands, so as to maintain the horizontal alignment of the body in front crawl.     

Stroke frequency in front crawl: its mechanical link to the fluid forces required in non-propulsive directions

Two hypotheses were tested: (a) stroke frequency is predictable from the amplitudes of bodyroll and the turning effect around the body's long-axis generated by the non-propulsive fluid forces (that is, the torque driving bodyroll), and (b) swimmers exhibit at least one alteration in the factors influencing the bodyroll cycle as they increase the stroke frequency for faster swimming, so that they can reduce the fluid forces "wasted" in non-propulsive directions. The mechanical formula that links stroke frequency and the kinetics of bodyroll was derived on the basis of Euler's equation of motion. Experimental data were collected from competitive swimmers to validate the derived mechanical relations and to examine the strategy that skilled swimmers would use to change the stroke frequency as they swam faster. A strong correlation (slow: r = 0.70 and fast: r = 0.85) and a non-significant difference between the observed stroke frequency and the formula-based estimates supported the first hypothesis. As the subjects increased stroke frequency (38%) for faster swimming, bodyroll decreased (19%) and the trunk twist increased (40%). The combined alterations resulted in a small reduction in the shoulder roll (12%), enabling the swimmers to gain the benefits associated with a large rolling action of the upper trunk, while limiting the amount of increase in the turning effect of fluid forces in non-propulsive directions (40%). The second hypothesis was, therefore, supported. The derived mechanical formula provides a theoretical basis to explore mechanisms underlying the interrelations among stroke frequency, stroke length and swimming speed, and sheds light on a possible reason that swimmers generally adopt six-beat kicks.     

Effect of body roll amplitude and arm rotation speed on propulsion of arm amputee swimmers

Only a limited amount of research has gone into evaluating the contribution made by the upper arm to the propulsion of elite swimmers with an amputation at elbow level. With assistance of computational fluid dynamics (CFD) modelling, the swimming technique of competitive arm amputee swimmers can be assessed through numerical simulations which test the effect of various parameters on the effectiveness of the swimming propulsion.This numerical study investigates the effect of body roll amplitude and of upper arm rotation speed on the propulsion of an arm amputee swimmer, at different mean swimming speeds. Various test cases are simulated resulting in a thorough analysis of the complex body/fluid interaction with a detailed quantitative assessment of the effect of the variation of each parameter on the arm propulsion. It is found that a body roll movement with an amplitude of 45° enhances greatly the propulsive contribution from the upper arm with an increase of about 70% in the propulsive force compared to the no roll condition. An increase in the angular velocity of the upper arm also leads to a concomitant increase in the propulsive forces produced by the arm.Such results have direct implications for competitive arm amputee front crawl swimmers and for those who coach them. One important message that emerges in this present work is that there exists, for any given swimming speed, a minimum angular velocity at which the upper arm must be rotated to generate effective propulsion. Below this velocity, the upper arm will experience a net resistive drag force which adversely affects swimming performance.     

Rolling rhythms in front crawl swimming with six-beat kick

The purpose of this study was to establish the rhythm characteristics of skilled front crawl swimmers using a six-beat kick. These included the amplitudes of the first three Fourier harmonics (H1, H2, H3) and their percent contributions to power contained in the angular displacement signals of the shoulders, hips, knees, and ankles with respect to the longitudinal axis in line with the swimming direction. Three-dimensional video data of seven national/international level swimmers were collected during simulated 200 m front crawl races in which swimmers maintained six-beat kicking patterns. Swimmers differed in all variables but had small variability across the four 50 m laps. Modest changes occurred during the 200 m, with the exception of shoulder roll, which remained constant and was represented almost entirely by a single sinusoid (H1). Changes across laps reached significance for swimming speed, stroke rate, hip roll, and H3 wave velocity between the knee and ankle. A H3 body wave of moderate and increasing velocity travelled caudally from hip to ankle. In the light of existing knowledge of aquatic locomotion this was compatible with the goal of generating propulsion in an efficient manner.     

Whole-body lift and ground effect during pectoral fin locomotion in the northern spearnose poacher (Agonopsis vulsa)

The northern spearnose poacher, Agonopsis vulsa, is a benthic, heavily armored fish that swims primarily using pectoral fins. High-speed kinematics, whole-body lift measurements, and flow visualization were used to study how A. vulsa overcomes substantial negative buoyancy while generating forward thrust. Kinematics for five freely swimming poachers indicate that individuals tend to swim near the bottom (within 1 cm) with a consistently small (less than 1°) pitch angle of the body. When the poachers swam more than 1 cm above the bottom, however, body pitch angles were higher and varied inversely with speed, suggesting that lift may help overcome negative buoyancy. To determine the contribution of the body to total lift, fins were removed from euthanized fish (n=3) and the lift and drag from the body were measured in a flume. Lift and drag were found to increase with increasing flow velocity and angle of attack (ANCOVA, p<0.0001 for both effects). Lift force from the body was found to supply approximately half of the force necessary to overcome negative buoyancy when the fish were swimming more than 1 cm above the bottom. Lastly, flow visualization experiments were performed to examine the mechanism of lift generation for near-bottom swimming. A vortex in the wake of the pectoral fins was observed to interact strongly with the substratum when the animals approached the bottom. These flow patterns suggest that, when swimming within 1 cm of the bottom, poachers may use hydrodynamic ground effect to augment lift, thereby counteracting negative buoyancy.     

Mechanical energy contribution of the metatarsophalangeal joint to running and sprinting

Despite the fact that a number of studies have investigated lower extremity energy generation during locomotion, the influence of the metatarsophalangeal (MP) joint remained unknown. The purpose of this study was to determine the relative contribution of the MP joint to the total mechanical energy in running and sprinting. A sagittal plane analysis was performed on data collected from 10 trained male athletes (five runners and five sprinters). The MP moment was assumed to be negligible until the ground reaction force acted distal to the joint. During running, once the ground reaction force crossed the MP joint, the MP moment was plantarflexor for the remainder of ground contact with average peak values of 59.9 Nm. The MP joint moment was plantarflexor throughout the stance phase for sprinting with average peak values of 112.4 Nm. Since the MP joint was dorsiflexing throughout the majority of the stance phase the joint absorbed large amounts of energy, on average 20.9 J during running and 47.8 J during sprinting. A lack of plantarflexion of the MP joint resulted in a lack of energy generation during take-off. Thus, the energy that was absorbed at the joint was dissipated in the shoe and foot structures.     

The foraging benefits of being fat in a highly migratory marine mammal

Foraging theory predicts that breath-hold divers adjust the time spent foraging at depth relative to the energetic cost of swimming, which varies with buoyancy (body density). However, the buoyancy of diving animals varies as a function of their body condition, and the effects of these changes on swimming costs and foraging behaviour have been poorly examined. A novel animal-borne accelerometer was developed that recorded the number of flipper strokes, which allowed us to monitor the number of strokes per metre swam (hereafter, referred to as strokes-per-metre) by female northern elephant seals over their months-long, oceanic foraging migrations. As negatively buoyant seals increased their fat stores and buoyancy, the strokes-per-metre increased slightly in the buoyancy-aided direction (descending), but decreased significantly in the buoyancy-hindered direction (ascending), with associated changes in swim speed and gliding duration. Overall, the round-trip strokes-per-metre decreased and reached a minimum value when seals achieved neutral buoyancy. Consistent with foraging theory, seals stayed longer at foraging depths when their round-trip strokes-per-metre was less. Therefore, neutrally buoyant divers gained an energetic advantage via reduced swimming costs, which resulted in an increase in time spent foraging at depth, suggesting a foraging benefit of being fat.     

SKIN DENSITY AND ITS INFLUENCE ON BUOYANCY IN THE MANATEE (TRICHECHUS MANATUS LATIROSTRIS), HARBOR PORPOISE (PHOCOENA PHOCOENA), AND BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS)

This study investigated how skin contributes to buoyancy control in the Florida manatee ( Trichechus manatus latirostris ), harbor porpoise ( Phocoena phocoena ), and bottlenose dolphin ( Tursiops truncatus ). Manatees are shallow divers and control their position in the water column hydrostatically. The two cetaceans are relatively deep divers that control their buoyancy hydrodynamically. Although the cetacean skin had been hypothesized to lower total body density ( e. g. , Dearolf et al. 2000, Nowacek et al. 2001), its buoyant force had not been calculated. The density of manatee skin, and its contribution to buoyancy, was unknown. Skin densities of 27 manatees, five harbor porpoises, and five bottlenose dolphins were measured volumetrically. Skin mass and density were used to calculate buoyant force. Harbor porpoise (952 kg/m³) and bottlenose dolphin (969 kg/m³) skins were less dense than seawater, and added 9 and 25 N of positive buoyant force, respectively, to total body buoyancy. By contrast, manatee skin (1,121 kg/m³) contributed 56 N of negative buoyant force, which equaled 70% of the negative buoyant force of their dense, pachyosteosclerotic ribs. Calculation of buoyant forces of the skeleton, skin and lungs demonstrates that the manatee is positively buoyant at the surface and negatively buoyant at depths of less than 10 m.     

Mechanisms and implications of animal flight maneuverability

Accelerations and directional changes of flying animals derivefrom interactions between aerodynamic force production and theinertial resistance of the body to translation and rotation.Anatomical and allometric features of body design thus mediatethe rapidity of aerial maneuvers. Both translational and rotationalresponsiveness of the body to applied force decrease with increasedtotal mass. For flying vertebrates, contributions of the relativelyheavy wings to whole-body rotational inertia are substantial,whereas the relatively light wings of many insect taxa suggestthat rotational inertia is dominated by the contributions ofbody segments. In some circumstances, inertial features of wingdesign may be as significant as are their aerodynamic propertiesin influencing the rapidity of body rotations. Stability inflight requires force and moment balances that are usually attainedvia bilateral symmetry in wingbeat kinematics, whereas bodyroll and yaw derive from bilaterally asymmetric movements ofboth axial and appendicular structures. In many flying vertebrates,use of the tail facilitates the generation of aerodynamic torquesand substantially enhances quickness of body rotation. Geometricalconstraints on wingbeat kinematics may limit total force productionand thus accelerational capacity in certain behavioral circumstances.Unitary limits to animal flight performance and maneuverabilityare unlikely, however, given varied and context-specific interactionsamong anatomical, biomechanical, and energetic features of design.     

Helical swimming can provide robust upwards transport for gravitactic single-cell algae; a mechanistic model

In still fluid, many phytoplankton swim in helical paths with an average upwards motion. A new mechanistic model for gravitactic algae subject to an intrinsic torque is developed here, based on Heterosigma akashiwo, which results in upwards helical trajectories in still fluid. The resultant upwards swimming speed is calculated as a function of the gravitactic and intrinsic torques. Helical swimmers have a reduced upwards speed in still fluid compared to cells which swim straight upwards. However a novel result is obtained when the effect of fluid shear is considered. For intermediate values of shear and intrinsic torque, a new stable equilibrium solution for swimming direction is obtained for helical swimmers. This results in positive upwards transport in vertical shear flow, in contrast to the stable equilibrium solution for straight swimmers which results in downwards transport in vertical shear flow. Furthermore, for strong intrinsic torque, when there is no longer a stable orientation equilibrium, we show that the average downwards transport of helical swimmers in vertical shear flow is greatly suppressed compared to straight swimmers. We hypothesise that helical swimming provides robustness for upwards transport in the presence of fluid shearing motions.     

A continuous dynamic beam model for swimming fish

When a fish swims in water, muscle contraction, controlled by the nervous system, interacts with the body tissues and the surrounding fluid to yield the observed movement pattern of the body. A continuous dynamic beam model describing the bending moment balance on the body for such an interaction during swimming has been established. In the model a linear visco-elastic assumption is made for the passive behaviour of internal tissues, skin and backbone, and the unsteady fluid force acting on the swimming body is calculated by the 3D waving plate theory. The body bending moment distribution due to the various components, in isolation and acting together, is analysed. The analysis is based on the saithe (Pollachius virens), a carangiform swimmer. The fluid reaction needs a bending moment of increasing amplitude towards the tail and near-standing wave behaviour on the rear-half of the body. The inertial movement of the fish results from a wave of bending moment with increasing amplitude along the body and a higher propagation speed than that of body bending. In particular, the fluid reaction, mainly designed for propulsion, can provide a considerable force to balance the local momentum change of the body and thereby reduce the power required from the muscle. The wave of passive visco-elastic bending moment, with an amplitude distribution peaking a little before the mid-point of the fish, travels with a speed close to that of body bending. The calculated muscle bending moment from the whole dynamic system has a wave speed almost the same as that observed for EMG-onset and a starting instant close to that of muscle activation, suggesting a consistent matching between the muscle activation pattern and the dynamic response of the system in steady swimming. A faster wave of muscle activation, with a variable phase relation between the strain and activation cycle, appears to be designed to fit the fluid reaction and, to a lesser extent, the body inertia, and is limited by the passive internal tissues. Higher active stress is required from caudal muscle, as predicted from experimental studies on fish muscle. In general, the active force development by muscle does not coincide with the propulsive force generation on the tail. The stiffer backbone may play a role in transmitting force and deformation to maintain and adjust the movement of the body and tail in water.     

Observations of neutral buoyancy in diapausing copepods Calanoides acutus during Antarctic winter

The herbivorous Antarctic copepod Calanoides acutus overwinters inactively in a resting stage (diapause) at depths below 500 m. It is assumed that during diapause C. acutus is neutrally buoyant in order to retain energy reserves otherwise depleted by swimming activities. However, so far, no experimental observations on its buoyancy have been reported and our knowledge of buoyancy regulation mechanisms is incomplete. In the present study, species-specific differences in buoyancy were assessed visually. Observations were made of specimens from the diapausing cohort of C. acutus and compared to another herbivorous copepod Calanus propinquus, which overwinters actively feeding in the upper water layers. Freshly caught copepods were anaesthetized in a 3-amino-benzoic acid ethyl ester (MS222) in seawater solution in order to exclude the influence of swimming movements on buoyancy control. It was shown that C. propinquus was negatively buoyant, whereas diapausing C. acutus remained neutrally buoyant. This is the first record that neutral buoyancy in diapausing copepods is maintained by the biochemical body composition without the additional need of swimming movements.     

Costs of swimming measured at optimum speed: scale effects, differences between swimming styles, taxonomic groups and submerged and surface swimming

1. Data on swimming energy expenditure of 30 submerged and nine surface swimmers, covering different swimming styles and taxonomic groups, are selected from the literature. 2. The costs of transport at the optimum speed are compared and related to body mass and Re numbers. 3. Fish and turtles use relatively less and most surface swimmers slightly more energy than the other submerged swimmers; man and mink are poorly adapted to swimming. 4. The metabolic rate in W at optimum speed is approximately equal to the body mass in kg for fish and turtles and three times the mass figure for the other submerged swimmers.     

To Bend a Dolphin: Convergence of Force Transmission Designs in Cetaceans and Scombrid Fishes

The similarity in swimming style and external body shape betweendolphins and scombrid fishes, especially tunas, is a textbookexample of evolutionary convergence. I identify additional morphologicalfeatures of the musculoskeletal system shared by dolphins andtunas. Specifically, these swimmers share a pattern of forcetransmission through a complex, three-dimensional system ofcollagenous fabrics, which are stiffened by muscular hydrostaticpressure. This force transmission system increases both thedisplacement advantage and moment arm of contracting axial muscle.These features represent a functionally significant design forsteady swimming vertebrates.     

Direct and indirect effects of joint torque inputs during an induced speed analysis of a swinging motion

This study proposed a method to quantify direct and indirect effects of the joint torque inputs in the speed-generating mechanism of a swinging motion. Linear and angular accelerations of all segments within a multi-linked system can be expressed as the sum of contributions from a joint torque term, gravitational force term and motion-dependent term (MDT), where the MDT is a nonlinear term consisting of centrifugal force, Coriolis force and gyroscopic effect moment components. Direct effects result from angular accelerations induced by a joint torque at a given instant, whereas indirect effects arise through the MDT induced by joint torques exerted in the past. These two effects were quantified for the kicking-side leg during a rugby place kick. The MDT was the largest contributor to the foot centre of gravity (CG)’s speed at ball contact. Of the factors responsible for generating the MDT, the direct and indirect effects of the hip flexion-extension torque during both the flight phase (from the final kicking foot take-off to support foot contact) and the subsequent support phase (from support foot contact to ball contact) were important contributors to the foot CG’s speed at ball contact. The indirect effect of the ankle plantar-dorsal flexion torque and the direct effect of the knee flexion-extension torque during the support phase showed the largest positive and negative contributions to the foot CG’s speed at ball contact, respectively. The proposed method allows the identification of which individual joint torque axes are crucial and the timings of joint torque exertion that are used to generate a high speed of the distal point of a multi-linked system.     

How early reactions in the support limb contribute to balance recovery after tripping

Tripping causes a forward angular momentum that has to be arrested to prevent a fall. The support limb, contralateral to the obstructed swing limb, can contribute to an adequate recovery by providing time and clearance for proper positioning of the recovery limb, and by restraining the angular momentum of the body during push-off. The present study investigated how such a contribution is achieved by the support limb in terms of response times and muscle moment generation, in order to provide more insight in the requirements for successful recovery after tripping. Twelve young adults repeatedly walked over a platform in which 21 obstacles were hidden. Each subject was tripped over one of these obstacles during mid-swing in at least five trials. Kinematics, dynamics and muscle activity were measured. Very rapid responses were seen in the muscles of the support limb (approximately 65 ms), causing fast increases in muscle moments in the joints during the primary phase of recovery. Especially a large ankle plantar flexion moment (204 Nm), a knee flexion moment (-54 Nm) and a hip extension moment (52 Nm), generated by triceps surae and hamstring muscle activity, brought about the necessary push-off reaction and simultaneously caused a restraining of the forward angular momentum of the body. These required joint moments could be a problem for the elderly, who might not be able to generate such powerful moments. Strength training in these muscle groups may be indicated in elderly subjects to reduce the risk of falling after a trip.     

Comparing different approaches for determining joint torque parameters from isovelocity dynamometer measurements

Strength, or maximum joint torque, is a fundamental factor governing human movement, and is regularly assessed for clinical and rehabilitative purposes as well as for research into human performance. This study aimed to identify the most appropriate protocol for fitting a maximum voluntary torque function to experimental joint torque data. Three participants performed maximum isometric and concentric-eccentric knee extension trials on an isovelocity dynamometer and a separate experimental protocol was used to estimate maximum knee extension angular velocity. A nine parameter maximum voluntary torque function, which included angle, angular velocity and neural inhibition effects, was fitted to the experimental torque data and three aspects of this fitting protocol were investigated. Using an independent experimental estimate of maximum knee extension angular velocity gave lower variability in the high concentric velocity region of the maximum torque function compared to using dynamometer measurements alone. A weighted root mean square difference (RMSD) score function, that forced the majority (73-92%) of experimental data beneath the maximum torque function, was found to best account for the one-sided noise in experimental torques resulting from sub-maximal effort by the participants. The suggested protocol (an appropriately weighted RMSD score function and an independent estimate of maximum knee extension angular velocity) gave a weighted RMSD of between 11 and 13 Nm (4-5% of maximum isometric torque). It is recommended that this protocol be used in generating maximum voluntary joint torque functions in all torque-based modelling of dynamic human movement.     

The effect of external dummy transmitters on oxygen consumption and performance of swimming Atlantic cod

Decreased critical swimming speed and increased oxygen consumption (     ) was found for externally tagged Atlantic cod Gadus morhua swimming at a high speed of 0·9 body length (total length, L _T) s⁻¹. No difference was found in the standard metabolic rate, indicating that the higher     for tagged cod was due to drag force rather than increased costs to keep buoyancy.     

Unexpected Positive Buoyancy in Deep Sea Sharks,Hexanchus griseus,and a Echinorhinus cookei

We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two species of deep-sea sharks (bluntnose sixgill sharks, Hexanchus griseus, and a prickly shark, Echinorhinus cookei) in their natural habitat, we used accelerometer-magnetometer data loggers to measure their swimming performance. Both species of deep-sea sharks showed similar diel vertical migrations: they swam at depths of 200–300 m at night and deeper than 500 m during the day. Ambient water temperature was around 15°C at 200–300 m but below 7°C at depths greater than 500 m. During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats. This positive buoyancy may adaptive for stealthy hunting (i.e. upward gliding to surprise prey from underneath) or may facilitate evening upward migrations when muscle temperatures are coolest, and swimming most sluggish, after spending the day in deep, cold water. Positive buoyancy could potentially be widespread in fish conducting daily vertical migration in deep-sea habitats.