Swimming Mechanism Research Articles

Swimming posture recognition using inertial sensors and CNN-SVM: Unveiling the cellular molecular biomechanics nexus

The biomechanical mechanisms of swimming involve a number of aspects. The forces exerted by muscles during different swimming postures are crucial. These muscle contractions and relaxations follow specific biomechanical principles. This work aims to develop a swimming posture recognition system based on inertial sensors and a Convolutional Neural Network-Support Vector Machine (CNN-SVM) to improve the accuracy and real-time performance of posture recognition. First, an inertial sensor system to be worn on swimwear is designed to collect three-axis motion data, including acceleration, angular velocity, and magnetometer readings. The collected data are then preprocessed through denoising, normalization, and feature extraction steps to ensure high-quality input data. Next, a Convolutional Neural Network (CNN) is constructed to automatically extract high-level features from the preprocessed sensor data. The CNN model, through multi-layer convolution and pooling operations, effectively captures the spatiotemporal patterns in the motion data, extracting highly distinguishable features for posture recognition. To further improve the model’s classification performance, a Support Vector Machine (SVM) classifier is applied based on the CNN model. Specifically, CNN is responsible for feature extraction, while the SVM handles the final posture classification. Cross-validation is used to train and validate the model, assessing its performance. Experimental results show that the model achieves a 95% accuracy rate on the training dataset and maintains an accuracy rate above 93% on the test dataset. The system can accurately and in real-time recognize various swimming postures, including freestyle, breaststroke, backstroke, and butterfly. The recognition accuracy for all four swimming styles exceeds 91%. Understanding these biomechanical mechanisms helps in improving the accuracy of the recognition system. In summary, the proposed method for swimming posture recognition based on inertial sensors and CNN-SVM has significant advantages in accuracy and real-time performance. It allows for better interpretation of the sensor data and more precise identification of different postures. The high accuracy and generalization ability of the proposed system suggest that it can effectively capture and analyze the biomechanical nuances of swimming, providing valuable insights for swimming training and performance evaluation, and opening up new avenues for intelligent sports monitoring. evaluation.

A fins-shaped bidirectional linear ultrasonic motor driven by a single-phase signal

Abstract Inspired by the propulsion mechanism of fish swimming through tail-fin oscillations, we have developed and experimentally studied a high-performance miniature bending vibration linear ultrasonic motor driven by a single-phase standing wave. A symmetrical structural design achieves bidirectional motion with consistent output characteristics. Two lead zirconate titanate (PZT) piezoelectric patches are symmetrically distributed on a metal elastic body’s upper and lower sides. In the stator, a sinusoidal signal is applied only to the piezoelectric patch on one side, while the patch on the opposite side is either left open or short-circuited to the ground. This results in an asymmetry in the structural damping between the upper and lower sides of the stator. The first-order bending vibration of the stator generates displacement in an inclined direction at the end of the driving foot. The motor achieves forward and backward motion by applying the same voltage to the piezoelectric patches on different sides. The dimensions of the stator have been finalized, and a prototype of the designed motor has been fabricated. Subsequent experimental tests were conducted to evaluate the prototype’s performance, including its mechanical output characteristics. At a frequency of 138.3 kHz, the maximum speed of the motor without load is 134.58 mm s−1. Under a voltage of 200 Vp-p and a preload of 3 N, the motor’s maximum output thrust is approximately 0.3 N. The actual mass of the motor is approximately 1.2 g, resulting in a thrust-to-weight ratio of 25.

Swimming by Spinning: Spinning-Top Type Rotations Regularize Sperm Swimming Into Persistently Progressive Paths in 3D.

Sperm swimming is essential for reproduction, with movement strategies adapted to specific environments. Sperm navigate by modulating the symmetry of their flagellar beating, but how they swim forward with asymmetrical beats remains unclear. Current methods lack the ability to robustly detect the flagellar symmetry state in free-swimming spermatozoa, despite its importance in understanding sperm motility. This study uses numerical simulations to investigate the fluid mechanics of sperm swimming with asymmetrical flagellar beats. Results show that sperm rotation regularizes the swimming motion, allowing persistently progressive swimming even with asymmetrical flagellar beats. Crucially, 3D sperm head orientation, rather than the swimming path, provides critical insight into the flagellar symmetry state. Sperm rotations during swimming closely resemble spinning-top dynamics, with sperm head precession driven by the helical beating of the flagellum. These results may prove essential in future studies on the role of symmetry in microorganisms and artificial swimmers, as body orientation detection has been largely overlooked in favor of swimming path analysis. Altogether, this rotational mechanism provides a reliable solution for forward propulsion and navigation in nature, which would otherwise be challenging for flagella with brokensymmetry.

Autonomous phototaxis of hydrogel swimmers

The design of synthetic soft matter capable of emulating the complex behaviors of living organisms, such as sensing and adapting to their environment, remains an important challenge in developing biomimetic materials. Functionalized hydrogels are ideal candidates for such materials since they are highly responsive to their environment and can be operated in water. In this work, we investigate a hybrid bonding hydrogel composed of peptide amphiphile supramolecular nanofibers covalently attached to a photoresponsive network, in which high-aspect-ratio ferromagnetic nanowires are aligned along the length of the sample, designed to swim under oscillating magnetic fields. This hybrid hydrogel swimmer can autonomously swim toward a light source by utilizing photoinduced interactions between supramolecular and covalent networks reminiscent of phototactic swimming in living systems. Using a combination of experimental techniques and a continuum model incorporating photochemistry, magnetoelasticity, and hydrodynamics, we explain the swimming mechanism and predict phototactic behavior. Our work highlights the potential role of hybrid bonding polymers, which leverage the interplay between supramolecular assemblies and covalent networks. We demonstrate how these polymers can be tailored to react dynamically to their environment, paving the way for developing intelligent and autonomous robotic systems.

Numerical simulation of the self-propelled swimming performances and mechanisms of a biomimetic robotic fish with undulating fins under different fin waveforms

To study the self-propelled swimming performances and mechanisms of biomimetic robotic fish with undulating fins (BRFUF) under different waveforms, a numerical simulation system coupled with body dynamics and fluid dynamics was established to study the starting, accelerating, and cruising processes of a biomimetic robotic fish in a median/paired fin swimming mode. A systematic parametric study was carried out on the swimming performance of a BRFUF under the cooperative propulsion of two fins, and the mechanism of thrust generation and the influence mechanisms of waveform and kinematic parameters of fins on swimming performance were analyzed based on the hydrodynamic performance, surface pressure distribution, vortex dynamics, and longitudinal velocity iso-surface of the flow field. The results showed that a larger fin ray oscillation angle amplitude increased the acceleration and cruising velocity of the BRFUF from the static state to the cruising stage. A highly concentrated vortex generated at the trough of the fin creates a jet mass that generates a reactive (added-mass) force perpendicular to the propulsive element, which is the mechanism by which the high pressure always covers the trough of the fin. Driven by the flexible fluctuations of the fins, the high-pressure region continuously moves toward the trailing edge along with the vortex. Along with the generation and shedding of the vortex, the high-pressure region is constantly generated, moving and disappearing on the surface of the fin and providing continuous thrust for BRFUF self-propulsion.

Steady motions of single spherical microswimmers in non-Newtonian fluids

In biological systems, microswimmers often propel themselves through complex media. However, many aspects of swimming mechanisms in non-Newtonian fluids remain unclear. This study considers the propulsion of two types of single spherical microswimmers (squirmers) in shear-thickening and shear-thinning fluids. The slip-driven squirmer propels faster/slower in shear-thickening/thinning fluids than in Newtonian fluids [C. Datt et al., J. Fluid Mech. 784, R1 (2015)]. In contrast, we discovered that a traction-driven squirmer exhibits the opposite trend, moving slower/faster in shear-thickening/thinning fluids than in Newtonian fluids. In addition, we have shown theoretically that Purcell's scallop theorem does not hold in non-Newtonian fluids when a squirmer with reciprocal surface motions is used. The present findings open up possibilities for the design of new types of microswimmers that can achieve translational motion from a single reciprocal motion in non-Newtonian fluids. Furthermore, we demonstrated that traction-driven squirmers swim faster and more efficiently in shear-thinning fluids than in Newtonian fluids. These findings highlight how the non-Newtonian rheology enhances both swimming speed and efficiency, suggesting further potential for optimizing locomotion performance otherwise impossible in Newtonian fluids.

Ups and downs: copepods reverse the near-body flow to cruise in the water column

Copepods are negatively buoyant organisms actively participating in large-scale vertical migrations as primary consumers in marine ecosystems. As such, these organisms need to overcome their own weight to swim upwards, incurring extra energy costs that are not offset by any mechanism intrinsic to drag-based propulsion. While copepod vertical migrations are well documented, it is still unclear how they achieve extensive upward cruising despite this limitation. In this study, we found suction to be a compensatory mechanism enhancing thrust in upward-swimming copepods. Using experimentally derived velocity and pressure fields, we observed that copepods pull water in front of them to generate sub-ambient pressure gradients when cruising upward, thereby inducing an upstream suction force to complement the thrust produced by the legs. Contrary to expectations that drag always dominates the leg recovery phase, we found that the upstream suction generates net thrust for about a third of the recovery stroke. In contrast, downward-swimming copepods push rather than pull on water and do not benefit from thrust-enhancing suction effects during the recovery stroke. Differences in the induced flows are associated with contrasting leg kinematics, indicating a response to the body orientation rather than a fixed effect. These results offer insights into an important swimming mechanism that can inform the role mesozooplankton play in biogenic hydrodynamic transport and its impact on marine biogeochemistry.

Numerical simulation on the interaction of median fins for enhancing vortex dynamics and propulsion performance in fish self-propelled swimming

Studies have shown that fish can enhance propulsion performance by utilizing the interaction between median fins (dorsal, anal, and caudal fins), compared to fish with only caudal fin. However, most of the current studies are based on the fish oscillating in-place, and the analysis of median fins interaction to improve swimming propulsion performance is still insufficient, and the mechanism needs further study. This study applied three-dimensional numerical simulation methods to solve the process of grass carp accelerating from a stationary state to cruising state under different body and median fins combination, as well as different motion parameter models. A comparative quantitative analysis of different models was conducted to assess the impact of median fins interactions on enhancing swimming performance, with a detailed analysis of the hydrodynamic mechanisms and their relationship with vortex dynamics. The results indicated that interactions between median fins could generate significant hydrodynamic benefits, with the fish's average swimming speed increasing up to 4.6 times, thrust up to 33.47%, and swimming efficiency up to 25.48%. This study found that the enhancement of propulsion performance was due to the formation of high-intensity and persistent posterior body vortices by the movements of the dorsal and anal fins, which were captured by the leading-edge of the caudal fin, greatly enhancing the strength of the leading-edge vortex. This study elucidates the hydrodynamic mechanisms of the interaction between median fins and could provide new insights into the efficient swimming mechanism of fish in nature.

Hydrodynamic Simulation and Experiment of a Self-Adaptive Amphibious Robot Driven by Tracks and Bionic Fins.

Amphibious robots have broad prospects in the fields of industry, defense, and transportation. To improve the propulsion performance and reduce operation complexity, a novel bionic amphibious robot, namely AmphiFinbot-II, is presented in this paper. The swimming and walking components adopt a compound drive mechanism, enabling simultaneous control for the rotation of the track and the wave-like motion of the undulating fin. The robot employs different propulsion methods but utilizes the same operation strategy, eliminating the need for mode switching. The structure and the locomotion principle are introduced. The performance of the robot in different motion patterns was analyzed via computational fluid dynamics simulation. The simulation results verified the feasibility of the wave-like swimming mechanism. Physical experiments were conducted for both land and underwater motion, and the results were consistent with the simulation regulation. Both the underwater linear and angular velocity were proportional to the undulating frequency. The robot's maximum linear speed and steering speed on land were 2.26 m/s (2.79 BL/s) and 442°/s, respectively, while the maximum speeds underwater were 0.54 m/s (0.67 BL/s) and 84°/s, respectively. The research findings indicate that the robot possesses outstanding amphibious motion capabilities and a simplistic yet unified control approach, thereby validating the feasibility of the robot's design scheme, and offering a novel concept for the development of high-performance and self-contained amphibious robots.

Numerical Simulation of Bionic Underwater Vehicle Morphology Drag Optimisation and Flow Field Noise Analysis

The study of aquatic organisms’ ectomorphology is important to understanding the mechanisms of efficient swimming and drag reduction in fish. The drag reduction mechanism in fish remains unknown yet is needed for optimising the efficiency of bionic fish. It is thus crucial to conduct drag tests and analyses. In this paper, an optimal dolphin morphological model is constructed taking the beakless porpoise as the research object. A numerical simulation of the dolphin body model is carried out for different combinations of pitch angle and speed adopting computational fluid dynamics, and the flow field noise of the dolphin body model is solved for different speeds using the FW-H equation. When the dolphin model is oriented horizontally, the differential pressure drag accounts for approximately 20–25% of the total drag as airspeed increases. As both the pitch angle and airspeed increase, the differential pressure drag and friction drag decrease with increasing airspeed. Moreover, the acoustic energy is mainly concentrated at low frequencies for both the dolphin and Bluefin-21 models. The dolphin body model has better noise performance than the Bluefin-21 model at the same speed. The optimisation of the external morphology of the bionic underwater submarine and the analysis of the shape drag are thus important for revealing the drag reduction mechanism, reducing noise in the flow field and provide guidance for research on bionic fish.

Investigating the Influence of Counterflow Regions on the Hydrodynamic Performance of Biomimetic Robotic Fish.

Biomimetic robotic fish are a novel approach to studying quiet, highly agile, and efficient underwater propulsion systems, attracting significant interest from experts in robotics and engineering. These versatile robots showcase their ability to operate effectively in various water conditions. Nevertheless, the comprehension of the swimming mechanics and the evolution of the flow field of flexible robots in counterflow regions is still unknown. This paper presents a framework for the self-propulsion of robotic fish that imitates biological characteristics. The method utilizes computational fluid dynamics to analyze the hydrodynamic efficiency of the organisms at different frequencies of tail movement, under both still and opposing flow circumstances. Moreover, this study clarifies the mechanisms that explain how changes in the aquatic environment affect the speed and efficiency of propulsion. It also examines the most effective swimming tactics for places with counterflow. The results suggest that the propulsion effectiveness of robotic fish in counterflow locations does not consistently correspond to various tail-beat frequencies. By utilizing vorticity maps, a comparative analysis can identify situations when counterflow zones improve the efficiency of propulsion.

Hydrodynamic performance analysis of swimming processes in self-propelled manta rays

To fill the research gap regarding the whole process (steady-state and nonsteady-state phases) of median and/or paired fin (MPF) mode swimming in underwater organisms, a two-degree-of-freedom self-propelled coupling method of motion and hydrodynamics based on user-defined functions of Fluent software was established, and numerical simulations were carried out for the startup, acceleration, and steady-state phases of manta rays. The interaction mechanism among the hydrodynamic characteristics, vortex evolution, and pressure distribution was investigated in the mentioned phases. We concluded that the negative pressure zone generated by the leading edge vortex and the shear layer contributes to thrust generation and changes in swimming velocity dominate the hydrodynamic characteristics by affecting the evolution of the shear layer and the leading edge vortex, with a 17.54% increase in forward average velocity in the fourth cycle compared to the third cycle and a consequent 9.5% increase in average thrust. In the end, the relationship between the formation of trailing edge vortex rings and changes in thrust was revealed. The vortex ring contributes to the increase in thrust, but the formation of the vortex ring comes at the cost of the loss of the leading edge vortex negative pressure zone, which greatly affects thrust, decreasing to 38.3% of its peak. The swimming mechanism revealed in this study provides a reference for the study of MPF-driven biodynamics and a new simulation strategy for the prediction of bionic navigator motions.

Hydrodynamic Performance Research of Underwater Oscillating Fin With the Compound Locomotion of Two Modes

Abstract The fish-like propulsion robot is becoming a profound intelligent equipment due to its excellent swimming ability and good environmental adaptability. In this paper, we propose the oscillating fin based on the fish swimming mechanism, which is compounded with the locomotion modes of sway and yaw. The kinematic and dynamic models are established to study the locomotion mechanism of the oscillating fin. The hydrodynamic performance of underwater locomotion is investigated to analyze the velocity, the propulsive force, the pressure, the propulsive efficiency, and the vortices property. Finally, the experimental measurements of the robot with oscillating fin propulsion are carried out to analyze the underwater propulsion of the oscillating fin and the unsteady fluid flow with Strouhal number. The results illustrate that the propulsive force is fluctuating, and the velocity is increasing to the maximum value. The underwater propulsion velocity could reach 1.2 m/s in a period of 0.4 s. Besides, the high- and low-pressure regions change alternatively, and the fin deforming process illustrates the vortices property and the locomotion mechanism analyses. The propulsive efficiency of the oscillating fin with compound waves is increased by 11% compared with that of the one without deformation. The experiments of the robot prototype verify the numerical simulation, and the propulsive velocity with a period of 0.4 s is two times larger than that of a period of 0.8 s. The Strouhal number of each motion mode is obtained through theoretical and experimental analyses.

Fast-Swimming Soft Robotic Fish Actuated by Bionic Muscle.

Soft underwater swimming robots actuated by smart materials have unique advantages in exploring the ocean, such as low noise, high flexibility, and friendly environment interaction ability. However, most of them typically exhibit limited swimming speed and flexibility due to the inherent characteristics of soft actuation materials. The actuation method and structural design of soft robots are key elements to improve their motion performance. Inspired by the muscle actuation and swimming mechanism of natural fish, a fast-swimming soft robotic fish actuated by a bionic muscle actuator made of dielectric elastomer is presented. The results show that by controlling the two independent actuating units of a biomimetic actuator, the robotic fish can not only achieve continuous C-shaped body motion similar to natural fish but also have a large bending angle (maximum unidirectional angle is about 40°) and thrust force (peak thrust is about 14 mN). In addition, the coupling relationship between the swimming speed and actuating parameters of the robotic fish is established through experiments and theoretical analysis. By optimizing the control strategy, the robotic fish can demonstrate a fast swimming speed of 76 mm/s (0.76 body length/s), which is much faster than most of the reported soft robotic fish driven by nonbiological soft materials that swim in body and/or caudal fin propulsion mode. What's more, by applying programmed voltage excitation to the actuating units of the bionic muscle, the robotic fish can be steered along specific trajectories, such as continuous turning motions and an S-shaped routine. This study is beneficial for promoting the design and development of high-performance soft underwater robots, and the adopted biomimetic mechanisms, as well as actuating methods, can be extended to other various flexible devices and soft robots.

Swimming Anatomy and Lower Back Injuries in Competitive Swimmers: A Narrative Review.

Competitive swimmers are at high risk of overuse musculoskeletal injuries due to their high training volumes. Spine injuries are the second most common musculoskeletal injury in swimmers and are often a result of the combination of improper technique, high loads on the spine in strokes that require hyperextension, and repetitive overuse leading to fatigue of the supporting trunk muscles. The purpose of this review is to summarize the current evidence regarding swimming biomechanics, stroke techniques, and common injuries in the lumbar spine to promote a discussion on the prevention and rehabilitation of lower back injuries in competitive swimmers. From a PUBMED/MEDLINE search, 16 articles were identified for inclusion using the search terms "swimming," "low back" or "lumbar," and "injury" or "injuries." Narrative review. Levels 4 and 5. The trunk muscles are integral to swimming stroke biomechanics. In freestyle and backstroke, the body roll generated by the paraspinal and abdominal muscles is integral to efficient stroke mechanics by allowing synergistic movements of the upper and lower extremities. In butterfly and breaststroke, the undulating wave like motion of the dolphin kick requires dynamic engagement of the core to generate repetitive flexion and extension of the spine and is a common mechanism for hyperextension injuries. The most common lower back injuries in swimming were determined to be lumbar strain, spondylolysis and spondylolisthesis, facet joint pain, and disc disease. Most overuse swimming injuries can be treated conservatively with physical therapy and training adjustments. Managing swimmers with low back pain requires a basic knowledge of swimming technique and a focus on prevention-based care. Since most swimming injuries are secondary to overuse, it is important for providers to understand the mechanisms underlying the swimming injury, including an understanding of the biomechanics involved in swimming and the role of spine involvement in the 4 strokes that assist in stabilization and force generation in the water. Knowledge of the biomechanics involved in swimming and the significant demands placed on the spinal musculoskeletal system will aid the clinician in the diagnosis and management of injuries and assist in the development of a proper rehabilitation program aimed at correction of any abnormal swimming mechanics, treatment of pain, and future injury prevention. B. Recommendation based on limited quality or inconsistent patient-oriented evidence.

AI-aided geometric design of anti-infection catheters.

Bacteria can swim upstream in a narrow tube and pose a clinical threat of urinary tract infection to patients implanted with catheters. Coatings and structured surfaces have been proposed to repel bacteria, but no such approach thoroughly addresses the contamination problem in catheters. Here, on the basis of the physical mechanism of upstream swimming, we propose a novel geometric design, optimized by an artificial intelligence model. Using Escherichia coli, we demonstrate the anti-infection mechanism in microfluidic experiments and evaluate the effectiveness of the design in three-dimensionally printed prototype catheters under clinical flow rates. Our catheter design shows that one to two orders of magnitude improved suppression of bacterial contamination at the upstream end, potentially prolonging the in-dwelling time for catheter use and reducing the overall risk of catheter-associated urinary tract infection.

Target-Following Control of a Biomimetic Autonomous System Based on Predictive Reinforcement Learning.

Biological fish often swim in a schooling manner, the mechanism of which comes from the fact that these schooling movements can improve the fishes' hydrodynamic efficiency. Inspired by this phenomenon, a target-following control framework for a biomimetic autonomous system is proposed in this paper. Firstly, a following motion model is established based on the mechanism of fish schooling swimming, in which the follower robotic fish keeps a certain distance and orientation from the leader robotic fish. Second, by incorporating a predictive concept into reinforcement learning, a predictive deep deterministic policy gradient-following controller is provided with the normalized state space, action space, reward, and prediction design. It can avoid overshoot to a certain extent. A nonlinear model predictive controller is designed and can be selected for the follower robotic fish, together with the predictive reinforcement learning. Finally, extensive simulations are conducted, including the fix point and dynamic target following for single robotic fish, as well as cooperative following with the leader robotic fish. The obtained results indicate the effectiveness of the proposed methods, providing a valuable sight for the cooperative control of underwater robots to explore the ocean.

Bacterial surface swimming states revealed by TIRF microscopy.

Motility near solid surfaces plays a key role in the life cycle of bacteria and is essential for biofilm formation, biofilm dispersal, and virulence. The alignment of the cell body with the surface during surface swimming impacts bacterial surface sensing. Here, we developed a high-throughput method for characterizing the orientation of the cell body relative to the surface using total internal reflection fluorescence (TIRF) microscopy. The angle between the cell body and the surface was determined by maximizing image cross-correlations between the TIRF image of the cell and a reference library. Utilizing this technique, we surprisingly identified six distinct surface swimming states of Pseudomonas aeruginosa according to the body alignment and the flagellar position. Furthermore, we observed that the near-surface swimming speed is greater in the pull state than in the push state, attributed to hydrodynamic effects near the liquid-solid interface. Hydrodynamic force analysis of the swimming states provided rich insights into the mechanics of bacterial surface swimming. Our technique is readily applicable to the study of surface motility across a wide spectrum of bacterial species.

Acoustically Powered Nano- and Microswimmers: From Individual to Collective Behavior.

Micro- and nanoscopic particles that swim autonomously and self-assemble under the influence of chemical fuels and external fields show promise for realizing systems capable of carrying out large-scale, predetermined tasks. Different behaviors can be realized by tuning swimmer interactions at the individual level in a manner analogous to the emergent collective behavior of bacteria and mammalian cells. However, the limited toolbox of weak forces with which to drive these systems has made it difficult to achieve useful collective functions. Here, we review recent research on driving swimming and particle self-organization using acoustic fields, which offers capabilities complementary to those of the other methods used to power microswimmers. With either chemical or acoustic propulsion (or a combination of the two), understanding individual swimming mechanisms and the forces that arise between individual particles is a prerequisite to harnessing their interactions to realize collective phenomena and macroscopic functionality. We discuss here the ingredients necessary to drive the motion of microscopic particles using ultrasound, the theory that describes that behavior, and the gaps in our understanding. We then cover the combination of acoustically powered systems with other cross-compatible driving forces and the use of ultrasound in generating collective behavior. Finally, we highlight the demonstrated applications of acoustically powered microswimmers, and we offer a perspective on the state of the field, open questions, and opportunities. We hope that this review will serve as a guide to students beginning their work in this area and motivate others to consider research in microswimmers and acoustic fields.

Skeletal convergence in thunniform sharks, ichthyosaurs, whales, and tunas, and its possible ecological links through the marine ecosystem evolution

Tunas, lamnid sharks, modern whales, and derived ichthyosaurs converged on the thunniform body plan, with a fusiform body, lunate caudal fin, compressed peduncle, and peduncle joint. This evolutionary convergence has been studied for a long time but little is known about whether all four clades share any skeletal characteristics. Comparisons of vertebral centrum dimensions along the body reveal that the four clades indeed share three skeletal characteristics (e.g., thick vertebral column for its length), while an additional feature is shared by cetaceans, lamnid sharks, and ichthyosaurs and two more by lamnid sharks and ichthyosaurs alone. These vertebral features are all related to the mechanics of thunniform swimming through contributions to posterior concentration of tail-stem oscillation, tail stem stabilization, peduncle joint flexibility, and caudal fin angle fixation. Quantitative identifications of these features in fossil vertebrates would allow an inference of whether they were a thunniform swimmer. Based on measurements in the literature, mosasaurs lacked these features and were probably not thunniform swimmers, whereas a Cretaceous lamniform shark had a mosaic of thunniform and non-thunniform features. The evolution of thunniform swimming appears to be linked with the evolution of prey types and, in part, niche availability through geologic time.