Differences in kinematics and energy cost between front crawl and backstroke below the anaerobic threshold

Stroke-Specific Swimming Critical Speed Testing: Balancing Feasibility and Scientific Rigour

This study aimed to assess the reliability of a two-distance critical speed protocol in the specialist strokes of national-level swimmers and understand the practical feasibility of extending the protocol to increase its validity. Thirty-two national-level swimmers (butterfly n = 7; backstroke n = 8; breaststroke n = 7; front crawl n = 10) swum three 200-m and three 400-m performance trials over a three-week period. Critical speed and supra-critical speed distance capacity were computed from the linear modelling of the distance-time relationship. Swimmers were subsequently asked whether they felt they could or would want to complete an 800-m trial as part of a three-distance critical speed protocol to enhance validity. Both 200-m and 400-m performances (coefficient of variation of < 2%) and derived critical speed (typical error of ≤ 0.04 m·s-1; coefficient of variation of < 4%) were reliable for all strokes, while supra-critical speed distance capacity (typical error from 4 to 9 m; coefficient of variation from 13 to 45%) was not reliable. Response rates to the follow-up questions were 100%. Few butterfly swimmers said they felt they could complete an 800-m performance trial (39%), with more positive responses for breaststroke (71%), backstroke (100%), and front crawl swimmers (100%). Butterfly swimmers were significantly less likely to say they could or would want to complete an 800-m trial than backstroke and front crawl swimmers (p < 0.05). Including a third distance 800-m trial to increase critical speed validity would not be acceptable to butterfly swimmers, would be challenging to breaststroke swimmers, but would be acceptable to front crawl and backstroke swimmers.

Differences in the rotational effect of buoyancy and trunk kinematics between front crawl and backstroke swimming

The purpose of the present study was to investigate differences between front crawl and backstroke swimming in hydrodynamic (produced by swimmers) and buoyant torque around the transverse axis. Ten swimmers performed 50 m front crawl and backstroke at four selected velocities (same velocities for both techniques). All trials were recorded by four underwater and two above-water cameras to collect data for three-dimensional whole-body motion during one stroke cycle (defined as a period between two consecutive wrist entries to the water). The inverse dynamics approach was applied to obtain buoyant and hydrodynamic torque around the transverse axis. The differences between front crawl and backstroke techniques across four levels of velocity were assessed with a two-way repeated-measures ANOVA. There was a main effect of technique on the mean buoyant and hydrodynamic torque, with 30-40 % larger leg-raising buoyant torque and leg sinking hydrodynamic torque in front crawl than in backstroke (p ≤ 0.001). The time-series data revealed that the hydrodynamic leg-sinking torque had its peaks during the first half of the underwater upper-limb motion in front crawl, but that was not observed in backstroke, implying that the strategy of counterbalancing the buoyant torque is different between the techniques.

Golden ratio and self-similarity in swimming: breast-stroke and the back-stroke

INTRODUCTION

Dynamics-on-graph concepts and generalized finite-length Fibonacci sequences have been used to characterize, from a temporal point of view, both human walking & running at a comfortable speed and front-crawl & butterfly swimming strokes at a middle/long distance pace. Such sequences, in which the golden ratio plays a crucial role to describe self-similar patterns, have been found to be subtly experimentally exhibited by healthy (but not pathological) walking subjects and elite swimmers, in terms of durations of gait/stroke-subphases with a clear physical meaning. Corresponding quantitative indices have been able to unveil the resulting hidden time-harmonic and self-similar structures.

RESULTS

In this study, we meaningfully extend such latest findings to the remaining two swimming strokes, namely, the breast-stroke and the back-stroke: breast-stroke, just like butterfly swimming, is highly technical and involves the complex coordination of the arm and leg actions, while back-stroke is definitely similar to front-crawl swimming. An experimental validation with reference to international-level swimmers is included.

Froude Efficiency and Velocity Fluctuation in Forearm-Amputee Front Crawl: Implications for Para Swimming Classification

PURPOSE

The impact of physical impairment on Froude efficiency and intracyclic velocity fluctuation in Para swimmers is not well documented. Identification of differences in these variables between disabled and nondisabled swimmers could help develop a more objective system for assigning Para swimmers to classes for competition. This study quantifies Froude efficiency and intracyclic velocity fluctuation in unilateral forearm-amputee front crawl swimmers and evaluates associations between these variables and performance.

METHODS

Ten unilateral forearm-amputee swimmers completed front crawl trials at 50- and 400-m pace; three-dimensional video analysis provided mass center, and wrist and stump velocities. Intracyclic velocity fluctuation was calculated as follows: 1) maximum-minimum mass center velocity, expressed as percent of mean velocity, and 2) coefficient of variation in mass center velocity. Froude efficiency was the ratio between mean swimming velocity and wrist plus stump velocity during each segment's respective 1) underwater phase and 2) propulsive underwater phase.

RESULTS

Forearm amputees' intracyclic velocity fluctuation (400 m: 22% ± 7%, 50 m: 18% ± 5%) was similar to published values for nondisabled swimmers, whereas Froude efficiencies were lower. Froude efficiency was higher at 400-m (0.37 ± 0.04) than 50-m pace (0.35 ± 0.05; P < 0.05) and higher for the unaffected limb (400 m: 0.52 ± 0.03, 50 m: 0.54 ± 0.04) than the residual limb (400 m: 0.38 ± 0.03, 50 m 0.38 ± 0.02; P < 0.05). Neither intracyclic velocity fluctuation nor Froude efficiency was associated with swimming performance.

CONCLUSIONS

Froude efficiency may be a valuable measure of activity limitation in swimmers with an upper limb deficiency and a useful metric for comparing swimmers with different types and severity of physical impairment.

Is the use of the coefficient of variation a valid way to assess the swimming intra-cycle velocity fluctuation?

OBJECTIVES

Swimming intra-cycle velocity fluctuation has often been assessed using the coefficient of variation, which requires a mathematical assumption of a positive linear relationship between the velocity mean and standard deviation. As this assumption has never been tested, the current study aimed to investigate the within-participant relationship between the mean and standard deviation of the intra-cycle velocity.

DESIGN

Cross-sectional study.

METHODS

The intra-trial mean and standard deviation of one stroke cycle centre of mass velocity (vCM_mean and vCM_SD, respectively) were obtained from 80 front crawl trials (10 participants × eight swimming speeds) using whole-body three-dimensional motion analysis. The linear mixed-effect model and intra-class correlation analysis were used to test the linear relationship between vCM_mean and vCM_SD (n = 80) and the absolute agreement between vCM_mean and vCM_SD relative to those during the fastest trial (n = 70).

RESULTS

Neither the linear regression model (95 % confidence interval range of the fixed effect of vCM_mean: -0.003-0.031) nor the intra-class correlation coefficient (ICC = 0.07; p = 0.26) verified linear relationships between vCM_mean and vCM_SD, which violated the background assumption of coefficient of variation calculation.

CONCLUSIONS

When investigating the intra-cycle velocity fluctuation, the coefficient of variation should not be used alone. Researchers and practitioners should always interpret/report the obtained results together with the mean and standard deviation to avoid misleading conclusions and feedback because the coefficient of variation obtained from one cycle velocity data is likely biased by mean velocity.

Intracycle Velocity Variation in Swimming: A Systematic Scoping Review

Intracycle velocity variation is a swimming relevant research topic, focusing on understanding the interaction between hydrodynamic propulsive and drag forces. We have performed a systematic scoping review to map the main concepts, sources and types of evidence accomplished. Searches were conducted in the PubMed, Scopus and Web of Science databases, as well as the Biomechanics and Medicine in Swimming Symposia Proceedings Book, with manual searches, snowballing citation tracking, and external experts consultation. The eligibility criteria included competitive swimmers' intracycle velocity variation assessment of any sex, distance, pace, swimming technique and protocol. Studies' characteristics were summarized and expressed in an evidence gap map, and the risk of bias was judged using RoBANS. A total of 76 studies, corresponding to 68 trials involving 1440 swimmers (55.2 and 34.1% males and females), were included, with only 20 (29.4%) presenting an overall low risk of bias. The front crawl was the most studied swimming technique and intracycle velocity variation was assessed and quantified in several ways, leading to extremely divergent results. Researchers related intracycle velocity variation to coordination, energy cost, fatigue, technical proficiency, velocity, swimming techniques variants and force. Future studies should focus on studying backstroke, breaststroke and butterfly at high intensities, in young, youth and world-class swimmers, as well as in IVV quantification.

Young Swimmers' Classification Based on Performance and Biomechanical Determinants: Determining Similarities Through Cluster Analysis

The aim of this study was to classify and identify young swimmers' performance, and biomechanical determinant factors, and understand if both sexes can be clustered together. Thirty-eight swimmers of national level (22 boys: 15.92 ± 0.75 years and 16 girls: 14.99 ± 1.06 years) were assessed. Performance (swim speed at front crawl stroke) and a set of kinematic, efficiency, kinetic, and hydrodynamic variables were measured. Variables related to kinetics and efficiency (p < .001) were the ones that better discriminated the clusters. All three clusters included girls. Based on the interaction of these determinant factors, there are girls who can train together with boys. These findings indicate that not understanding the importance of the interplay between such determinants may lead to performance suppression in girls.

Three-dimensional front crawl arm-stroke efficiency and hand displacement in male and female swimmers

This study aimed (i) to verify if underwater horizontal, vertical and medio-lateral hand displacements (HD), in pull and push phases of the front crawl stroke, can be associated with arm-stroke efficiency (ƞp) and (ii) to compare np and selected kinematic variables between male and female swimmers. Ten male and 10 female swimmers performed an all-out front crawl 25-m test. Data were obtained with six synchronised video cameras (60 Hz) and analysed with a three-dimensional method. Results for males and females were respectively, as follows: (i) horizontal HD: 0.55 ± 0.06 m and 0.61 ± 0.09 m (p = 0.062; d = 0.78); vertical HD: 0.68 ± 0.06 m and 0.58 ± 0.07 m (p < 0.001; d = 1.53); and medio-lateral HD: 0.22 ± 0.07 m and 0.16 ± 0.03 m (p = 0.012; d = 1.11); (ii) ƞp: 0.33 ± 0.02 and 0.32 ± 0.03 (p = 0.48; d = 0.39); (iii) vCOM: 1.77 ± 0.06 m∙s-1 and 1.55 ± 0.10 m∙s-1 (p < 0.001; d = 2.42). Multiple linear regression (p = 0.019) indicated that horizontal and medio-lateral HD were able to predict np. The lower the horizontal hand displacement, the higher the ƞp.

Are Young Swimmers Short and Middle Distances Energy Cost Sex-Specific?

This study assessed the energy cost in swimming (C) during short and middle distances to analyze the sex-specific responses of C during supramaximal velocity and whether body composition account to the expected differences. Twenty-six swimmers (13 men and 13 women: 16.7 ± 1.9 vs. 15.5 ± 2.8 years old and 70.8 ± 10.6 vs. 55.9 ± 7.0 kg of weight) performed maximal front crawl swimming trials in 50, 100, and 200 m. The oxygen uptake (V˙O₂) was analyzed along with the tests (and post-exercise) through a portable gas analyser connected to a respiratory snorkel. Blood samples were collected before and after exercise (at the 1st, 3rd, 5th, and 7th min) to determine blood lactate concentration [La^–]. The lean mass of the trunk (LM_Trunk), upper limb (LM_UL), and lower limb (LM_LL) was assessed using dual X-ray energy absorptiometry. Anaerobic energy demand was calculated from the phosphagen and glycolytic components, with the first corresponding to the fast component of the V˙O₂ bi-exponential recovery phase and the second from the 2.72 ml × kg^–1 equivalent for each 1.0 mmol × L^–1 [La^–] variation above the baseline value. The aerobic demand was obtained from the integral value of the V˙O₂ vs. swimming time curve. The C was estimated by the rate between total energy releasing (in Joules) and swimming velocity. The sex effect on C for each swimming trial was verified by the two-way ANOVA (Bonferroni post hoc test) and the relationships between LM_Trunk, LM_UL, and LM_LL to C were tested by Pearson coefficient. The C was higher for men than women in 50 (1.8 ± 0.3 vs. 1.3 ± 0.3 kJ × m^–1), 100 (1.4 ± 0.1 vs. 1.0 ± 0.2 kJ × m^–1), and 200 m (1.0 ± 0.2 vs. 0.8 ± 0.1 kJ × m^–1) with p < 0.01 for all comparisons. In addition, C differed between distances for each sex (p < 0.01). The regional LM_Trunk (26.5 ± 3.6 vs. 20.1 ± 2.6 kg), LM_UL (6.8 ± 1.0 vs. 4.3 ± 0.8 kg), and LM_LL (20.4 ± 2.6 vs. 13.6 ± 2.5 kg) for men vs. women were significantly correlated to C in 50 (R²_adj = 0.73), 100 (R²_adj = 0.61), and 200 m (R²_adj = 0.60, p < 0.01). Therefore, the increase in C with distance is higher for men than women and is determined by the lean mass in trunk and upper and lower limbs independent of the differences in body composition between sexes.

Front Crawl Is More Efficient and Has Smaller Active Drag Than Backstroke Swimming: Kinematic and Kinetic Comparison Between the Two Techniques at the Same Swimming Speeds

The purpose of this study was to investigate differences in Froude efficiency (η _F ) and active drag (D _A ) between front crawl and backstroke at the same speed. η _F was investigated by the three-dimensional (3D) motion analysis using 10 male swimmers. The swimmers performed 50 m swims at four swimming speeds in each technique, and their whole body motion during one upper-limb cycle was quantified by a 3D direct linear transformation algorithm with manually digitized video footage. Stroke length (SL), stroke frequency (SF), the index of coordination (IdC), η _F , and the underwater body volume (UWV _body ) were obtained. D _A was assessed by the measuring residual thrust method (MRT method) using a different group of swimmers (six males) due to a sufficient experience and familiarization required for the method. A two-way repeated-measures ANOVA (trials and techniques as the factors) and a paired t-test were used for the outcomes from the 3D motion analysis and the MRT method, respectively. Swimmers had 8.3% longer SL, 5.4% lower SF, 14.3% smaller IdC, and 30.8% higher η _F in front crawl than backstroke in the 3D motion analysis (all p < 0.01), which suggest that front crawl is more efficient than backstroke. Backstroke had 25% larger D _A at 1.2 m⋅s^-1 than front crawl (p < 0.01) in the MRT trial. A 4% difference in UWV _body (p < 0.001) between the two techniques in the 3D motion analysis also indirectly showed that the pressure drag and friction drag were probably larger in backstroke than in front crawl. In conclusion, front crawl is more efficient and has a smaller D _A than backstroke at the same swimming speed.

In-Water and On-Land Swimmers' Symmetry and Force Production

Although performance and biomechanical evaluations are becoming more swimming-specific, dryland testing permits monitoring of a larger number of performance-related variables. However, as the degree of comparability of measurements conducted in-water and on land conditions is unclear, we aimed to assess the differences between force production in these two different conditions. Twelve elite swimmers performed a 30 s tethered swimming test and four isokinetic tests (shoulder and knee extension at 90 and 300°/s) to assess peak force, peak and average torque, and power symmetry index. We observed contralateral symmetry in all the tests performed, e.g., for 30 s tethered swimming and peak torque shoulder extension at 90°/s: 178 ± 50 vs. 183 ± 56 N (p = 0.38) and 95 ± 37 vs. 94 ± 35 N × m (p = 0.52). Moderate to very large direct relationships were evident between dryland testing and swimming force production (r = 0.62 to 0.96; p < 0.05). Swimmers maintained similar symmetry index values independently of the testing conditions (r = −0.06 to −0.41 and 0.04 to 0.44; p = 0.18–0.88). Asymmetries in water seems to be more related to technical constraints than muscular imbalances, but swimmers that displayed higher propulsive forces were the ones with greater force values on land. Thus, tethered swimming and isokinetic evaluations are useful for assessing muscular imbalances regarding propulsive force production and technical asymmetries.

The energy cost of swimming and its determinants

The 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.

Upper body kinematic differences between maximum front crawl and backstroke swimming

The purpose of this study was to investigate why front crawl is faster than backstroke from a kinematic perspective. Three-dimensional kinematics were obtained from one upper-limb cycle of ten male competitive swimmers performing 50 m front crawl and backstroke trials at maximum speed. Swimmers achieved faster centre of mass velocity in front crawl than backstroke (1.70 ± 0.04 vs 1.54 ± 0.06 m·s^-1; p < 0.01) with no difference in stroke length (2.00 ± 0.25 vs 2.07 ± 0.17 m·cycle^-1), while stroke frequency in front crawl was higher than that in backstroke (51.67 ± 6.38 vs 44.81 ± 4.68 cycles·min^-1; p < 0.01). Maximum shoulder roll angle in front crawl was larger than that in backstroke (52.88 ± 4.89 vs 49.73 ± 5.73°; p < 0.05), while swimmers had smaller maximum hip roll in front crawl than backstroke (33.79 ± 6.07 vs 39.83 ± 7.25°; p < 0.05). Absolute duration of the release phase (from the last backward movement to the exit from the water of the wrist) and relative duration of the recovery phase were shorter in front crawl than backstroke (0.07 ± 0.03 vs 0.26 ± 0.08 s; p < 0.01, and 28.69 ± 2.50 vs 33.21 ± 1.43%; p < 0.01, respectively). In conclusion, front crawl is faster than backstroke because of its higher stroke frequency due to the shorter absolute release phase and relative recovery phase durations.

Do swimmers conform to criterion speed during pace-controlled swimming in a 25-m pool using a visual light pacer?

The purpose of this study was to investigate whether swimmers follow the instructed speed (v_target) accurately with the aid of a commercial visual light pacer during front crawl and backstroke swimming in a 25 m pool. Ten male swimmers performed 50 m front crawl and backstroke at different speeds (controlled by a visual light pacer) in a 25 m pool. The mean speed during the 50 m swimming (v_S) was quantified from the time measured by a stopwatch. The mean speed of the centre of mass during a stroke cycle in the middle of the pool (v_COM) was calculated from three-dimensional coordinates obtained from Direct Linear Transformation of two-dimensional digitised coordinates of 19 segment endpoints for each of six cameras. Swimmers achieved accurate v_S in front crawl and backstroke (ICC = 0.972 and 0.978, respectively). However, v_COM for the single mid-pool sample had lower correlations with v_target (ICC = 0.781 and 0.681, respectively). In backstroke, v_COM was slower by 4.1-5.1% than v_target. However, this was not the case in front crawl (1.0-2.7%). With the use of a visual light pacer, swimmers can achieve accurate mean speed overall but are less able to achieve the target speed stroke by stroke.