The effect of body position and mass centre velocity at toe off on the start performance of elite swimmers and how this differs between gender

ABSTRACT

The start in swimming is a crucial phase of a race, where improvements in performance can be made. Twenty-four elite swimmers race pace starts were recorded from five above and below water 50 Hz video cameras. Body position at toe off was calculated from the recordings and consisted of the two-dimensional mass centre position at toe off, and the arm, trunk, front leg and rear leg angles.Horizontal, vertical and resultant velocity of the mass centre at toe off, time to 5 m, 10 m and 15 m were also determined. Whilst time to 5 m (starting performance) differed by 0.17 s between genders, body position at toe off showed no significant differences. The difference in start performance was mainly due to a difference in horizontal velocity at toe off. The relationship between arm angle and start performance warrants further investigation as there was a range of techniques adopted but no clear link to performance. The trunk angle at toe off was correlated to starting performance for both males and females. This study demonstrates that the body position at toe off is no different between genders but is a critical determinant of starting performance for both males and females.

KEYWORDS:

• Biomechanics
• kinematics
• swimming
• start
• body position

Introduction

Medal positions at international swimming competitions have become increasingly competitive as the top positions are split by hundredths of a second. For example, during the Rio 2016 Olympics three males tied for a silver medal in the 100 m Butterfly and only 0.01 s separated the silver medal position and fourth place (International Olympic Committee, 2016). It is therefore extremely important to consider all aspects of the race that contribute to performance. Whilst free swim speed is important, the technical skills of starts and turns can also affect the race outcome (Morais et al., 2019). The start accounts for 0.8–26.1% of the total race time, depending on race distance (Cossor and Mason, 2001), highlighting its importance, particularly in the shorter distance events.

Overall start performance is commonly measured as the time taken to reach 15 m and can be divided into three primary phases: the block, flight and underwater phases (Slawson et al., 2013). Another phase that has not been considered by Slawson et al. (2013) is the breakout into free swimming, which could also be considered an important factor in time to 15 m. The block and flight phase will typically contribute to 17% of total start time to 15 m for males, and 15.1% for females (Tor et al., 2014). The underwater phase contributes around 84% to the total time taken to reach 15 m (Slawson et al., 2013) and therefore is a questionable metric to use when analysing the block and flight-phase performance of a swimmer, as it greatly depends on the swimmer’s underwater capabilities. As such, time to 5 m is deemed more suitable as around 83% of the variances in it results from the block time and flight distance (Peterson Silveira et al., 2018).

Many variables affect a swimmer's start performance such as horizontal take off velocity, flight time and entry distance. The entry distance can be defined as the horizontal distance between the starting wall and the swimmers vertex at water entry (Tor et al., 2015). A greater horizontal take-off velocity improves both time to 5 m and 15 m (Barlow et al., 2014; Tor et al., 2015) and male swimmers typically have a faster horizontal velocity at toe off than female swimmers (Tor et al., 2015)(4.85 ± 0.17 m/s and 4.33 ± 0.19 m/s, respectively) (Honda et al., 2012; Tor et al., 2014). It is likely that a faster take off velocity in males is correlated to males producing greater peak forces on the block, per body weight, than females (Slawson et al., 2013). However, no correlation co-efficient has been stated in the literature. Flight time is correlated with time to 5 m (Peterson Silveira et al., 2018) and is typically between 0.22 and 0.35 s (Peterson Silveira et al., 2018; Tor et al., 2014).The difference in flight time is likely due to slightly different definitions of the entry phase (Peterson Silveira et al., 2018; Tor et al., 2014). Conversely, Ruschel et al. (2007) reported that flight time did correlate with time to 15 m but that entry distance was one of the variables that determined start performance (r = −0.482). Given air resistance is less than water resistance it is reasonable to maximise the entry distance and typical distances have been reported between 2.7 and 3.3 m (Peterson Silveira et al. 2018).

To achieve an optimal take off velocity, entry distance and flight time, the swimmer will need to be in an ideal body position whilst still producing enough forward force.Take-off angle is defined as the angle between the horizontal and the line connecting a swimmers mass centre to a reference point on their foot (Beretić et al., 2012; Vantorre et al., 2010). When stood in the anatomical position there are differences in mass centre location between genders (Summers et al., 2010), it can therefore be expected that mass centre position at toe off will also show differences. The angle between the hip, toe and the horizontal has also been used by Seifert et al. (2010) which will produce similar results; however, this could cause confusion if comparing values between researchers. Kibele et al. (2015) defined the take off angle as the inclination of the mass centre trajectory during the first 3 frames of the flight phase. This definition of take off angle will give very different results as it’s measuring the initial trajectory of the flight, not the trajectory of the body from the block as in the other definitions. None of these definitions identify the overall position of the body at take off. The mass centre location will not identify whether the arms are horizontal in front of their head, at their hips or even pointing vertically down at the water. Seifert et al. (2010) calculated extra body angles alongside the take off angle; angle between the ankle, hip and shoulder and the angle between the hip, shoulder and wrist. Whilst this gives an insight into the body orientation of the swimmer, it does not provide information on the position and orientation of the swimmer in relation to the global co-ordinate system.

The purpose of this study was to understand how a swimmer’s body segment orientations and mass centre velocity at toe off, entry distance and flight time affect start performance. Furthermore, the differences in start performance and body position at toe off between males and females will be considered. It is hypothesised that starting performance and take off velocity of the mass centre will differ between the genders, while body position at the toe-off will not; and orientation of the trunk and arms will be correlated with start performance.

Materials and methods

Twenty-four elite sprint swimmers (14 females (171.42 ± 6.81 cm, 65.72 ± 5.71 kg), 10 males (184.68 ± 5.65 cm, 81.99 ± 6.09 kg)) from the Loughborough University Swimming programme, with their highest ranked event ranging from 698 to 945 FINA points (mean = 790 FINA points), aged 22 ± 4 years took part in this study. The swimmers selected specialised in events of 200 m or shorter, in a mixture of Butterfly, Breaststroke, Freestyle or Individual Medley. The study was approved by the Loughborough University Ethical Advisory Committee and, in accordance with protocol, subjects provided informed consent to participate and were made aware of their right to withdraw.

Swimmers, wearing competition racing suits, were asked to complete three maximum effort swimming starts through to 20 m with each start recorded using five Basler acA1920-50gc cameras recording at 50 Hz (above water side and rear views, and three underwater views at 5 m, 10 m and 15 m, all perpendicular to the plane of motion). Twenty-nine anthropometric measures for each swimmer were taken using a simplified version of Yeadon’s inertia model with a reduced number of measurements (Yeadon, 1990) to enable participant-specific mass centre positions to be determined.

Fifteen key landmarks on the body (vertex, shoulders (2), elbows (2), wrists (2), hips (2), knees (2), ankles (2) and toes (2)) were manually digitised for each swimmers fastest dive to 5 m (side camera recording) using the Quintic Biomechanics V31 software. Three frames for each trial were digitised: toe off, the final frame where the whole body was visible prior to water entry and the last frame where the head was visible prior to water entry. A Direct Linear Transformation (DLT) procedure was used to reconstruct digitised points as projections onto a vertical plane through the body when the swimmer was at toe off (Sayyah et al., 2018).

For each digitised trial the following 14 kinematic variables were calculated: Block time (first movement to toe off), flight time (toe off to head entry), time to 5, 10 and 15 m (first movement to when their swimming goggles reached 5, 10 and 15 m, respectively), entry distance (from pool edge to vertex at water entry) and velocity of the mass centre at toe off (horizontal, vertical and resultant). At toe off, the angles of the swimmers body segments relative to the horizontal were determined; arm (shoulder to wrist), trunk (shoulder to hip), front leg (hip to toe), rear leg (hip to toe) and mass centre y, z position (y, horizontal distance from starting edge of pool; z, vertical distance from water level) were calculated.

All 14 variables were tested to determine if the data were parametric or non-parametric. An independent samples t-test was used to compare between genders when the data were parametric. If data were non-parametric for one or both genders, then a Mann Whitney–U test was used to compare. When analysing the relationships between variables and start performance a Pearson’s correlation test was run when both metrics were parametric, and Spearman’s rank correlation was used when one or both metrics were non-parametric. All correlations were analysed with a significance level of 0.05. Cohen’s d effect sizes were calculated where d = 0.2 is a small effect size, d = 0.5 is medium and d = 0.8 is large.

Results

The performance metrics (time to 5 m, 10 m, 15 m, flight time and entry distance) were all different between the genders (Table 1) while mass centre vertical height at take off was the only different body position metric. The horizontal take off velocity and resultant take-off velocity were also different between genders, but not the vertical take off velocity (Table 1).

Table 1. Summary of male and females results and the difference between genders.

variable	Male	Female	Difference in mean	ρ	Effect size
Time to 5 m (s)	1.27 ± 0.05	1.44 ± 0.07	−0.17	<0.0005*^†	2.79
Time to 10 m (s)	3.51 ± 0.24	3.96 ± 0.21	−0.45	< 0.0005*	2.00
Time to 15 m (s)	6.41 ± 0.67	6.96 ± 0.55	−0.55	0.039*	0.90
Flight time (s)	0.34 ± 0.05	0.30 ± 0.06	0.04	0.050	0.72
Entry distance (m)	3.17 ± 0.22	2.81 ± 0.22	0.36	0.001*	1.64
Arm angle^a (°)	−86.3 ± 105.2	−60.8 ± 63.4	−25.5	0.143^†	0.29
Trunk angle^a(°)	−2.7 ± 9.3	−4.5 ± 8.5	1.8	0.631	0.20
Back leg angle^a (°)	10.8 ± 10.3	10.0 ± 5.5	0.8	0.819	0.10
Front leg angle^a (°)	−26.8 ± 4.8	−24.5 ± 5.2	−2.3	0.287	0.46
Mass centre y position^a (m)	1.03 ± 0.08	1.03 ± 0.06	0.00	0.849	0.00
Mass centre z position^a (m)	1.28 ± 0.06	1.17 ± 0.10	0.11	0.006*	1.33
Horizontal velocity^a (m/s)	4.58 ± 0.15	4.19 ± 0.18	0.39	< 0.0005*	2.35
Vertical velocity^a (m/s)	−0.61 ± 0.37	−0.91 ± 0.43	−0.30	0.086	0.75
Resultant velocity^a (m/s)	4.63 ± 0.15	4.31 ± 0.20	0.32	0.005*	1.81

*Significant at 0.05 level, ^† Mann Whitney-U test due to non-normality of data

^aat toe off.

Time to 5 m was negatively correlated with entry distance for all swimmers (Table 2) with the coefficient of determination suggesting that a change in entry distance could explain ~49% of the variation in time to 5 m for males (r² = 0.493) and ~30% for females (r² = 0.305).

Table 2. Correlation analysis of variables to the performance outcome, time to 5 m.

determining factor	Males		Females
	r/rho^†	ρ	r/rho^†	ρ
Entry distance	−0.702*	0.024	−0.552*^†	0.041
Flight time	−0.535	0.111	−0.260^†	0.370
Horizontal velocity^a	−0.754*	0.012	−0.663*^†	0.010
Vertical velocity^a	−0.510	0.132	−0.454^†	0.103
Resultant velocity^a	−0.652*	0.041	−0.347^†	0.224
Trunk angle^a	−0.694*	0.026	−0.420^†	0.134
Arm angle^a	0.571^†	0.084	0.051^†	0.862
Back leg angle^a	0.207	0.565	−0.378^†	0.182
Front leg angle^a	−0.099	0.785	0.078^†	0.791
Mass centre z position^a	−0.530	0.115	−0.227^†	0.435
Mass centre y position^a	0.174	0.631	−0.076^†	0.797

*Significant at 0.05 level, ^† Spearman’s rank test due to non-normality of data.

^aat toe off.

Between males and females there were no differences in arm angle, trunk angle, front leg angle or back leg angle, demonstrating that a swimmer’s toe off position does not differ with gender. At toe off, trunk angle was negatively correlated with time to 5 m for males (r = −0.694, p = 0.026); however, it was not correlated for females (rho = −0.420, p = 0.134). Despite seeing a range of arm positions at toe off across the 24 subjects (Figure 1) arm angle for both males and females did not correlate with time to 5 m (Table 1) or entry distance (rho = −1.76, p = 0.627, rho = 0.240, p = 0.409). Arm positions similar to Figure 1(a) were seen from zero males and six females, Figure 1(b) four males and five females and Figure 1(c) six males and three females.

Figure 1. Typical arm positions at toe off: (a) arms stretched out in front (b) arms pointing down towards the water (c) arms back by the hips. x denotes mass centre position, • denotes head position.

Discussion and implications

This study investigated the effect a swimmer’s body position and mass centre velocity at toe off had on start performance and the differences between male and female swimmers. While all metrics were not correlated to starting performance for both genders some important relationships were established.

Inter-gender comparisons

Males starting performance to 5 m was 0.17 s faster than females. Since their body positions at toe off were not different, the faster time for males was best explained by their greater horizontal take off velocity (assumed to be due to increased strength characteristics that allow them to produce greater force, and thus impulse, off the starting block).

There were no differences between genders for the vertical take off velocity, corresponding to the findings of Tor et al. (2014). However, a difference was found for the mass centre height above water at toe off; for males the average mass centre height was 1.28 ± 0.06 m while the females was 1.17 ± 0.10 m. This could simply be that the stature and limb lengths for male’s are greater than that of females, along with a naturally higher mass centre position (Summers et al., 2010). This is supported by the correlation for both genders between front leg angle and mass centre height at toe off; females (r = -0.853, p < 0.0005), males (r = -0.635, p = 0.048). There were no differences between the male’s and female’s front leg angle at toe off, further suggesting that mass centre height differences were due to gender stature variance. The difference in mass centre height was therefore explained and should not be used as a performance determinant.

Despite the mass centre height at toe off not correlating with time to 5 m, it did have a correlation to flight time (males: r = 0.909, p < 0.0005, females: r = 0.971, p <0.0005), and entry distance (males: r = 0.927, p < 0.0005, females: r = 0.830, p < 0.0005) which does correlate with time to 5 m. This would suggest that greater mass centre height at toe off could improve start performance through increased flight time and entry distance (Peterson Silveira et al., 2018; Vantorre et al., 2014). However, there must be a limit to this correlation since the overall goal of a start is for horizontal displacement, not vertical. While the mass centre height at toe off is correlated with front leg angle, and thus the swimmers height, the use of their arms could also alter the mass center height. Without needing to change their front leg angle, those swimmers that have their arms pointing down towards the water at toe off could increase their mass centre height by altering their arm angle, which is coachable.

While there were differences between the genders for the performance metrics (time to 5 m, flight time, entry distance), there were no differences between body positions at toe off. At toe off, arm angle, trunk angle, front leg angle, back leg angle and mass centre horizontal position showed no differences between the genders. As previously stated, there was a difference between mass centre height at toe off. This would suggest that the difference in start performance between males and females is due to variables other than their body position at toe off. Horizontal take off velocity, which will be related to strength and power characteristics, is likely to be a key factor.

Intra-gender correlations

When investigating the relationships, within gender, to start performance there were some relationships that correlated for males, but not females and vice versa. It is unlikely that certain variables would enhance start performance for only a single gender. Since recruiting elite level athletes has its challenges, a sample size of 24 elite swimmers was pleasing. However, once separated into males (10) and females (14) the sample sizes and variance in data meant that the conclusions that could be drawn were limited by the sample size.

An example of this would be that resultant take off velocity showed a correlation to time to 5 m for males (r = -0.652, p = 0.041), but not for females (r = -0.461, p = 0.097). This is interesting since there was a greater sample size of females than males. It was expected that resultant take off velocity correlated to time to 5 m across both genders, not just one. This was expected since resultant take off velocity has a strong correlation to horizontal take off velocity where a relationship with starting performance has been established previously by other authors (Vantorre et al., 2010; Vantorre, Chollet and Seifert, 2014; Dragunas, 2015; Tor, Pease and Ball, 2015). However, analysis of body position at toe off is limited and therefore understanding how body position affects start performance will provide greater insight into the determining factors.

The back leg angle showed no correlations for females, but correlated with flight time (r = −0.655, p = 0.040) and vertical take off velocity for males (r = -0.773, p = 0.009). However, for females only the front leg angle correlated with flight time (r = −0.847, p = <0.0005), entry distance (r = −0.617, p = 0.019), mass centre height at toe off (r = -0.853, p < 0.0005) and vertical take off velocity (r = -0.796, p = 0.001). For males, the trunk angle correlated to time to 5 m, but not entry distance (r = 0.479, p = 0.162) or flight time (r = 0.325, p = 0.359). While for females, the trunk angle did not correlate with time to 5 m or entry distance (r = 0.508, p = 0.063), but it did correlate to flight time (r = 0.630, p = 0.016). Since a swimmer’s aim is to complete a horizontal distance the quickest, it is more important to maximise horizontal travel rather than vertical travel. Swimmers want to maximise horizontal force on the starting block as that will translate directly to horizontal velocity and movement. Consequently, at toe off swimmers and coaches typically aim for a horizontal position (0ᵒ trunk angle) to allow the flight time and entry distance to be maximised, improving time to 5 m. It was therefore unexpected that the male’s flight time and entry distance did not correlate to trunk angle.

However, for females, trunk angle correlated to flight time (r = 0.630, p = 0.016) but not to entry distance or time to 5 m. However, a single female subject stands out as a possible outlier in the correlation between trunk angle and time to 5 m. When this subject was removed a correlation was found for females (rho = −0.605, P = 0.005). This shows that for males, and possibly females, time to 5 m is slowest when there is a negative trunk angle and gets faster as the trunk angle increases towards 0° and above. In theory, a quadratic correlation should be seen since a trunk angle that is too high will have a negative effect on entry distance, just as a trunk angle that is too low does, thus an optimum needs to be found. However, for this study only a linear correlation was found. This is likely due to the sample size and the small variation in the trunk angle data.

Since the trunk angle correlated to performance it was unexpected that the arm angle did not, for either gender. It is clear from the data that there were two distinct techniques used across the 24 swimmers. Some swimmers bring their hands up towards their hips before bringing their arms forwards, as shown in Figure 1(c) (6 males, 4 females), while others, at varying speeds, move their arms forwards from the block towards a streamline position (4 males, 10 females) as shown in Figure 1(a,b). It was expected that at toe off, if the arms were stretched out in front of the body then the entry distance would be greater than if the arms were pointing down towards the water; however, no previous research has been conducted in this area. It is these two different techniques that cause such a large variation in the arm angle at toe off and possibly why no relationship was found between arm angle and start performance. Arm angle was also one of the few metrics that was non-normally distributed. If each gender was separated into groups based on which technique they used and the correlations retested within the groups, the results may be different. However, this study did not have a large enough sample size for grouping further within each gender. Arm angle at toe off needs further investigation, with a larger sample size, or with a single group of swimmers that can execute different techniques through an intervention study.

Future studies should collect data on factors not examined in this study, such as strength characteristics, block phase and setup position, which will have an effect on start performance. Whilst this study did not show correlations for all aspects of the swimmers body position to start performance, the trunk angle significance demonstrates the importance of body position and confirms the need for further investigation of other body position factors with a larger sample size. A priori test showed for a medium effect size (r = 0.5), 80% power and alpha value of 0.05, a sample size of 29 would be required. These swimmers would need to all be of the same gender for comparisons, and to look at arm orientation correlations they would all need to be using the same arm technique. It is however understood that achieving a sample size this large in elite sport is challenging. The swimmers set up position and block phase could also determine how they reach their body position at toe off and would be interesting to explore.

Conclusion

In conclusion, despite the differences between start performance in males and females, this study suggests that the difference is unlikely to be caused by differences in body position at toe off, but by the differences in horizontal take off velocity orother factors not examined in the study. The trunk angle correlated with start performance for both males and females. Therefore, it is advised that the trunk angle of swimmers in this study should be adjusted where required. Since technique changes can be difficult to conduct into established movement patterns the education of younger swimmers in their developmental phase is crucial. The swimmers arm angle did not correlate to starting performance. However, it is suspected that a lack of correlation between arm angle and performance may not represent start performance determinants and needs further investigation with a larger sample size. The start remains an area of the race where gains can be made and body position at toe off has the potential to provide this.

Acknowledgements

The authors would like to acknowledge the support of the Loughborough University Swimming programme in allowing their athletes to participate in this study. The performance analysis system and programmes used were available through British Swimming, the English Institute of Sport and Sheffield Hallam University.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the English Institute of Sport [grant number J16236]; British Swimming [grant number J16236].