The main goal of human competitive swimming is to diminish drag and increase propulsion to achieve a higher swim velocity and, therefore, travel a given distance in the shortest possible time. In this context, an in-depth analysis of key variables is performed regularly to advise swimmers about ways to progress (Barbosa et al., 2021). In the last couple of decades, there has been a boost in technological advances to get a more friendly and ecological assessment in the water. A large set of devices was developed in a diversity of areas, which allowed researchers to carry out a proper assessment of the various factors that influence swimming performance.
One of the recent areas of scientific research includes swimming kinetics (Santos et al., 2021). The ability to produce propulsive force in the water has been a topic of great interest. A differential pressure sensors system (Aquanex System, Swimming Technology Research) was designed to measure swimmers’ propulsive force. This is a user-friendly set-up that allows the swimmer’s displacement throughout the water in a very similar condition to “free swimming” and delivers real-time feedback (Santos et al., 2021). This commercially available hydrodynamic system measures water pressure differences between the palmar/plantar and dorsal surface (Barbosa et al., 2020) of each body limb (i.e., hands and feet), and hence provides force output (N, newton) as the product of pressure and the area.
Previous studies used the Aquanex System to understand the behaviour of propulsive forces generated by the upper and lower limbs during front-crawl (e.g., Barbosa et al., 2020; Morais et al., 2020; Ng et al., 2019) and the butterfly stroke (e.g., Morais et al., 2021; Pereira et al., 2015). Some of them also reported the assessment of kinematic variables while propulsive force was retrieved (e.g., Morais et al., 2021). Although considered accurate, carrying these tiny pressure sensors can impose some mechanical constraints leading to an underestimation or overestimation of kinematic and efficiency data. Since the change of the hand area surface can occur from additional body salience promoted by the sensors, resistive forces, such as pressure drag, can increase and affect arm stroke motion.
The constraints imposed by several devices during underwater testing have already been a topic of interest. Slight changes in the biomechanical pattern have been found when swimmers used the AquaTrainer® snorkel for physiological purposes (Barbosa et al., 2010; Conceição et al., 2013; Ribeiro et al., 2016; Szczepan et al., 2018). However, to date, there is no evidence of whether the Aquanex System impairs the swimming pattern, and what are the constraints derived from using it. This kind of feedback will help researchers and coaches to be comfortable when using this system in their daily tasks.
The aim of this study was twofold: (i) to analyse and compare the mechanical and efficiency constraints between free swim and the Aquanex System; and (ii) to understand if there are differences in response between sexes. It was hypothesised that: (i) swimming with the Aquanex System would impose slight constraints in the front crawl; and (ii) boys and girls would show similar constraints while using the device.
Thirty young swimmers (14 boys and 16 girls) were recruited to participate in this study (Table 1). Swimmers were assessed at the end of the third macrocycle (peak form) and the inclusion criteria consisted of: (i) being a competitive swimmer; (ii) having at least two years of experience competing in regional or national events; (iii) completing more than four swim training sessions per week; and (iv) not having suffered from any injury in the past six months. Swimmers’ parents or legal guardians were informed about the benefits and experimental risks before signing a written informed consent form. All procedures were in accordance with the Declaration of Helsinki and approved by the Institutional Ethics Committee of the University of Beira Interior (code: CE-UBI-Pj-2020-058).
Demographics of competitive swimmers.
Overall (n = 30) M ± 1SD | Boys (n = 14) M ± 1SD | Girls (n = 16) M ± 1SD | |
---|---|---|---|
Age (years) | 12.31 ± 0.67 | 12.58 ± 0.64 | 12.07 ± 0.59 |
Body mass (kg) | 48.53 ± 8.43 | 50.75 ± 7.57 | 46.62 ± 8.65 |
Body height (cm) | 157.54 ± 7.48 | 159.63 ± 8.38 | 155.76 ± 6.06 |
Arm span (cm) | 158.05 ± 8.34 | 160.82 ± 9.67 | 155.68 ± 6.06 |
Dominant upper-limb (cm) | 71.02 ± 4.18 | 72.53 ± 4.54 | 69.73 ± 3.33 |
FINA points (50-m freestyle) | 270.17 ± 62.27 | 278.30 ± 75.06 | 263.92 ± 49.35 |
kg, kilogram; cm, centimeter.
The in-water testing took place in a 25-m indoor swimming pool (mean water temperature: 27.5°C) during two consecutive days (24 h apart) in the afternoon period. Swimmers were randomly assigned (first bout) to perform 25-m all-out sprints in front crawl (full stroke), after a standard warm-up previously reported for sprinting events (Neiva et al., 2015). Each swimmer undertook three maximal bouts per each selected condition on separate days: free swim and swimming with sensors. All in-water bouts started by a push-off and swimmers were instructed to maintain their normal breathing pattern for sprinting events. To ensure full recovery, a 30-min rest interval between bouts was applied. All swimmers were encouraged to avoid intense exercise on the data collection days, as well as the day before. The in-water data were assessed in all bouts for both conditions and the best result was considered for further analysis. Under the condition with sensors, swimmers wore a differential pressure system composed of two hand pressure sensors (Type A, Swimming Technology Research, Richmond, VA, USA) positioned between the third and fourth metacarpals (Figure 1).
Figure 1
Swimmer carrying the hand differential pressure system with Type A sensors.

The shoulders and arms elastic straps allowed the system to be carried during the swimmer's displacement throughout the water and the sensors were connected to an interface connected to a laptop with Aquanex software (v.4.1, Model DU2, Swimming Technology Research, Richmond, VA, USA). The time spent (in s) to cover the predefined distance (i.e., 25 m) was manually assessed by two experts (ICC: 0.97), each with a stopwatch (FINIS 3x100, Finis Inc., USA), and it was considered as a swimming performance variable (T25). The stroke mechanics comprised the swimming velocity (swimming
in which
The normality of the data distribution was checked with the Shapiro-Wilk test. The mean and one standard deviation (M ± 1SD) were computed for all variables, as well as the mean percentage of change (Δ). The dataset for each condition was split into three groups: overall (n = 30), boys (n = 14), and girls (n = 16). The paired sample
The comparison of swimming performance under both conditions is shown in Figure 2. Overall, there was an increase in T25 when swimming with sensors (
Figure 2
Comparison of swimming performance between free swim and sensors in the front crawl. *p ≤ 0.05 or **p ≤ 0.01, denotes a significant difference to sensors.

Figure 3 depicts the comparison between free swim and swimming with sensors according to the stroke mechanics variables. The
Figure 3
Comparison between free swim and sensors in stroke mechanics variables in the front crawl. Panel A: swimming velocity (v); Panel B: stroke rate (SR); Panel C: stroke length (SL). *p ≤ 0.05 or **p ≤ 0.01, denotes a significant difference to sensors

The swimming efficiency variables are shown in Figure 4. There was a significant decrease in the girls’ SI (Panel A) with sensors (
Figure 4
Comparison between free swim and sensors in swimming efficiency variables in the front crawl. Panel A: stroke index (SI); Panel B: arm stroke efficiency (

This study considered the technical constraints induced by the Aquanex System when swimming front crawl. The main finding was that swimming with sensors imposed trivial constraints on swimming performance and
Front crawl has been recognised as the fastest and most economical swimming stroke (Barbosa et al., 2010; Deschodt et al., 1999), being the most reported for field-oriented research purposes and for tracking swimming performance. Sprint events in short- and long-course swimming pools are characterised by generating a greater amount of propulsion in the water to reach higher velocity (Seifert et al., 2007). Thus, this kind of assessment is crucial and needs to be as accurate as possible, imposing the least constraints in the various aspects of the stroke.
Overall, front crawl swimming performance decreased significantly (1.30%) by adding the sensors (i.e., T25 increase), and thereby the
Another important aspect is how the all-out effort was performed. Swimmers were assessed in a short distance (i.e., 25 m) with an in-water start. This was performed equally under both conditions without diving and adding a dolphin kick. When using sensors, it can be argued that the decrease found in swimming performance and
The SF was assessed considering the 11th and the 24th m of the pool. It seems that swimmers were able to maintain their motion with and without the system. Theoretically,
Boys and girls were analysed together at a first stage, since the sex gap is not an issue in this age group, at least with regard to pre-adolescence (Seifert et al., 2011; Zuniga et al., 2011). However, this does not mean that, at some point, the behaviour between boys and girls will not be interpreted separately (Barbosa et al., 2014). Within this approach, while girls showed decreases in
We may point out few limitations in the present research: (i) the
The Aquanex System seems not to induce constraints on the mechanics and efficiency of young swimmers, which can allow coaches to use it in their daily practice for monitoring of the training process. Despite that, coaches and researchers are advised to take some care in its application because during all-out efforts the initial velocity of the test can be compromised. As the cable can be an issue, a necessary quick adaptation to the device after the start is needed. As such, this can slightly compromise the mean velocity if we consider the overall distance covered for velocity estimation. Thus, measures such as swimming velocity, mechanics of the stroke, and efficiency, along with propulsive force should be retrieved further in the test for a more accurate assessment.
Figure 1

Figure 2

Figure 3

Figure 4

Demographics of competitive swimmers.
Overall (n = 30) M ± 1SD | Boys (n = 14) M ± 1SD | Girls (n = 16) M ± 1SD | |
---|---|---|---|
Age (years) | 12.31 ± 0.67 | 12.58 ± 0.64 | 12.07 ± 0.59 |
Body mass (kg) | 48.53 ± 8.43 | 50.75 ± 7.57 | 46.62 ± 8.65 |
Body height (cm) | 157.54 ± 7.48 | 159.63 ± 8.38 | 155.76 ± 6.06 |
Arm span (cm) | 158.05 ± 8.34 | 160.82 ± 9.67 | 155.68 ± 6.06 |
Dominant upper-limb (cm) | 71.02 ± 4.18 | 72.53 ± 4.54 | 69.73 ± 3.33 |
FINA points (50-m freestyle) | 270.17 ± 62.27 | 278.30 ± 75.06 | 263.92 ± 49.35 |
Relationship among the Change of Direction Ability, Sprinting, Jumping Performance, Aerobic Power and Anaerobic Speed Reserve: A Cross-Sectional Study in Elite 3x3 Basketball Players Construct Validity and Applicability of a Team-Sport-Specific Change of Direction Test Change of Direction Deficit: A Promising Method to Measure a Change of Direction Ability in Adolescent Basketball Players Effects of Arm Dominance and Decision Demands on Change of Direction Performance in Handball Players Effectiveness and Kinematic Analysis of Initial Step Patterns for Multidirectional Acceleration in Team and Racquet Sports Change of Direction Ability as a Sensitive Marker of Adaptation to Different Training Configurations, and Different Populations: Results from Four Experiments Lower Limb Skeletal Robustness Determines the Change of Directional Speed Performance in Youth Ice Hockey Reactive Agility in Competitive Young Volleyball Players: A Gender Comparison of Perceptual-Cognitive and Motor Determinants The Relationship among Acceleration, Deceleration and Changes of Direction in Repeated Small Sided Games Change of Direction Performance and its Physical Determinants Among Young Basketball Male Players Training to Improve Pro-Agility Performance: A Systematic Review Relationships between Sprint, Acceleration, and Deceleration Metrics with Training Load in Division I Collegiate Women’s Soccer Players