Relationship Between Front Crawl Trunk Incline and Lower Limbs’ Biomechanics in Non-Expert Swimmers

Abstract

This study examined the relationship between front crawl trunk incline and the lower limbs’ biomechanics in non-expert swimmers. Eighteen male participants (19.22 ± 1.11 years) were recorded in the sagittal plane performing 2 × 25 m of front crawl at maximum intensity to analyze their trunk incline (TI), maximum knee angle (KneeMax), minimum knee angle (KneeMin), knee range of motion (KneeROM), kicking duration (KickDur), descendent phase duration (DurDesc), and ascendant phase duration (DurAsc). They also performed towing for passive drag measurements and a 20 s lower limbs’ tethered test while connected to an electromechanical device and grabbing a floating board to collect the maximum (Fmax) and mean (Fmean) kicking forces. Pearson’s correlation coefficient (r) was used to compute the relationships between all variables. For kinematics, a negative association was found between the TI and v (r = −0.64), KneeMin (r = −0.68), KneeRoM (r = −0.74), and SI (r = −0.52). Regarding kinetics, a single association was found between TI and Fmean (r = −0.52). The results indicate that a greater TI in non-expert swimmers may be a consequence of weaker knee action, which compromises their mean force application and negatively affects velocity and efficiency.

1. Introduction

Front crawl is a complex swimming technique requiring precise coordination between different body segments to generate efficient propulsion while maintaining optimum stability [1]. Due to the instability induced by the aquatic environment, specific muscular activations and kinematic adaptations help in maintaining smooth water displacement and a steady body position [2]. In fact, the position of the swimmer’s body, more precisely the sagittal tilt of the torso around the medio-lateral axis, has been proven to be one of the crucial aspects for ensuring coordination between different body segments [3]. So, the ability to remain as horizontal as possible can determine a swimmer’s success in generating effective propulsive force while reducing the resistance to forward motion.

In the specific case of swimming, the trunk incline (TI) is the angle between the shoulder (acromion), hip segment (greater trochanter), and the horizontal, which is defined by the water line, and is crucial for water locomotion [3]. Several anatomical points, like the relative positional relationship between the center of mass and the center of buoyancy (i.e., when those are too far apart) [4], the lung volume [5], or the projected frontal area [6] have been found to increase the TI. The fact is that the greater the TI, the less horizontal the swimmer is and the greater are the drag forces acting on their body, which impacts the energy cost of swimming [7].

The contribution of the lower limbs to forward displacement has attracted a great deal of interest over the years, as they account for more than 50% of the velocity in the different swimming strokes [8,9,10]. The segmental actions involving the lower part of the body increase swimming speed, improve the swimmer’s overall stability, and reduce their TI, resulting in a more horizontal position in the water [11]. When testing competitive swimmers, the TI seems to remain quite stable, near 10° at a range of different velocities [3,6,12] or even when additional equipment (e.g., fins) is used [13]. A lower TI is expected in these swimmers as they have a high level of expertise in detailed aspects of technique, derived from a large experience in swim training. However, this is not the case for recreational swimmers, as they can take a longer time to learn, which may influence their overall technical ability.

The teaching contents in swimming are arranged in terms of complexity, starting with work related to body position, which is then followed by the inclusion of limbs’ actions and breathing [14]. Because recreational swimmers regularly show deficits in global aspects of their swimming technique, their body position and lower limbs’ actions should receive greater attention from teachers. Indeed, the overall stroke mechanics and efficiency may be influenced by how they position themselves in the water and the biomechanical characteristics of their lower limbs.

Although several studies have highlighted the importance of the lower limbs’ action and its effects on the TI in front crawl, they failed to assess the relationships between TI and kicking from a deeper kinematic and kinetic standpoint. Moreover, most of the studies were conducted using competitive swimmers as participants [3,6,12] and not those who are more affected by TI (i.e., recreational swimmers). So, understanding the mechanisms underlying how these movements cause interactions between different segments of the swimmer’s body is essential to developing effective pedagogical or training strategies for more efficient front crawl swimming.

This study aimed to analyze the relationship between the lower limbs’ kinematics or kinetics and trunk inclination in front crawl swimming. It was hypothesized that the angle of trunk inclination would be more directly related to the kinetics than the kinematics of the swimmer’s lower limbs.

2. Materials and Methods

2.1. Participants

A total of 18 non-expert male swimmers (19.22 ± 1.11 years, 75.32 ± 7.89 kg of body mass, 178 ± 4.92 cm of body height, and 179.61 ± 6.34 cm of arm span) volunteered to take part in this study. The inclusion criteria were defined as follows: (i) not having any previous experience in swimming training or competitive events; (ii) not taking part in any swimming club affiliated with the national swimming federation; and (iii) not having had any injury in the six months before data collection. The swimmers were recruited from a local higher education institution and were part of the same dynamics of swimming lessons, having two sessions per week of a 45 min duration. They were on the basic level of teaching, where the main objective was to teach front crawl and backstroke techniques concurrently, as previously described [14]. All participants signed an informed consent form before the beginning of data collection. The experimental procedures were in full compliance with the Helsinki Declaration for human research and approved by an Institutional Ethics Committee.

2.2. Study Design and Data Collection

A cross-sectional research design was applied, with the swimmers having one testing session in a 25 m indoor pool with a water temperature of 27 °C and humidity of 60%. The session started with an assessment of the swimmers’ anthropometric characteristics while they were wearing only a textile swimsuit and a cap. Body mass (in kg) was assessed using a scale (TANITA, BC-730, Amsterdam, The Netherlands), while body height (in m) was measured using a digital stadiometer (SECA, 242, Hamburg, Germany). Swimmers were marked at four anatomical points on the right sagittal plane (acromion, hip, knee, ankle) using an oil-based black makeup and waterproof wax cream suitable for aquatic environments (Make-Up Pure Cream, 101, Grimas, Utrecht, The Netherlands). Then, the swimmers completed a standardized warm-up of 200 m of easy swimming—100 of kicking, 4 × 25 m with the increasing speed between each 25 m set—and 100 m at a recovery pace. After a passive rest of 10 min, the swimmers were randomly assigned to complete the in-water protocol (Figure 1), which consisted of three tests for kinematic and kinetic measurements, each followed by 10 min of recovery: (i) 25 m maximal front crawl; (ii) 25 m towing; and (iii) 20 s full tethered kicking.

In the first test (Figure 1, panel A), the swimmers performed a 25 m maximal-effort front crawl starting with a push-off against the wall after an audible signal, maintaining their normal breathing pattern. Each swimmer performed this test alone, without any other disturbances in the nearest lanes. The 25 m time (T25, in s) was manually assessed by a certified coach using a stopwatch (FINIS 3 × 100, Finis Inc.) and considered the time taken (in s) to cover 25 m and the mean velocity (v, in m/s) was estimated thereafter. The stroke rate (SR, in Hz) was assessed using a frequency chronometer (FINIS 3 × 300, Finis Inc.) activated over three consecutive stroke cycles. The stroke length (SL, in m) was then estimated (SL = v/SR), along with the stroke index (SI, in m²/s) (SI = v·SL), as previously described [15]. This test was recorded on video (at a frequency of 120 Hz) in the sagittal plane (right side) with a camera (Go Pro 6, resolution 1920 × 1080 pixels, San Mateo, CA, USA) mounted on a rigid iron fixed at 0.26 m, providing a view below the water’s surface. The system was positioned at around 15 m from the starting wall, on the side of the pool, perpendicular to the line of movement and 5 m away from the swimmer, providing a range of views between the 11th and 24th m. The images were stored on a USB memory stick, enabling data to be collected and processed using a computer and a video annotation tool (Kinovea^®, v. 0.8.15).

All the kinematic variables were assessed over two non-respiratory consecutive stroke cycles between the 11th and 24th m using the “angle” or “time” options from the software and the mean values from these cycles were considered for further analysis. The trunk inclination (TI, in °) was taken at the moment where the swimmer’s right hand started the entry phase of the stroke cycle and was determined to be the angle between the shoulder (acromion), hip segment (greater trochanter) and the horizontal, as previously described [3]. For the lower limbs’ kinematics, the maximum (KneeMax, in °) and minimum (KneeMin, in °) knee angles (increasing with extension and reducing with flexion), as well as the knee’s range of motion (KneeROM, in °), were retrieved from the right side. The deepest point of the feet in relation to the water surface (full extension of the lower limb) and the nearest point to the water surface (maximum flexion of the lower limb) were considered as the instantaneous timings for KneeMax and KneeMin measurements, respectively. The duration of the ascendant phase (DurAsc, in s) and the descendant phase (DurDesc, in s) of the kicking cycle were collected by considering the intervals in which the lower limb was moving upward or downward, respectively. Then, the duration of the overall kicking cycle (DurKick, in s) was considered to be the sum of DurAsc and DurDesc.

In the second test (Figure 1, panel B), the swimmers were passively towed on the water’s surface across the entire length of the pool twice at a velocity of 1.5 m/s, with each length separated by 3 min of rest. This velocity was chosen as it represents the average velocity achieved in the 25 m maximum effort at front crawl by this cohort of swimmers (1.51 ± 0.14 m/s). They were connected via a non-elastic wire to an electromechanical device (Swim-Spektro, Talamonti, Pescara, Italy) positioned at the edge of the pool. The device was calibrated on different ranges of velocities using a 2D video protocol and on the towing force while suspending vertically known masses, as described by Cortesi et al. [16]. The swimmers were instructed to adopt their best hydrodynamic position with arms fully extended above their head and in contact with the sides of the head (one hand over the other), while keeping the feet together with ankles in plantar flexion. The hydrodynamic position of the swimmers was carefully checked through visual observation. While being towed, they were instructed to hold their breath and not perform any propulsive actions. Data was captured using dedicated software (DB:4, Talamonti Spa, Ascoli Piceno, Italy). The passive drag (Dp, in N) was equal to the instantaneous force required for towing and was measured from 15 to 5 m from the wall, which was when the velocity was constant. A Matlab routine was used to filter the signal (10 Hz) and resample the data points (100 Hz). A correction angle was accounted for when calculating the horizontal component of the velocity (i.e., considering the height of the device relative to the water’s surface and the towing distance). The maximum (Dpmax, in N), minimum (Dpmin, in N), and mean (DPmean, in N) passive drag were recorded for each trial.

The third test (Figure 1, panel C) consisted of 20 s of full tethered kicking at maximum intensity. The same system used for towing was used for measuring the in-water force of the lower limbs. The swimmers held onto a floating board above their heads in a ventral and horizontal position, with their faces in contact with the water and their upper limbs fully extended. They were allowed to raise their heads to breathe when necessary. To avoid the inertial effect, participants started the test by performing lower limbs’ actions for 5 s at a low intensity before an auditory signal was given for them to change to maximum effort for 20 s. This data was also imported from the manufacturer’s software (DB:4, Talamonti Spa, Ascoli Piceno, Italy). The main kinetic variables retrieved from this test were the maximum force (Fmax, in N) and mean force (Fmean, in N) during the 20 s of kicking.

2.3. Statistical Procedures

Shapiro–Wilk and Levene tests were used to assess the normality and homoscedasticity of the data, respectively. The mean plus one standard deviation (M ± 1SD), maximum, and minimum were computed as descriptive statistics. The Pearson Correlation Coefficients (r) between the trunk incline and the remaining variables were calculated and interpreted as high if r ≥ 0.60, moderate if 0.30 ≥ r < 0.60, and low if r < 0.30 [17]. All statistical analyses were performed in the SPSS software (v.27, IBM, SPSS Inc., Chicago, IL, USA) and graphical design was carried out using GraphPad Prism (v.9, GraphPad Software, San Diego, CA, USA). The statistical significance was set at p ≤ 0.05.

3. Results

Table 1. Descriptive analysis of kinematic and kinetic variables of non-expert swimmers.

Variables	Mean ± SD	Max	Min
Kinematics
TI (°)	15.64 ± 3.16	25.9	12.5
v (m/s)	1.51 ± 0.14	1.71	1.21
SR (Hz)	0.88 ± 0.12	1.17	0.68
SL (m)	1.74 ± 0.09	2.17	1.29
SI (m2/s)	2.66 ± 0.66	3,42	1.75
KneeMax (°)	178.31 ± 2.68	180.00	172.00
KneeMin (°)	121 ± 11.45	141.75	102.70
Knee ROM (°)	56.83 ± 10.83	72.65	38.25
DurKick (s)	0.41 ± 0.14	0.96	0.30
DurDesc (s)	0.18 ± 0.07	0.46	0.13
DurAsc (s)	0.22 ± 0.07	0.50	0.16
Kinetics
Fmax (N)	130.31 ± 21.78	168.80	88.55
Fmean (N)	61.19 ± 12.78	83.52	38.98
Dp max (N)	160.02 ± 15.20	186.99	131.15
Dp min (N)	110.13 ± 13.77	134.43	81.86
Dp mean (N)	135.09 ± 11.00	153.99	115.54

Legend: TI, trunk incline; v, mean velocity; SR, stroke rate; SL, stroke length; SI, stroke index; KneeMax, maximum knee angle; KneeMin, minimum knee angle; KneeROM, knee range of motion; DurKick, kicking cycle duration; DurDesc, descendent phase duration; DurAsc, ascendant phase duration; Fmax, maximum kicking force; Fmean, mean kicking force; Dp max, maximum passive drag; Dp min, minimum passive drag; Dp mean, mean passive drag.

presents a descriptive analysis of kinematic and kinetic variables. The time taken to complete the 25 m front crawl test was 16.66 ± 1.67 s.

Figure 2 shows the correlation matrix between the TI and the kinematic variables. A negative association was found between the TI and v (r = −0.64, p = 0.002), SI, (r = −0.52, p = 0.03), KneeMin (r = −0.68, p = 0.002), and KneeROM (r = −0.73, p = 0.001). The remaining kinematic variables showed no association with TI.

Figure 3 shows the correlation matrix between the TI and the kinetic variables. A negative association was only found between the TI and Fmean (r = −0.52, p = 0.02). The remaining kinetic variables showed no association with the TI.

4. Discussion

The aim of this study was to analyze the relationship between the lower limbs’ kinematics or kinetics and trunk inclination in front crawl performed by non-expert swimmers. Main findings indicate that a greater TI in non-expert swimmers can result from a weaker knee motion, which negatively affects velocity and efficiency. The mean kicking force is a kinetic measure that seems to determine the degree of TI in non-expert swimmers. As such, our hypothesis was partially confirmed, as both the kinematics and kinetics of the lower limbs were related to the angle of the trunk’s inclination to a similar extent.

The swimmers participating in this study exhibited trunk inclination angles of approximately 15–16° during front crawl swimming. Those values came from a 25 m maximal-effort test, which likely influences how their body segments behaved. It is likely, under lower-intensity efforts (e.g., a submaximal training pace) or longer-distance swimming, where fatigue comes into play, these TI values could become higher. Still, this finding highlights the fundamental difference in body position between non-expert and elite swimmers, who typically demonstrate inclination values closer to 10° [3]. While direct comparisons within a similar non-expert sample are unavailable due to a lack of studies on this specific metric, previous research on elite swimmers suggests that their better position in the water allows them to swim more smoothly than non-expert swimmers [18]. A greater TI in non-expert swimmers may contribute to increased frontal drag, decelerating the body, which leads to higher energy demands and reduced swimming efficiency. This steeper angle suggests that these swimmers may struggle to maintain a streamlined posture, potentially due to a lack of body awareness, insufficient technical control, or suboptimal buoyancy distribution. Although this has been considered by the swimming science community, no attempts have been made until now to a deeper understanding of how TI is affected by the lower limbs’ behavior in these swimmers.

A negative correlation was observed between the TI and knee range of motion (ROM) during the kicking action. Specifically, swimmers with a greater TI exhibited a reduced KneeROM and a reduced KneeMin during kicking. This suggests that a more inclined body position may be, at least in part, a consequence of a weaker or less effective knee action, ultimately impairing velocity. An increased KneeROM could be a counterproductive strategy, limiting the ability to generate sufficient propulsion, as happens in other cyclic actions like running [19]. An effective kick relies on a well-coordinated movement of the hip, knee, and ankle, where the propulsive force is part of a seamless cycle reflecting the synergy of the lower extremities’ muscles [20]. When knee flexion is restricted, the foot may not achieve an optimal position for generating thrust. When knee flexion is exaggerated, the legs may increase hydrodynamic resistance during the ascendant phase, reducing the overall effectiveness of the kick. So, some kind of a comfortable interval of kicking should be found for each swimmer where he/she will benefit by remaining more horizontal.

This relationship between TI and the lower limbs’ action also highlights a potential cycle of inefficiency, as seen in the negative association between TI and SI. A steeper trunk angle increases drag [21], requiring greater propulsion to maintain speed. However, if the knee action is insufficient, the swimmer may be unable to counteract this increased resistance, further reinforcing a suboptimal body position. This finding aligns with a previous study on elite swimmers, where a more streamlined posture was associated with an effective kicking technique. For Kadi et al. [22], the kicking action in front crawl had a positive effect on reducing the pressure drag acting on the trunk, thereby allowing swimmers to achieve a given velocity with less propulsive force from their hands, with this effect being most notable in low-velocity ranges.

Our third finding reveals a negative relationship between the mean kicking force and TI, suggesting that non-expert swimmers with a lower kicking force tend to exhibit a steeper trunk angle. This insight provides a possible explanation for the first two findings, reinforcing the idea that ineffective lower limb action contributes to a suboptimal body position and, ultimately, reduced swimming efficiency. A strong and continuous kicking force plays a crucial role in maintaining a more horizontal body position in the water and not accounting for drag increases [23]. In well-trained swimmers, the propulsive force generated by the legs counteracts the natural tendency of the lower body to sink, allowing for a more streamlined posture with minimal trunk inclination [11]. However, in non-expert swimmers, a weaker kicking force may fail to provide adequate lift, leading to a greater trunk inclination and increased hydrodynamic resistance. This aligns with our second finding, where a restricted knee range of motion and exaggerated knee actions were associated with a greater TI. These factors likely contribute to a less effective kick overall, reducing both propulsion and the swimmer’s ability to sustain an optimal body position.

This relationship supports the idea that non-expert swimmers rely more on their upper body for propulsion while their lower limbs play a limited role in both forward movement and stability. This imbalance may lead to compensatory techniques, such as increased lateral actions or exaggerated arm strokes directed to the bottom, which could further impair their swimming efficiency. In contrast, elite swimmers demonstrate a well-integrated kicking pattern where the lower limbs not only contribute to propulsion but also help maintain body alignment when coupled with the arms’ action [10]. As such, any teaching model for this kind of swimmers should include a longer-lasting phase that works on body alignment derived from the lower limbs’ actions alone, which may create a more solid foundation when breathing and the upper limbs come into play later in the teaching model.

In the end, these findings benefit the swimming professionals working daily on the pool deck. Since non-expert swimmers are the “core” of their population, they should keep their focus on trunk incline behaviors while progressing through each component of a structured swimming program. Some teaching strategies should focus on finding the ideal knee ROM based on in-water contrast drills, emphasizing exaggerated or restricted kick mechanics. This dual work on kick frequency and amplitude can help swimmers generate a more proprioceptive stimulus in order to readjust their trunk inclination based on several task constraints.

Some limitations can be assumed while conducting this study: (i) the assessed non-expert swimmers were 19 years of age, so these results cannot be extrapolated to younger ages; (ii) kicking kinematics and kinetics were assessed independently through separate tests, which may hide potential links between both domains and TI. Future studies should try to avoid these issues and explore whether additional interventions may benefit this kind of swimmers. Targeted interventions, such as core stability exercises, flotation drills, or proprioceptive feedback, could help non-expert swimmers achieve a more hydrodynamic and favorable posture. Moreover, research focusing on how individual factors such as anthropometry, buoyancy, and breathing mechanics influence TI could provide further insights into optimizing the body alignment of swimmers at different skill levels.

5. Conclusions

The results indicate that a greater TI in non-expert swimmers could be a consequence of a weaker knee action, which negatively affects velocity and efficiency. Also, the mean kicking force is a clear kinetic measure that determines the degree of TI. Thus, both the kinematics and kinetics of the lower limbs are related to the angle of trunk inclination. This demonstrates that the knee movement and the mean force applied for propulsion are crucial to maintaining a horizontal position and should be a constant focus in the initial stages of teaching swimming.