Keywords

1 Introduction

It has been shown that individual swimmers vary considerably with regard to swim technique and style [1]. It has also been reported that swim athletes display specific training adaptations in comparison to land based athletes and triathletes [2]. Specifically, it has been reported that triathletes and swimmers vary in swim velocity and propulsion efficiency [3]. It is therefore of interest to understand the different physiological responses that may occur within different swim athletes. Roels et al. demonstrated that during an incremental swim test, competition swimmers produced significantly higher heart rate maximum, maximal oxygen consumption (VO2max) and maximal swim velocities in comparison with competition triathletes. Current physiological assessment during swim exercise has predominately focused upon global measurements such as heart rate [4], blood lactate [5] and VO2max [6]. Systemic measurements of oxygen status have been made possible through the development of specialized snorkels such as the Aqua Trainer Valve Cosmed technologies©, which now allows for breath-by-breath analysis within the pool [7]. It has been suggested that a local measurement of muscle oxygenation during swim exercise would contribute to a multi-modality approach that is perhaps warranted, to enable a more detailed evaluation of swim athletes [8].

Near-infrared spectroscopy (NIRS) has been used successfully in the non-invasive observation of changes in local muscle oxygenation and hemodynamic responses in tissue within a variety of dynamic land based sports including; short track speed skating [9], downhill skiing [10] and sprint running [11] and recently the NIR technique has been utilised with swim athletes [12], albeit during incremental cycle exercise. To the best of our knowledge the concurrent observation of the muscle oxygenation in both the vastus lateralis (VL) and latissimus dorsi (LD) muscles, using NIRS, during underwater swimming has not been carried out. The development of a portable waterproof NIRS device, utilising an optically clear, waterproof, silicone covering, has enabled the provision of a local measurement of muscle oxygenation that could provide novel insight into the physiological changes that occur during swim exercise. Therefore, the aims of the study were: to advance the assessment and utility of an underwater near-infrared spectroscopy device and to monitor the differences (if any) in muscle oxygenation changes within two different cohorts of swim athletes in the upper and lower extremities.

2 Methods

Club level swimmers and triathletes from Essex based (United Kingdom) swim clubs were asked on a voluntary basis to take part in a 200 m maximal swim effort in their club training pool. Ten subjects, five club level swimmers and five club level triathletes, (three men and seven women) volunteered for assessment. Swim group; mean ± SD = age 21.2 ± 1.6 years; height 170.6 ± 7.5 cm; weight 62.8 ± 6.9 kg; vastus lateralis skin fold 13.8 ± 5.6 mm; latissimus dorsi skinfold 12.6 ± 3.7. Triathlete group; mean ± SD = age 44.0 ± 10.5 years; height 171.6 ± 7.0 cm; weight 68.6 ± 12.7 kg; vastus lateralis skin fold 11.8 ± 3.5 mm; latissimus dorsi skinfold 11.2 ± 3.1. All subjects were experienced swim athletes who had been involved in swim training for 11.1 ± 3.4 years. Subjects were asked not to perform any strenuous activity 24 h prior to the observational assessment. The study received ethical approval and all subjects provided informed consent.

Each subject was asked to perform a maximal 200 m freestyle swim (8 × 25 m) in a short course swimming pool. Subjects were made to complete a 400 m warm up comprised of 300 m (freestyle) at a self-selected pace and 100 m (freestyle) consisting of 15 m hard and 35 m easy × 2. Following warm up, subjects passively stood in the shallow end of the swimming pool (waist immersion), with their arms by their sides and their body weight evenly distributed over each leg, for a 2-min period to establish a baseline NIR reading. Subjects then started freestyle swimming without diving from the pool edge and were able to perform regular (‘tumble turn’) turning motions at the end of the lane. Upon completion subjects passively stood for 3 min at the shallow end of the pool, following the same procedure as for the baseline NIR attainment.

The portable NIRS apparatus (PortaMon, Artinis, Medical Systems, BV, the Netherlands) used in this study was a dual wavelength continuous system, which simultaneously uses the modified Beer-Lambert law and spatially resolved spectroscopy (SRS) methods. Changes in tissue oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb) and total hemoglobin (tHb) were measured using the difference in absorption characteristics of wavelength at 750 and 850 nm. The tissue hemoglobin saturation index (TSI) (expressed in % and calculated as [O2Hb]/([O2Hb] + [HHb]) × 100), was calculated using the SRS method [13]. During testing, the PortaMon stored the data within the device’s internal memory capacity. These data were subsequently downloaded onto a personal computer for analysis through an online software program. Data acquisition was sampled at a rate of 10 Hz for analogue-to-digital conversion and subsequent analysis. Baseline and minimum TSI (%) values were calculated as the 30-s average prior to the start of the exercise and the 3-s average surrounding the lowest value during the 200 m swim effort. The ΔTSI was therefore calculated as the baseline value minus the minimum value e.g. TSI Baseline − TSI Min = ΔTSI. The NIRS devices were positioned on the belly of the vastus lateralis muscle, midway between the greater trochanter of the femur and the lateral femoral epicondyle and for the latissimus dorsi muscle, at the midpoint between the mid-axilla and the spinal column. To ensure the optodes and detector did not move relative to the subject’s skin, the device was fixed into position using sports waterproof adhesive tape and secured using the subject’s own specialist swim apparel.

Descriptive statistics are presented as a mean ± SD unless otherwise stated. Each variable was examined with Kolmogorov-Smirnov normality test. ΔTSI %, between muscle groups and time to swim completion (s) were analysed using either paired (2-tailed) t-tests or Wilcoxon signed rank test when the sample normality test failed as per Bravo and colleagues [14]. The level of significance for analyses was set at p < 0.05. All analyses were performed using Graphpad Prism 6 (Graphpad Software, San Diego, CA).

3 Results

Figures 4.1a–e and 4.2a–e show the individual tissue saturation index (TSI %) trends for the vastus lateralis and latissimus dorsi muscle for the club level swimmers and club level triathletes groups, respectively. Club level swimmers completed the 200 m freestyle swim significantly faster (p = 0.04) than the triathlete group. Swim group time; 172.4 ± 14.6 (s), triathlete group time; 232.6 ± 39.6 (s).

Fig. 4.1
figure 1figure 1

Individual (subject’s (a–e)) swim group ΔTSI (%) for the vastus lateralis and latissimus dorsi muscles. Group average data show there was no significant difference (p = 0.686) between the extent of desaturation in the upper and lower extremity; VL ΔTSI % = −9.44 ± 1.50, LD ΔTSI %= −9.00 ± 3.27

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Fig. 4.2
figure 2figure 2

Individual (subject’s (a–e)) triathlete group ΔTSI (%) for the vastus lateralis and latissimus dorsi muscles. Group average data show there was a significant difference (p = 0.043) between the extent of desaturation in the upper and lower extremity; VL ΔTSI % = −3.27 ± 2.74, LD ΔTSI % = −8.79 ± 5.24

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4 Conclusions

This pioneering study has demonstrated how, for the first time, a waterproofed NIR device was able to report on muscle oxygen changes at the local level in both the upper and lower body in club level swimmers and triathletes. Furthermore, the data reported in this study show that, during a maximal 200 m swim, club level swimmers displayed no significant difference in ΔTSI (%) in the upper and lower body (p = 0.686), whereas the club level triathletes experienced a significantly greater drop in TSI (%) in the upper body compared with the lower (p = 0.043). These data suggest that club level swimmers use both the upper and lower body muscles to a similar extent during a maximal 200 m swim effort. Club level triathletes, however, predominantly use the upper body for propulsion during the same exercise. A significant difference in swim time (p = 0.04) was seen, with the club level swimmers completing the 200 m swim distance faster than the triathlete group. As previously mentioned, notable differences in swim velocity and propulsion [3] and marked systemic differences [2] have previously been observed between these swim groups. The most likely explanation for the difference in swim time would be related to the contribution of the lower body (legs) to the swim exercise. It has been shown that when the leg kicking action is added to the arm action, a 10 % increment in swim speed can be seen during swim sprint efforts [15]. However, even when told to swim as fast as possible, triathletes used only the upper body, presumably due to the need to spare the leg muscles for the later cycling and running stages of their event. It must be stated that all subjects were asked to swim maximally and ‘normally’ during the swim task; the differences in swim style were, however, very apparent within this particular level (club) of triathlete and the NIRS data reflected these biomechanical differences. In conclusion, the waterproofed NIR device was able to report upon muscle oxygenation changes in the upper and lower extremities in club level swimmers and triathletes. The ΔTSI % changes indicated that club level swimmers use both the upper and lower muscles to a similar extent during a maximal 200 m swim, whereas there was minimal desaturation in the lower body muscles of the triathletes.