Cold water endurance swims (CWESs) such as the English Channel (EC) crossing, like many endurance sporting events, are increasing in popularity [1–4]. The EC is seen as the pinnacle in endurance swimming [5]. At the shortest distance, the EC spans 33.5 km (20.8 min) [4, 6] from England to France. The average successful cross-channel swimmer takes approximately 13 and a half hours to complete the swim crossing [7]. The fastest recorded time is 6 h and 55 min and was achieved in 2012; the slowest recorded time is 28 h and 44 min, completed in 2010 [7].
Hypothermia is a significant potential risk for open water (OW) swimmers [8–11] and has been recorded at water temperatures as high as 22 °C [12]. Hypothermia is broadly defined by core temperature (TC) of less than 35 °C (95 °F). To reduce the risk of hypothermia in participants in their events, the Fédération Internationale de Natation (FINA) has established a lower water temperature limit of 16 °C [13] for OW swimming events. Similarly, in events longer than 1500 m, the International Triathlon Union (ITU) has made wetsuits compulsory if the water temperature is below 16 °C [14]. In contrast, for an EC swim to be ratified by the Channel Swimming Association (CSA) or the Channel Swimming and Pilot Federation (CSPF), regardless of the water temperature [15] swimmers are not permitted to wear a wetsuit and are limited to a standard non-buoyant swimsuit, goggles, ear plugs and one swim cap [15, 16]. The water temperature during the Channel season is usually between 11 and 19 °C, with most swims occurring between 14 and 16 °C [17]. In addition, all swimmers are required to complete a 6-hour qualifying swim in water less than 15.5 °C for CSA-ratified [15] and 16 °C for CSPF-ratified [17] swims.
The duration of exposure to cold water and the conditions of the EC place swimmers at an increased risk of developing hypothermia [12, 18]. Hypothermia has undesirable physiological responses which have previously been shown to contribute to premature swim termination [11]. As a cold water swimmer (CWS) TC approaches hypothermia, physiological mechanisms (nervous, endocrine, muscular and integumentary systems) are initiated in an attempt to maintain core body temperature.
Minor risks of attempting the EC include feeling cold, nausea, emesis, jellyfish stings, salt mouth, otitis externa and musculoskeletal overuse injuries [19]. There are also major health risks which may be associated with hypothermia including blood flow restriction, pulmonary oedema, increased cardiac afterload, myocardial ischaemia, heart failure and even death [11, 20–22]. Since 1926, eight swimmers have lost their life attempting to swim across the EC, with the two most recent in 2012 and 2013 [23]. Both of these deaths occurred within one mile of completing the crossing.
In 2013, approximately 60Â % of EC attempts were successful, highlighting that failure is frequent [24]. In preparation for an EC attempt, aspiring EC swimmers invest significant time training and financial resources to cover pilot boat fees, support crew, coaching and professional advisor fees, and where required to cover flights and accommodation. Many aspirants also increase body weight and/or fat mass to a potentially unhealthy level in preparation for the cold water temperature [15, 19, 25].
Body composition of marathon swimmers has been previously reported as being higher in body fat (BF) and shorter in stature than competitive pool swimmers, and the differences are attributed to OW swimmers usually being of lower standard than pool swimmers [26]. The CSA and CSPF recommend EC aspirants to increase their body weight and adiposity to increase the safety of their swim in an attempt to reduce the risk of developing hypothermia [4, 6]. In non-elite marathon swims, higher BF may be associated with improvements in body temperature regulation when swimming in cold water [19, 27] and BF acts as an insulator against the cold [9]. It has been reported that athletes with higher subcutaneous fat [9, 22, 28, 29] and higher BMIs [10, 12] are at lower risk of developing hypothermia [22] and are more likely to endure cold water swims for longer periods [22, 30].
Other strategies that have been employed to reduce the likelihood of developing hypothermia include cold water adaptation [31–33] and hypothermic exercise training [20, 34, 35]. Factors that have been previously shown to increase hypothermia risk are older age [36, 37], wind chill [11], motion sickness [38] and local muscular fatigue [11].
Potential disadvantages to gaining weight to an ‘overweight’ or ‘obese’ level of BF may include health risks [39] and detriments to swimming performance [40, 41]. It is well established that being ‘overweight’ is a risk factor for chronic diseases such as type two diabetes mellitus, cancers (i.e. colorectal and breast cancer) and cardiovascular diseases (i.e. atherosclerosis, stroke and coronary heart disease) [39]. However, exercise is known to reduce the risk for these health conditions [39] and some research shows that this is independent of fatness level [42].
To measure swim performance, indicators of swimming velocity, stroke rate and/or stroke length are commonly used in both elite pool swimming [43–45] and cold water swimming [28, 46]. In freestyle swimming, increased SR is associated with increased metabolic heat production and typically results in an increased velocity [47]. Stroke rate is also related to energy output, and thus a decline in SR is generally indicative of a swimmer’s state of fatigue and performance [48].
Given the health, medical, time and financial risks, along with the increasing popularity of the EC swim [4], this study aimed to observe the TC changes in CWS in an EC qualifying swim of 6Â h. We investigated whether factors such as adiposity, lean mass, SR, age or training volume affected TC change, and if any of these factors were associated with EC success and EC crossing time.