Background: The purpose of this study was to determine if cold water swimmers (CWS) developed hypothermia

over a 6-h cold water endurance swim and whether body composition, stroke rate (SR) or personal characteristics

correlated with core temperature (TC) change. Nine experienced male and female CWS who were aspiring English

Channel (EC) swimmers volunteered to participate. Subjects aimed to complete their 6-h EC qualifying swim (water

15–15.8 °C/air 15–25 °C) while researchers intermittently monitored TC and SR. Data obtained included anthropometry (height, mass, segmental body composition), training volume and EC completion.

Results: Of the nine swimmers who volunteered, all successfully completed their EC qualifying swim. Six CWS had

complete data included in analysis. One CWS demonstrated hypothermia (34.8 °C) at 6-h. TC rate of decline was

slower in the first 3 h (−0.06 °C/hr) compared to the last 3 h (−0.36 °C/hr) of the swim. Older age was significantly

correlated to TC change (r = −0.901, p < 0.05) and SR change (r = −0.915, p < 0.05). Absolute and percentage body fat (BF) were not significantly associated with higher TC. Mean SR over the 6-h swim was 57.8 spm (range 48–73 spm), and a significant (p < 0.05) decline in SR was observed over the 6 h (−9.7 %). A strong, positive correlation was found between SR change between 3 and 6 h and TC over the 6 h (r = 0.840, p < 0.05) and TC from 3–6 h (r = 0.827, p < 0.05). Seven of the nine participants (77.8 %) in this study successfully completed the EC crossing. Successful EC swimmers swam in the pool and open water (OW); however, they swam significantly [t (7) = −2.433, p < 0.05] more kilometres (M = 19.09 km/wk ± 5.55) in OW than unsuccessful (M = 9 km/wk ± 1.41) EC swimmers. There was a significant relationship between EC crossing time and height (r = −0.817, p < 0.05), but no other variables and EC crossing time.

Conclusions: Cold water endurance swim (CWES) of 6-h duration at 15–16 °C resulted in TC reduction in the majority

of swimmers regardless of anthropometry. More research is required to determine why some CWS are able to maintain

their TC throughout a CWES. Our results indicate that older swimmers are at greater risk of developing hypothermia,

and that SR decline is an indicator of TC decline. Our results also suggest that OW swimming training combined

with pool training is important for EC swim success.