Some concepts or acts can be so fundamental to a communal activity that the members of that community can forget that they once may not have known of the concept, let alone the meaning as the activity becomes second nature. These acts, which we might call unconscious assumptions, are not ignored, but accepted, language grows around them that the community understands and uses. But that language may be opaque to outsiders, or newcomers. It can be difficult for outsiders to differentiate between such words or phrases and buzzwords. One clarifies and conveys a common if specific meaning (afterdrop) whereas the other (“synergistic metamorphosis”, “going forward” etc) only excludes because such phrases are intended to convey status and a secret language of initiates or simply a lack of understanding of the word meanings.
You can differentiate because the users of the first will happily and easily explain the meaning, whereas almost few ever question the ludicrous nature of the second and in general the people using them also don’t really know what they mean.
There are a few aspects of open water swimming that fall into the unconscious assumption. Like well, being able to swim and having access to open water that are the two foundational requirements of everything else, and the prime feature of open water ocean swimming which is the variation of experience from swim to swim.
There are others less accessible to outsider understanding, like the requirement for chaffing lubrication for long swims, or how long distance swimmers go to the toilet.
For cold water swimmers, such implicit knowledge and essential terminology includes The Claw and afterdrop.
Afterdrop has appeared in so many articles here, and more importantly, to every cold water swimmer, that it deserves its own entry and some exploration. For those many of you who are familiar with it, hopefully I might have some new information you didn’t know, so I hope you stick with me.
What is afterdrop?
Afterdrop is the term used by swimmers AND those associated with cold exposure, such as medical and rescue personnel. Afterdrop is defined as “continued cooling following removal from cold stress“. It is not purely a swimming term.
For open water swimming, and what all cold swimmers become aware of, is this usually means the swimmer gets colder after they exit the water.
I say “usually” because the minor but not unimportant exception is swimmers who may stay in the water after finishing swimming (for whatever reason, such as thinking they are further acclimatising, or want to spend a specific time in the water).
People who do this should be especially aware of the increased danger posed by afterdrop to themselves while still immersed.
How soon does afterdrop occur, and how long does it last?
For most people afterdrop begins immediately after emerging from the water. However the effect may take up to a few minutes for their core temperature to drop sufficiently to be noticed. It is typical in the Irish open water swimming community that most people say it takes four to five minutes before it starts to to be become noticeable.
As a consequence, this is why most experienced cold water swimmers will recommend getting dressed promptly.
Afterdrop lasts until core body temperature starts to recover. The definition means that once the body stops cooling, it is generally in recovery. How and why this happens, and how rewarming should be done, and what should be avoided will be in a followup article on rewarming.
Who is affected by afterdrop?
Once the water is sufficiently cold, everyone is affected by afterdrop. There is no-one who isn’t. It may take some people longer to get sufficiently cold to the point when it happens, but it will always happen eventually.
Because folks, of what has been called the “dead hand” of the second law of thermodynamics: “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time“.
If the water is colder than a human body, unless heat is put into the person in some form, then heat will leave the body, making the water a tiny bit warmer and the body colder. Since the mass of a body is virtually negligible in comparison to the size of a larger body of water, the effect on the person is far larger (than the effect on the water).
Heat flows from warmer to colder in the same way water flows down a hill. There are no exceptions. It cannot be stopped, it can only be slowed.
The reasons wetsuits are effective is they act as insulators, separating the cold water from human skin by virtue of the composition of neoprene and the thin layer of body-warmed water which sits between it and the skin, reducing contact with cold water and greatly slowing heat from leaving the body.
The chart below, taken from Nuckton et al’s Hypothermia and Afterdrop following open water swimming, 2000*, shows post-swim rectal temperature measurements of ten swimmers, across a range of ages, speeds and sizes. It took two swimmers three to six minutes before afterdrop begins, but all went onto to develop afterdrop. The two swimmers denoted by asterisks below were not measured to the end of the 45 minute measurement period, the lower due to dropping below the 35 degree celsius (95 degrees fahrenheit) lower limit. However, this is not a record of a sinister outcome.
(Note: Average adult body temperature is 37 degrees celsius). It can be seen that six of the nine swimmers immediately entered afterdrop, while the remaining three took over from over three minutes to over six minutes to develop.
Is afterdrop hypothermia?
It is unusual for open water swimmers to use the phrase hypothermia for the aftermath of regular cold open water swimming. Not admitting to being particularly cold is a negative aspect of the sport, that I have long said is disingenuous, especially when more experienced swimmers use it with beginner swimmers. “Shivers” is the most common term, with “shakes” being occasionally added as a further qualifier or indicator that the afterdrop was more severe or recovery took longer. In my opinion, an experienced cold water swimmer who says they have “shivers and shakes” almost certainly has hypothermia.
The Nucton study uses two largely denotations of hypothermia, a “traditional” definition of below 35 degrees celsius (95 degrees fahrenheit)and an American Heart Association definition of 36 degrees. In the study, when the higher limit of 36 degrees was used, five swimmers dropped below the limit, when the traditional limit of 35 was used, one person was hypothermic. But of course, that is only relevant to those people, those particular conditions and swim times and is not indicative nor a guide to anything.
I have used both definitions here, but more commonly use 35 degrees celsius. I personally have come to favour higher limits because they will increase safety considerations and better outcomes by virtue of making people more aware of risk, and move away from the typical macho “I never get hypothermia” attitude that doesn’t serve newcomers or the sport well.
There are further definitions of mild hypothermia and moderate and severe hypothermia I covered many years back. From the point of view of new and learning cold water swimmers, my article on mild hypothermia is the entry stage.
- Mild hypothermia; 36 down to 35 degrees celsius ( 96.8 to 95 degrees fahrenheit)Moderate hypothermia; Definitions are even more unclear. From 35C (95F) down to 32C (89.6F), in some definitions down to as low as 29C (84.2F)
- Severe hypothermia; Below 29C. (84.2F)
Afterdrop therefore is part of the process of hypothermia.
Are there any lasting or negative effects of afterdrop?
In a medical sense, there are no lasting physiological effects of afterdrop, providing that it is only mildly hypothermic. Any negative physiological effects, and any positive mental or emotional effects are temporary.
This is important for two reasons:
1. Most obviously, we can endure afterdrop repeatedly, without concern.
2. We must return to the water if we wish to re-experience the positive and energising effects.
However, once we begin to consider that afterdrop can actually mean hypothermia, we must be cognisant that hypothermia itself impacts both physical and mental capacity while the person is hypothermic. So judgement or muscle control may be affected and this should always be remembered.
Afterdrop and the extent to which a person will suffer it, requires a safety-minded approach. While experienced swimmers will generally be able to deal with the effects of mild or possibly moderate core temperature drop due to afterdrop, afterdrop can be severe and dangerous. The afterdrop caused after or as part of the hypothermia caused during an Ice Mile can be extreme, as I experienced myself, and have seen in others, with loss of muscle control, inability to communicate or think clearly. Therefore the swimmer should consider either the necessity of having additional assistance on land after a swim, or based on previous experience, either lessening the swim duration for the conditions, or not swimming at all.
Afterdrop that occurs as part of person dropping into moderate or severe hypothermia, carries increased risks, which are beyond the scope of this article.
What are the factors affecting onset of afterdrop?
While I have so extensively covered cold water adaptation for so many years that I cannot select a single article link that condenses it all, nonetheless we can briefly itemise the major components that affect the onset of afterdrop. Essentially we are talking about the same list of items that could lead to hypothermia.
- Water temperature. The colder the water, the more likely is afterdrop.
- Exposure duration. The longer in the water, the more likely is afterdrop.
- Previous acclimation. (Experience and time spent in cold water resulting in physical and/or psychological adaptation).
- Ambient weather conditions. Especially wind strength and direction, insolation and ambient air temperature. (I avoid swimming off beaches in the Irish winter so the simple fact that it often takes longer to get back to my clothes so I get colder than if I can emerge to clothes which are only a few metres away.
- Metabolic rate. This is also a difficult one to condense. A fast swimmer swimming slow will get colder so afterdrop will be more profound.
- Body composition. Rather than simply list increased weight as a protective aspect, heat loss is a more complex function. The amount of both types of fat, the BMI (Body Mass Index) and overall body shape such as volume to surface area ratio are all important.
What is/are the cause/s of afterdrop?
The colder you get, the more likely you are to get afterdrop, due to the aforementioned Second Law.
Two types of heat loss occur:
- Conductive, where heat flows out of the body into the cold water, (as discussed)
- Convective, where cooled exterior blood flows into the warm core. These operate simultaneously.
Also relevant is the transition from the energetic swimming effort to a standing/walking position. While swimming we are engaged in physical effort in a prone position, who consumes energy and generates internal heat (thermogenesis). Once we emerge, the effort decreases and so does internal heat production.
Note: I have wondered about the immersion/exit transition. If I take the example where I swim 30 minutes in conditions where I know I can marginally manage 45 or even 60 minutes, nonetheless I will experience afterdrop after the 30 minute swim. I won’t experience it during the swim. I also won’t experience afterdrop in the 60 minute swim, until the swim is finished.
While this is in part due to the continuing energy contribution of swimming, is there a contributory factor from the transition itself, and the changing of body position in blood flow, especially impacting Convective cooling? I am unaware of any consideration of this.
Thanks to Thomas Nuckton MD and Dolphin Club swimmer for his paper, and not ignoring my questions. Any mistakes contained deriving from his study are entirely mine.
*Abstract at: Hypothermia and Afterdrop following open water swimming,: the Alcatraz/San Francisco Swim Study, – Thomas J. Nuckton, MD, David M. Claman, MD,Daniel Goldreich, PhD, Frederick C. Wendt MD, John G. Nuckton, MD, 2000.