If you have never felt the pain and discomfort of cramping during training or competition, consider yourself lucky. Cramping will slow you down; unfocus your mind and often leads to a dreaded DNF and perhaps the medical tent or nearest hospital. No doubt, many of you have experienced bouts of cramping, especially if you have been cranking out longer miles in high temperatures.

For as long as “hot endurance” events have been around, cramping has kept company. This is contrasted with “cold endurance” events, which tax the system in very different ways and are rarely associated with cramping.

Based on my experience with triathletes suffering from heat-related illnesses, the incidence of cramping in hot endurance sports is increasing. This would have to be related to the increasing length of endurance events, coupled with higher participation rates.

Where and why do you cramp?

Cramping typically occurs in muscles being used the most. For example, tennis players can experience cramping in their forearms, runners in their calves, and cyclists in their quads. Endurance athletes usually get nailed in the quads and calves, and the cramping typically hits during the run portion of the event, when fatigue and other physiological disturbances are at their peak.

There are two cramping theories. The more established theory, which I will call the “electrolyte theory,” relates cramping to electrolyte disturbances. These occur due to sweating. Sodium and potassium are the electrolytes mentioned most often, and normal levels of these ions are crucial for neuromuscular function.

Some relevant “sweat facts” are as follows:

Human sweat typically contains anywhere from 700 – 1200 mg of sodium per liter, and 180-400 mg of potassium per liter. (Though our sweat testing of many athletes with “heat” related cramping issues have revealed sodium losses of 3000mg and upward per litre of sweat loss in some individuals).
Your sports drink (considering many popular drinks) may only contain between 200-400 mg per liter of sodium, and 50-120 mg per liter of potassium.
Average sweat rates are about one liter per hour in the heat, though there will be wide variation from person to person.

Your gastrointestinal tract (stomach-intestines) is limited in the amount of fluid it can absorb, an average for most people would be around 1 liter per hour, though, again, wide variation does occur.

You probably don’t drink a liter of fluid per hour…

And therein lies the problem.

If cramping is due to electrolyte imbalances (and I believe very much that it plays a key roll), then the above numbers tell the story of how easy it is to drop your sodium and potassium levels with prolonged sweating. This is not helped by a low rehydration rate and the low electrolyte levels in your sports drink.

Furthermore, while sodium and potassium are likely the most important electrolytes to consider, calcium, zinc and magnesium are also worth mentioning, as there are may anecdotal reports of how supplementing with these minerals has helped cramping athletes. A newer theory, which I will call the “spinal reflex theory,” explains cramping as being the result of neurological alterations that occur at the spinal cord level, and these alterations secondarily cause muscles to cramp.

The physiology with this theory is somewhat involved, but the idea is that fatiguing muscles send carious nervous signals to the spinal cord, which secondarily sends reflex nervous impulses back to the muscle, and this causes spasm and cramping.

Both of these theories are relevant, but in my opinion, do not tell the whole story. Human physiology is complex and composed of many interdependent processes.

Other important factors to consider are:

Dehydration, resulting from large volumes of water lost in sweat.

Lactic acidosis (lactic acid build-up), caused by working close to or at your anaerobic-lactate threshold.

Hypoglycemia (low blood sugar), or bonking, or hitting the wall, resulting from inadequate replacement of easily absorbed, usable carbohydrates.
Combine these with electrolyte abnormalities, and you can see how messed up you really can become! Any “spinal reflex” contribution, if it exists, would have to be affected by theses abnormal physiological states.

Therefore, cramping, along with the many other heat- related symptoms ñ including headaches, nausea, dizziness, vomiting and diarrhea – is likely related to all of these abnormalities working together.


As these four abnormal physiological states are responsible for cramping; Prevention is a fourfold proposition:

To prevent dehydration, drink! If you lose a liter per hour, you must put back at least this amount. Weighing yourself before and after your event will give you an idea of how much water loss you have experienced. Remember, 1 liter of fluid = 1 kilogram = 2.2 pounds.

To reduce electrolyte depletion use a sports drink that contains physiological quantities of electrolyte (can it be any simpler?). As mentioned, you will run into problems using drinks that have significantly less electrolyte in them than what you are losing in your sweat. Furthermore, look for a drink that recognizes that the ration of sodium: potassium in sweat is about 3:1 to 5:1, which means that the ratio in the drink should be about the same. Some drinks have more potassium than sodium in them, ignoring these fundamental physiological ratios.

Also, the addition of zinc, magnesium and calcium is beneficial. Salt tablets are an inexact way of replenishing lost sodium and typically do not contain other electrolytes.

To reduce lactic-acid accumulation, use a sports drink that contains a lactic-acid buffer. Not many do! Citrate is an excellent buffer, as this compound is converted in your body to bicarbonate, and bicarbonate is the most important substance in the body for buffering lactic acid. Bicarbonate itself is not generally included in sports drinks because it is irritating to the gastrointestinal tract; therefore citrate is definitely the way to go.

To reduce hypoglycemia use a sports drink that has easily absorbed and readily useable carbohydrates. Simple sugars like glucose or dextrose are best. Dextrose is great for this (in fact, dextrose is the sugar used in intravenous fluids when rapid sugar replacement is required). Dextrose easily passes through your gut wall into the blood stream and requires no processing by your liver before you muscles can use it. It is also non- irritating to your gut and not too sweet.

Beware of “sports drinks” that contain fructose. Fructose, fruit sugar is very sweet, can cause gastrointestinal irritation. You want a sugar that gets in and keeps down so you can utilize it. Ideally it will require minimal processing so that energy is available when you need it. Sucrose tends not to be such an issue to the gut compared to plain fructose because it is cleaved in the small intestine.


The abnormal physiological processes that contribute to cramping are dehydration, electrolyte imbalances (especially sodium), lactic acidosis and hypoglycemia. When present together, all of these factors act synergistically to alter the way in which muscles contract and relax, leading to cramping. Optimal prevention means addressing all of these processes.

For tips on adding/customizing your electrolytes click here.

Dr. Douglas W. Stoddard is the medical director of the Toronto Sports & Exercise Medical Institute, eload sport nutriton.

“Inside Triathlon”