This post is outside my teaching program this term, but it should be a useful reminder for all the athletes out there:
Yesterday, after a soccer game in Kingstown, I talked to a student re. muscle cramping and later revived my understanding of the underlying causes – here is my summary:
Muscle cramping during exercise seems to have two major causes (and 100 suspected causes): (1) a nerve muscle fatigue syndrome (“sick muscle”) where you have to stretch against the contraction but won't be able to control the situation such as to continue exercising and (2) inappropriate systemic fluid and sodium levels, precipitated by inadequate pre-hydration, sodium intake or intermittent fluid intake during prolonged exercise. Forget about calcium, potassium and magnesium. Sweating looses a lot of water and sodium. Just taking water or juices may suffice but could not overcome the cramping risk if too much sodium has been lost: it is better then to take salt containing sports drinks or home-made fluid (½ tsp of salt/0.5 L). Definitely do not use pop drinks containing caffeine/theophylline: these methylxanthines cause diuresis and fluid and sodium loss, exactly the opposite of what you need.
When biking around the island I noted once that I precipitated some leg cramps within minutes after taking some fresh water from roadside taps. When repeating the exercise later and taking along some Gatorade, I had no problems. Unfortunately, the roadside rum shops carry no water or sparkling water – just pops.
Happy running, biking, playing!
Hi Dr. Baer :)
ReplyDeleteI was wondering, is the excess muscle contractions in cramping caused directly by imbalance of sodium ion and its effect on action potential generation or is it caused indirectly because of the disruption of normal osmolarity and its effects on different ion channels, such as calcium channels, in the muscle cells?
I'm leaning towards the latter option, but I can be missing the point entirely. Also a little off topic, do you think that patients with cystic fibrosis who have higher salt secretion in their sweat will be more prone to cramping (aside from the typical symptom presentation)? Thanks.
Concerning CF: cramping is not among the typical symptoms of CF (although that might be a possibility during exertion - but I doubt that CF pats. will exert themselves a lot), but in some blogs there is talk about cramping, maybe due to loss of Mg. Interesting thought, though.
DeleteRe. mechanisms, low sodium of course would reduce the Na gradient in excitable cells and thus reduce the driving force for Ca transport. Have to look into this some more.
The mechanism of lo-sodium-induced cramping is likely this: The sodium gradient is responsible for pumping Ca out of the cell against a Ca concentration gradient. When low external sodium diminuishes this gradient, not all calcium is pumped out and that leads to a phenomenon called Calcium-overloading, in turn maintaining contraction. This definitely happens in cardiac muscle (= adverse effect of cardiac glycosides) and probably also in skeletal muscle.
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