Re: mechanism of magnesium sulfate in a bath for sore muscles

Hello Holly,

I do not believe that soaking the body in Epsom salts would particularly 
help the muscles to relax for one reason.  The skin is designed to be an 
impermeable barrier between the inside of the body and the outside world.  
Therefore the magnesium sulfate in the bath would not be able to get to the 
muscles since they are behind the protective layer of skin.  Especially 
since they would contain ionic charges and be dissolved in water.  There 
are some modern medicines that have been designed to be absorbed through 
the skin (e.g. the nicotine patch and ibuprofen gel) but these drugs are  
in a special preperation that enables them to pass through lipid bilayer 
membranes, which I do not think magnesium sulfate would be able to do.  I 
would therefore think that it is most likely that it is the warm water that 
has the relaxing effect on the muscles.

Despite my determination that this is the case I have never the less gone 
on to have a look at what are the effects of magnesium sulfate in the body 
and I have found that this is sometimes used clinically as a muscle 
relaxant under certain circumstances.  Intravenous magnesium salts were 
once used as anaesthetic agents.  This is because the magnesium ions 
compete for calcium ions that are involved in the release of substances 
called neurotransmitters from nerve terminals.  This effect occurs both at 
the muscles and also centrally in the brain so it results in paralysis and 
unconsciousness.  This is interesting because this means that the magnesium 
salts do work as very strong muscle relaxants (I would say paralysis is a 
very strong muscle relaxant effect).  Nowadays we have much better 
anaesthetics so magnesium salts are no longer used for this effect.

A search of the recent literature suggests some more uses have been found 
for magnesium sulfate in modern medicine utilising its muscle relaxant 
properties.

Post operative analgesia
Acute bronchospasm and asthma
Inhibition of uterine contractions in premature labour
Acute myocardial infarction and emergency treatment of serious arrythmias
Ecalmsia 

Therefore magnesium is still used today for its muscle relaxant properties.

If you threw a large handful (say 500g) of magnesium sulfate into a bath of 
water (say 200 Litres) this would give a concentration of magnesium sulfate 
in the bath of around 10mM.  For myocardial infarcton magnesium sulfate can 
be given as an initial intravenous injection of magnesium sulphate 8 mmol 
Mg2+ over 20 minutes followed by an intravenous infusion of 65-72 mmol Mg2+ 
over the following 24 hours.  I very much doubt such quantities of 
magnesium could be absorbed from lying in a bath of 10mM magnesium sulfate. 
 Also note that magnesium sulfate is very poorly absorbed from the 
gastrointestinal tract and for this reason is used in the treatment of 
constipation.   It makes the gi tract retain water by an osmotic effect and 
therefore 'accidental' drinking of the bath water is also not going to have 
any desirable effects.

Magnesium sulfate is unlikely to have very much of a muscle relaxant effect 
in the bath water and I suspect that your suggestion that it is the warmth 
of the water that has the muscle relaxant effect is most likely to be 
correct.

Thank you for your interesting question.

Dave Burton