Hi Dan,
My mother used to tell the doc for years that she felt incredibly heavy all over, and he just acted like she was a loon. Doctors call this symptom neurological weakness. Your muscles are fine but it feel like a huge effort to move them because the nerves are not firing on all cylinders.
If you put neurological weakness into search engines you'll get a lot of info on potential causes.
http://www.asktheneurologist.com/neurological-weakness.html
This link gives you the basic definition which explains it better than I can.
Neurological weakness
This is a frequent symptom which may be defined as:- "decreased muscle strength compared to perceived effort"
When neurological weakness is suspected, the first thing a neurologist will ascertain is whether the complaint is of "general weakness or fatigue" rather than real muscle weakness which usually occurs in specific areas of the body or certain muscle groups.
"General fatigue" is not a specific complaint.
Although it can sometimes signify a diffuse (widespread) neurological condition, it is more commonly due to other conditions not considered to be "neurological" in the classical sense.
Such situations include,depression,"chronic fatigue syndrome" a general "internal medical" illness of any kind (eg.infections, liver or kidney problems, biochemical or hormonal imbalance, and even simple sleep deprivation to name but a few.
Neurological Muscle Weakness
In general,this type of problem preferentially affects some areas and not others.
Some conditions can ultimately deteriorate to severe lack of strength of the whole body (eg AIDP / Guillan Barre syndrome, myasthenia gravis )
However at the initial stages of the condition, the condition will almost invariably affect some areas whilst "sparing" others.
Systematic approach to diagnosing the site of the neurological weakness
e.g. "decreased leg strength"
Starting from the brain and moving down and out (proximal to distal) a problem in the following components can adversely affect leg strength:-
1) Brain (e.g.primary motor area of cerebral cortex)
2) Spinal cord
3) Nerve roots
4) Nerves
5) Neuromuscular junction (where the nerve meets and transmits messages to the muscle)
6) Muscle
It is only after a neurologist decides "where the lesion is" that they move on to investigate "what is the cause of the lesion"
In my own case, this weakness is helped a lot with magnesium. I take the max dose, which is 900mg daily. I also take 800mg daily of calcium, which is also essential for nerve health. Your body needs enough of these to do constant repairs to nerve damage, if it is caused by an infectious agent. In my case the infectious agents are bartonella and borrelia.
Some people also get this weakness from not enough B12, so if your dose is not very high, that could be another potential explanation. That's more likely to be the issue if you have poor coordination as well as weakness. In some people it is not a shortage of B12 per se, but of the other things it needs to work. Of course Richvank and Freddd are the ones to ask about that!
lastly, if you have adrenal burnout, then you need less potassium and more sodium. Having low cortisol means you leak sodium out constantly in your urine, and retain potassium. Having these two things unbalanced will certainly make you weak. Have you seen the thread I started on mending the adrenal glands? I basically summarised the whole book "Adrenal Fatigue" by Wilson, and other members added lots of other realy good info. You certainly need to get on the full nutritional therapy for adrenal burnout if you are weak, that has made a huge difference to me. (Sadly it is far from being a quick fix, it has taken me 9 months. It's about slow healing, and I still strongly believe that trying to take shortcuts by using adrenal extract or synthetic hormones is harmful and does ont lead to resolving all the problems; you need to mend your own glands by nourishing them and avoiding substances that harm them).