Jerky muscle movements

I think this is probably more of a doctor issue than a physio one. But it’s a muscle issue, numerous doctors have failed to explain it, and physios know all about muscles, so I thought I’d try here to see if I could get any pointers.

I have an issue where slow muscle contractions are normal but medium/fast movements are jerky. Visualise the following demonstration: me sitting in a chair, hands on knees; I can raise my hands slowly to any height, stop where I want under full control with no resting tremor, lift heavy things, all completely normal. But if I try and do the same thing quickly, my arms move in a series of jerks, though I can still stop where I want with no resting tremor.

The symptom affects both arms and legs – I get it trying to cycle where a slow cadence is OK but at high cadence my feet jerk off the pedals. I suspect it affects all muscles and maybe small ones more than big ones, but that’s hard to demonstrate.

I’ve seen a neurologist twice who first thought it might be Parkinson’s. I had nerve conduction studies (which were OK), then a year later he decided it wasn’t neurological in origin and there was no point in seeing him again. My GP doesn’t know who else to refer me to. I’ve looked at numerous videos and descriptions of tremors and muscle disorders and can find nothing to match – they all involve resting tremor or involuntary or uncontrolled movement, none of which I have.

My own theory is that it is a problem with contraction and relaxation of opposing muscles. Slow movement works fine, but when moving faster, the relaxing muscle can’t keep up with the contracting one, hence the jerk. My research suggested this could be magnesium deficiency so I’ve been taking a high dose supplement for quite a while – it still could be that, it takes a long time to treat a severe Mg deficiency via the gut.

You could sit drinking with me in a bar or go for a long walk and there'd be nothing to see. But it’s completely debilitating for many every day tasks. Watch me try to clean my teeth, beat an egg or just get dressed in slow motion and I look very old.

Any insight into what might cause jerkiness and imbalance of opposing muscles or what sort of specialist would deal with that would be greatly appreciated.