I'm wondering whether anybody can help me get my head around what the difference is between 'inconsistent' weakness and muscle fatiguability. I will give two examples from my recent neuro exam.

1. Trendelberg's sign (inability of left leg to sustain weight, causing collapsing left knee):

So, I have Trendelberg's sign. If I stand on my left leg the knee gives way and I fall over. When I walk I get a left sided limp due to this, which gets rapidly worse over space of a few minutes until the right knee gives way too and I fall. The only way of getting over this is is for me to keep my left knee locked while standing and walking which seems to get me a bit further but which still ultimately results in falling. For some reason I can't seem to just lock my left knee on my own, that's why it won't support the weight when standing just on that leg.

After I fell over from Trendelberg's I got up using my right leg and then got back to the bed by locking the left knee. Then they did the neuro exam and found I wasn't able to lift the left leg off the bed at all and had 'collapsing' weakness in the ankles. They said I would not be able to walk at all had my legs really been this weak.

I should mention three years ago I got an L5/S1 disc tear which has left me with a reduced reflex in my left ankle but which I am told does not explain the collapsing knee.

My feeling is, had they tested my walking after the neuro exam, the neuro exam would have been better and the walking worse. Does this mean the muscle weakness is 'inconsistent' or merely fatiguable? They told me it's not due to muscle fatiguability but didn't give an explanation as to why.

2. 'Distractable' tremor on holding arms up out in front of body

When holding my arms straight out in front of my body, which I find difficult, speech becomes incoherent within 30 seconds. Arms begin sideways circular tremor. After about a minute I collapse and fall off my chair. When asked to pat my left thigh with my left hand while continuing to hold the other upwards I had great difficulty in separating the arms. When I finally lowered one arm to do this the tremor in the right hand ceased. This is why they describe it as 'distractable'.

Holding two arms up in front is harder than holding one. The effort exerted in holding two might result in tremor but the removal of one arm, and therefore less exertion, could mean that the tremor disappears? Is this possible?

What I'm wondering with both these examples, is: Is it at all possible these signs can be, viewed from another perspective, be explained by muscle fatiguability? Is there any scientific explanation as to why this happens? Does this happen to anyone else?