What can cause sudden rubbery legs?

It happened to me just this morning; I was running up an escalator to catch a train. Near the top, my legs suddenly went completely rubbery. I took a few steps, but I knew I wasn’t going to make it. Indeed, I didn’t; I fell. From then on, I had trouble walking… because I struck my leg below my knee when I fell, and I’ve been in varying levels of pain all day. Because of that, I started wondering about it; it’s the first time it’d ever happened to me.

Since medical advice is off-limits, I’d like to keep it off my specific circumstances and in the realm of generalities. What can cause sudden rubbery legs like that?

Sounds like you ran your leg muscles out of glucose/glycogen.
It takes a while to recover from that, and also makes you susceptible to muscle cramps.

Perhaps cataplexy, which can manifest as “muscular weakness which may range from a barely perceptible slackening of the facial muscles to the dropping of the jaw or head, weakness at the knees, or a total collapse”?

Since the question has been answered seriously, I nominate this thread.

N.B. I actually like insects, and I know how to use it to my advantage. Heheheheh.

As a generality, how quick can that happen? Why does that happen?

Um, unless it took two hours to run up the escalator, I doubt it was glycogen depletion. (Unless the OP had not consumed any carbohydrates for a long period of time).

I’ve had that happen before, but usually only after a prolonged period of exertion, when my leg muscles just would not move anymore. And if I tried to keep using them they’d start shaking like mad every time they tried to contract. Does that mean they’ve run out of fuel?

Perhaps it has something to do with your creatine levels?

Another surefire cause of rubbery legs (far more pleasurable than the above): the knee trembler.

Simple dehydration can cause this. There is no warning, no “seeing it on the horizon”; just sudden rubber legs and a need to pass out. Been there, done that.

Sometimes, I think it is just a neural glitch - un-coordinated nerve action of some sort. It occasionally happens to me during exercise class, generally when I kick off for a sprint/shuttle run. I am certainly not glycogen depleted, my legs just fail to coordinate properly. It may be a confusion about which particular running action I should choose, particularly if it is part of a relay and there is additional pressure. Fortunately, I generally manage to stumble along until I get some rhythm, and then I am ok. I haven’t faceplanted yet.


I imagine that various electrolyte disturbances could manifest as “rubbery legs”, although I’d expect more than just the leg muscles to be affected. Low potassium, in particular, can cause muscles to go wonky.

Then, there are the various causes of “proximal muscle weakness” where weakness of the muscles closest to the trunk (and trunk muscles themselves) tend to be most affected. Typical symptoms are things like having trouble going up stairs, rising from a squatting position, and even rolling over in bed.

This happened to me last year playing baseball. I hit the ball and went to run to first. My second third and fourth step was like my legs were made of rubber and did not want to hold me up, very strange sensation for sure, but then on the fifth step my legs kicked back in and away
I went. It has never happened before or since this one time, but it definitely freaked me out.

What’s a “long period”? I had no idea that carb deficiency could do something like that.

If you take a statin for cholesterol, it could be rhabdomyolysis.

In this sort of instance, what happened is that your muscles used up their supply of ATP faster than you could replenish it; at that point the muscle is unable to work at all.

Muscles contract when the breakdown of adensonine triphosphate (ATP) to adenosine diphosphate (ADP) creates movement between actin and myosin molecules. There is only a tiny amount of ATP available at any given moment (perhaps a few seconds at maximal activity), and when you run out, the next quantity of ATP is obtained from anaerobic glycolysis–the breakdown of glucose without using oxygen. This only works for a short while (a minute or two, say, with maximal exercise). After that, you have to switch to aerobic (oxygen-rquiring) pathways to create more ATP.

If you burst up a flight of stairs, you can deplete your immediate ATP stores within seconds and your muscles will turn rubbery until your body can make more ATP. How rapidly you run out of ATP depends on the degree of effort as well as your underlying physiology, including the type and relative makeup of your muscle fibers. How rapidly you can replenish ATP also depends on your physiology and can be affected by such things as training and underlying ability to deliver oxygen to the muscles.

Note: This sort of problem is not related to nerves, electrolytes or muscle breakdown (rhabdomyolysis) if your physiology is otherwise normal.