Not sure if anyone is still dealing with this problem, but here is something for consideration.
Over the past decade or so, my stride had gone from somewhat wonky to completely disgraceful. Everyone has little form problems that they might want to address, but watching some finish line videos of myself, it was obvious that there was a huge problem of some kind. My right leg felt like it was doing most if not all of the work, while my left flared out badly. It got to the point where parts of my left side were going numb completely when running for anything over 3 miles or so. And I felt how I looked: it is hard to describe, but from the hips down, I felt "out of whack," like everything was crooked and imbalanced. I was beginning to get some fairly severe lower back pain as well.
I had tried addressing the problem with chiropractic care, which didn't really help. After this, I found an athletic trainer of some renown who was offering a functional movement assessment to help detect and address muscle weaknesses in athletes. I decided to get evaluated by this person. My own theory at this point was that years of having a very sedentary desk job had caused a lot of weakness in my hips, which was creating the issue.
In the process of the evaluation, he noticed that in fact my left leg was very noticeably weaker than my right, all the way down to my feet. For example, I could not resist my left foot being turned in no matter how hard I tried to resist, whereas I could quite easily resist with my right. I could also do things such as one legged squats, calf raises etc much more in my right leg than left. In addition he noticed what I had known about for years and ignored, that when put into certain positions my body would start experiencing fairly strong, uncontrollable tremoring/shaking.
This trainer referred my to an orthopedic doctor who also evaluated me. He was quite flummoxed by my symptoms, saying (in so many words to save time) that they didn't quite match up to the typical problems that might be associated with them. He expressed concern that the problem could be MS, and ordered an MRI of my brain and some other testing.
I was obviously saddened by this news but was relieved when the tests came back negative for MS. At the same time both the doctor and I wondered what the problem could be. He ordered an additional nerve test (EMG) to see if there was a problem on that level.
In the meanwhile a nutritionist friend of mine was talking about it with me and she said something to the effect of "I wonder if there isn't just some toxicity in your body at a very high level." I dismissed this as sounding too much like pseudoscience initially. However, as I continued to wonder about what was going on with my body, I thought about the fact that I had consumed massive amounts of aspartame for years. There was a time that I consumed about a 2 liter of diet soda per day, and although at this time I didn't get quite that much, it was still multiple cans of diet soda in the evening, as well as diet energy drinks during the day. Some internet research revealed that overconsumption of aspartame did lead people to have MS like symptoms, including weakness in one side of the body and tremors, in addition to a whole slew of other nasty symptoms.
I was still somewhat skeptical, but I decided to cut out not only aspartame but all artificial sweetners from my diet. I experienced some pretty strong withdrawal symptoms, but after about a month or so I noticed my legs feeling different. It no longer felt like my right leg was doing all the work, in fact my left leg was quite sore. I theorized that this was because for the first time, it was actually doing some work in carrying my body instead of flopping around ineffectually. In addition, the tremors greatly reduced both in frequency and strength--they do not come on as frequently and I can actually will them to go away with concentration--and the lower back problems that I had been experiencing disappeared completely.
I went through with all the tests recommended by my orthopedic doctor out of an abundance of caution, but was fairly sure I had identified the problem. And indeed, the multiple MRIs of my spine, brain, and the EMG all came back completely clear. It seemed that I was a perfectly healthy person with no medical problems that could cause my symptoms. In the end the orthopedic doctor recommended "deep breathing techniques" to control the tremoring, and going back to the athletic trainer for additional strengthening of the left leg. I didn't press the issue since in my mind, I had discovered the true cause.
It has been a bumpy road thus far, my left leg is still trying to catch up with my right. But I have felt better running than I have in years, and there are muscles being activated and worked that I can't remember the last time I felt. So if anyone who has posted on this issue in this thread has also consumed huge amounts of artificial sweeteners, I would definitely suggest looking at that as a possible cause.