Hi everybody,
Long post ahead. If you don't want to read all of it, I recommend skipping the part between the dashed lines!
I've been following this thread for some time. I'm 27, and I ran competitively from age 15 until last year (I hope to again when I can!). I got through college with mild ankle issues and one tibial stress fracture at age 20-21 that I made a full recovery from. I ran my first marathon when I was 24 and a marathon every 6 months for the next couple years (I've done 5, 2 of which were Boston). I distinctly remember two races at the end of 2016 where I felt the "loss of leg coordination" issue described in this thread, though I may have felt it on treadmills before that and dismissed it since it was exclusive to those "dreadmills". I had the sensation in a handful of harder training runs after 2016, particularly in the heat on long flat roads as well as treadmills. My way to describe the "loss of coordination": No pain, numbness, or tingling. Didn't feel like a tight muscle cramp kind of feeling. Felt like a feeling of heaviness and extreme muscle fatigue that prevented my quads from lifting my leg through the full motion of a stride. It was like my legs were locking and preventing me from moving, like a brake mechanism. Meanwhile I would try to fight this sensation, and the result was feeling incredibly awkward and feeling like I was losing my balance. Without being able to cycle all the way through my stride, my feet would jam into the ground and strike with much greater force, which made it that much harder to get my legs to lift again. I felt a kind of disconnect between my muscles and my brain, but I don't think there was actually a disconnect. I think it was just the muscles unable to respond to the messages my brain was sending. This was exclusive to my upper legs, and at first it was both, and then eventually I could tell it was more of an issue on the left side. I never felt like my foot splayed or dropped, just an awkwardly forceful landing on the ground. All signs point to classic muscle fatigue and my upper legs just not strong enough to sustain the paces I was trying to do. This was how I defined the issue until the fall of 2017, and I called it the dreaded "wobbly legs".
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I had just run my best marathon in Boston in April of 2017, and I didn't take the time off that summer that I should have. I ran a half marathon in the fall where I felt the wobbly legs after only about 4 miles on a long flat section of the course. When I tried to run the week after that, I felt like my left quad was strained. When I wasn't running, I felt like my left quad just wasn't all the way there, and the wobbly legs feeling began to be exclusive to that side. I developed a limp in that leg, but I thought I had some kind of nerve conduction issue because my pain had decreased and was replaced by a feeling of weakness even when I was walking. I took about 2 weeks off (I should have taken much more if I wanted to run Boston in 2018). After that time, the limp had subsided, and I began running again. I made it through about 6 weeks of training for another half, and the wobbly leg was back again. For a couple of strides near the end of the race, the left leg buckled, and the quad wouldn't lift properly again. I was able to finish and took about 3 weeks off before starting Boston training. Side note: I had weird anxiety symptoms after this race too, which I attribute to overtraining and onset of the injury it took me until February to start realizing what I was really dealing with.
Boston training went alright for a couple months before the wobbly legs, most particularly the left leg, began to set in sooner and sooner on my runs as I tried to push paces faster. I even would notice that I would get a sudden shortness of breath feeling along with it too, which was purely a nervous reaction, like a flight or fight response. The last race pace effort I did was a 10k in February of 2018, where I had another moment of wobbly leg and sudden shorter-ness of breath (Running of course makes me out of breath already, but this was like a sudden fear kind of gasp that stopped my breath for a second). I jogged a bit on some grass on the side of the road, and then I was able to finish out the race like normal. So weird, why did that help? A couple of weeks and a couple of long runs later, my quads, particularly my left one were just shot constantly. Worried about a potential injury, I tried to bike instead. That didn't help. I got off the bike one time, and my left leg felt heavy, and I even had tingling in my hands (Again, a nervous response. I was a nervous wreck and thought about everything from multiple sclerosis to dystonia).
I ended up seeing a chiro who thought it could be compartment syndrome, but he did a test that finally gave me a clue. He put a stethoscope on my pubic bone (I know, awkward), and tapped on each knee. And the left side was distinctly duller. STILL, the chiro suggested I see what running felt like since I'd taken a week off at that point. I ran and woke up the next morning with a distinct pain in my left quad. It got progressively worse just from walking throughout the week. It was so bad I went to urgent care just to get crutches. Okay, enough of this story. Is it obvious yet?
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I had a femoral stress reaction! Confirmed by MRI in March of 2018. No wonder soft surfaces helped! The doctor couldn't find an actual fracture line, but he did find a ton of bone swelling. And I believe that in my case, the wobbly legs were extreme muscle fatigue that contributed to bone stress. The locking and braking mechanism I mentioned earlier could be some kind of defense mechanism against injury. Eventually this became more localized to my weaker left side until I ended up actually injured. I also believe the anxiety issues are somehow related to the stress my body was under from overtraining and eventual injury.
I am now running 2-3 miles, and my leg still wobbles a bit from time to time as I try to push harder, and now I know this is where I should stop until my leg gets stronger. In my case, the loss of coordination was a sign that I was overloading the muscles in my leg and putting all of the force on the bone. Also, for me, time off has resulted in improvement, though I still have a long way to go. I realize that for a lot of these posts, time off didn't fix the issue. I still wanted to share in case anyone else is defining "loss of coordination" the way I did. It COULD just be run of the mill muscle fatigue from overload that can lead to serious injury. It COULD just be that more recovery is needed and more strength work needs to be done instead of just pounding the miles. I have found that glute and hip strengthening is very helpful. If those muscles are weak, the quads have to work too hard. Eventually they get overloaded, and say hello to wobbly legs. Also, I want to emphasize that I have learned from this thread that there are lots of issues that can cause "loss of coordination", and we all define "loss of coordination" differently.