Our expert says:
It could be any one of a number of possible things. The most obvious one, which may seem rather silly, but is quite common, is that your shoes are laced up really tightly cutting off circulation! So simply tying less tightly can help! Don't laugh, it happens often, even to experienced runners.
The second, also shoe related, is that your shoes are too narrow or too small, is there any chance of that.
Then I would be curious to know if they same thing happens when cycling, or if it's just running? If it is cycling too, then perhaps the muscle is compressing the nerve, occluding blood flow or something similar. This is unlikely though, I'd have to say it would be very unusual. But if the shoes doesn't work, then I would suggest you see a doctor, because it is possible that you have some sort of neural problem - that much is clear from the symptoms of numbness. that could reside in the lower back, the hamstrings or the calves and feet, so you'd need to be checked out to disocover which it is. But hopefully it's just the shoes! Another thing that does sometimes help is stretching the calves, which is something I would suggest you do, and see if this helps.
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