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My left pedal is sending a slight knock to the sole of my foot at every turn, at the 1 o'clock position. What could cause this? It happens on all chainring-sprocket combinations though not with the same intensity.

Edit: this is a flat pedal, JIS square taper bottom bracket.

Edit: By "1 o'clock" I mean there's a click in the left pedal when the right pedal is exiting its power sector/downward push, and the left pedal is about to come "over the top" ready for its own power stroke.

Edit 2: Solved! This is a brand new crank set that I installed onto a year-old bottom bracket. I must have installed the left arm wrong. I took it off, cleaned and re-greased the crank bolt tread and the faces of the square tapered spindle, put everything back together, tightened with care, and the knock is now gone.

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    My bet goes to the crank arm getting loose. Try to move the crank itself laterally, by holding on to the frame with your other hand. If it moves, you may just need to tighten the crank arm bolt.
    – njzk2
    Jan 13, 2021 at 21:27

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It can be many kinds of stuff. The obvious to look for are the pedal bearings, the bottom bracket bearings or the interface between the crankarms and the bottom bracket (for 3-piece cranks) or the interface between the crank spindle and the left crank (2-piece crank), but many other things could do it.

It is useful to test the pieces in isolation. Disconnect the chain and try to turn the crank. Try to turn the pedal on the pedal spindle. Do you feel any grinding or clicking? That would point to the right spot. Try to tighten the cranks to the required torque (depends on the exact type, 2-piece vs. 3-piece, the brand...).

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What kind of bike do you have?

Are you using clipless pedals?

It is hard to answer without knowing more.

Some things I have experienced to cause something similar are:

  • Issues with cleat and pedal interface - Some wet lube helps fix this on the cleat
  • Your left crank arm has play in it (never felt it first hand, but is possible)
  • Something is going on with your bottom bracket bearing only on the left side.

I usually would take the bike to a LBS to get better diagnosis when something like this happens. The last time that was for me was when I took a road bike through about 20 inches of water. I learned a lot about bearings that day - ¯\(ツ)/¯.

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