“Jerks”. One explanation
Jerking movement during the pedalling. This is seen very commonly.
This is why (seen on Electromyography).
This is an Electromyography (EMG) on a post-stroke person's legs (Left paralysis). Blue is his right leg, red is his left. We can clearly see the difference.
This difference causes the jerking movement. Without paralysis, both our legs will show the alternative rhythmical patterns. We now see how well we control our muscles without thinking about anything.
Improvement shown on EMG
By the way, there are 2 sets of EMGs in the photo above. The bottom is after 1 week of a cycling chair experience (20 mins every day). The paralysed left leg's signal hugely calmed down in one week. This makes sense. Straight after the first pedalling, people tend to strain their muscles too much, which does not lead to efficient pedalling. After a while, when the riders get used to the pedalling movement, they don't have to tense up so much, and they start not to use the unnecessary strength just to exhaust themselves. Can you also see the formation of rhythmic patterns on his paralysed side? This shows, he has actually gained a little amount of control on his paralysed leg.