system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do people start to vibrate when planking?
gpt
The muscles of the core become fatigued with prolonged contraction, and when they do they relax. 

However, your brain is still trying to keep them contracted, so they flex again and relax again once they  fatigue. As a result, you experience oscillating muscle contractions as your brain fights against your muscles.