Questions & Answers about yùndòng yǐhòu, jīròu hěn lèi.
Because 运动以后 means after exercising or after exercise.
In Mandarin, a time phrase like after doing something is often built as:
verb / event + 以后
So:
- 运动以后 = after exercising
- 吃饭以后 = after eating
- 下课以后 = after class
This whole phrase sets the time for the main statement: 肌肉很累.
The comma separates the time phrase from the main clause:
- 运动以后, = after exercising,
- 肌肉很累。 = the muscles are tired.
This is very common in Chinese when a sentence begins with a time or condition phrase. The comma is not always absolutely required in casual writing, but it is very natural and helps readability.