What causes bent knees to ache?

Factual Questions
1

I could go a lot further on my motorcycle if after a few hours my knees didn’t start aching. What’s the cause of the hurt?

2

WAG. The vibration maybe?

3

When your knee is bent, some of the muscles are stretched. This triggers a reflexive muscle movment to straighten out the knee, to return the muscles to their ‘natural’ position. This means that while your knee is bent, you have a continous/repetati9ve isometric excercise. After a while, your muscles ache. Sitting still, with your knees unable to stretch, is unatural.

I’m not a physiotheripist. Don’t quote me.

4

How old are you? I can do that sitting in an easy chair.

Tris