To jump on a bike and pedal will raise your core body temperature and get the blood pumping, although we need to make sure all our joints are ready for even this.
We need to think about the range of motion our joints move through when we exercise...or even going about our daily business
When coming from cold, a night asleep or a period of inactivity every joint in our body will somewhat 'stiffen' or lose this range of motion.
Even a slow start moving to a
faster pedal on a bike won't take your hips, knees and ankles through full ROM (Range of Motion).
On the other hand taking time to mobilise every joint in the body will ensure that each joint has a good ROM and nice and supple for the activity to come.
An excellent start point is just trying to move every joint round three 360 degree
circles at some point every day
Any good PT will tell you that the mobilisation part of any session is the ONLY part you can't cut back.
If I have only 40 min to work out, my warm up session will stay the same 15-20 minutes that it would be if I had 2 hours spare.
It also gives us a time to
connect to your body and feel for any niggles or twinges. I've often changed a program before the work out starts due to a tight Quad that hadn't been noticed until the mobilisation started.
It is why the first question ANY heath professional will ask when you walk through their door with an injury will be:
"Did you warm up first?!"
Here is a link to a basic intro to warming and mobilisation I use with my clients.