Warmups
Most warmups don’t take very long, just two-three minutes or so. To benefit us the most a warmup should work the same muscles we will be engaging during the main workout - they should include lighter exercises or a toned down version of the training ahead.
Examples
If you are a runner, your best warmup is a light jog.
If you are doing martial arts training a warmup should also include a few light kicks and punches.
If you are doing bodyweight workouts basic body (neck, arms, torso, legs) rotations will get your blood flowing.
With bodyweight training we can do without an actual warmup if we use one of the sets to warmup and get into it, moving at a slower pace, jumping lower and moving slower.
The temperature of when and where you workout plays a difference. In winter our body keeps most of the blood flow away from our extremities so warming up will take longer. In summer with higher ambient temperatures, the body is already half-warmed up and our muscles are loose. Many Olympic athletes tend to ‘chase the sun’ for that reason training in Australia and California so they spend all year in a summer environment.
Stretching
Everyone can get flexible if they work at it. Some people have a natural gift for flexibility others have to struggle to gain every little bit but everyone can get there. It takes persistence and regular training and the muscles adapt and respond.
The best time to stretch is post-workout when our muscles are completely relaxed, thoroughly warmed up and capable of giving us the most stretch, easiest. When muscles are really well warmed up they exhibit a high degree of plasticity. That means that not only do they stretch but after stretching and cooling down they maintain an increased range of movement and display greater flexibility.
This is the reason why everyone should stretch: range of movement. Whether you are a boxer or a ballet dancer your sport will necessitate your muscles performing by working through a range of motion. Muscles that are tight and constricted tend to work against each other to perform the move. That means that energy which should be used in the move itself is actually soaked up by fighting against the muscles themselves and the supporting muscle groups. This leeches away power.