This topic generated a lot of discussion in my Facebook group recently so I did some research to answer the main questions. This was initially shared as live which you can watch back here but as the sound quality is a bit sketchy I've blogged it as well.
What is the purpose of a warm up?
The main function is as it sounds, to warm the body up and get you into a state that is ready to do some exercise. Physiologically that includes lubrication of the joints, muscle temperature increasing, heart rate increasing blood pumping around the body, breathing rate increasing to ensure there is enough oxygen reaching muscles. This brings the body into a state where it can cope with the demands of the session. It might also help with mental focus, motivation and readiness to exercise. On a busy or stressful day the warm up can help to start to shift some of that energy around, again, so that you are more present and ready to train. There may also be an element of skill practice, for example, if you are going to be moving in a way that requires coordination and focus (think gymnastics or yoga or heavy weightlifting) you do simple versions of some moves building up to more advanced versions later in the session.
Static vs. Dynamic Stretching
Static is considered not helpful when warming up especially for running. A static stretch aims to lengthen the muscle and potentially increase range of motion around a joint, the way body achieves this is via relaxation of the nervous system and release of tension in the muscle which is the opposite of what is needed before a high impact activity. The motion of running comes from a spring-like system of tension and energy release so there does need to be a certain amount of stiffness in the system. Dynamic stretching, moving continuously throughout a stretch, is another way to help increase range of movement while also increasing blood and oxygen flow in order to be performance ready for a high energy or high impact activity. (See video linked below for a dynamic warm up routine).
What is the purpose of your workout?
Ideally the warm up should also be helping to improve the performance of your workout.
Low intensity, or low impact workouts might only need a gentle warm up. Bringing the body from being static (eg. behind a desk) and just opening up the hips and shoulders, with leg swings or arm circles, so that you are able to move more freely. Perhaps just walking for a few minutes. For high intensity or impact, e.g. HIIT or sprinting where bigger demands are placed on the muscles and joints would benefit from a longer more energetic workout. Specific dynamic stretching or explosive drills also help to fire up the nervous system. In the gym you might do lower weight versions of the main exercise, e.g. barbell squats or could use the crosstrainer as both would create warmth in the muscles. If your goal for the workout is to increase maximum weight lifted and to push the body hard you might find it more beneficial practicing the same movement patterns that you will be doing in training as part of your warm up and building up the weight gradually. Any explosive movements, such as sprinting or big lifts, benefit from the dynamic stretching mentioned above.
Summary
Do make sure your warm up is related to the workout you're about to do, for example, press ups would not be appropriate before a run. For a more general workout not at a high intensity something like a static bike or cross trainer would do the job of getting muscles warmer.
Do go straight from the warm up into your workout. Warming up then standing around chatting for five minutes is not helpful as the body will cool down again. Making what you do progressive, walk < jog < run, should help with this.
Don’t do something you hate. Don’t do something just because you know you need to do a warm up. Make sure it is enjoyable. Perhaps putting on some energising tunes and dancing around for a few minutes!
Do dynamic stretching rather than static before power / strength / sprint type workouts. Static stretching may be appropriate for a yoga practice or as a recovery activity.
Do keep it simple and avoid doing too much. You could easily spend 15-20 minutes doing all kinds of weird and wonderful warm up type exercises but then be fatigued and underperform in the main part of your session. The main purpose of a warm up is to enhance the quality of your main workout, not leave you too fatigued to perform.
Warm ups are about getting the body ready to exercise. Keep it relevant and match it to the level you're intending to train at. Make sure it is something you enjoy doing and that is going to help you feel ready to go!