How to warm up
Warming up before a strength session is very different to warming up before a more dynamic training session that may involve jumping, changing direction at speed and highly co-ordinated movements like Olympic Weightlfing.
Warming up to lift weights for strength and hypertrophy
A good warm up would involve doing 1-3 sets of 10-15 reps of the first exercise on your program for the day.
For example Bench press using Dumbbells.
Choose a lighter set of Dumbbells that you can lift comfortably for 10-15 reps as this will induce blood flow to the muscles about to be used and force the nervous system to start coordinating those joints.
Though these initial sets take time they will set you up for more focused result as the weights get heavier.
Warm up sets are also a chance to identify any issues you may be having that day such as tightness in one shoulder and can give you the opportunity to stretch it out between sets.
Warming up for a sports or CrossFit training session
In this kind as so many more movements rather than just body parts are involved the warm up needs to be longer and more dynamic.
Spending 3-5 mins doing aerobic work like on a stationary bike, rowing machine or light jogging will warm the joints, enhance circulation and produce blood flow to the working muscles. Next taking all joints through the range of motion that will be experienced during the workout is important and in this dynamic stretching takes place which is where muscles are taken to end range briefly, repeatedly but not held in a static position for extended time.
If changing direction is involved setting up an agility ladder or series of obstacles is beneficial during the warm up, and if weightlifting is also involved doing some reps at 30% of the weight being used during the session is also beneficial during a good warm up of this kind.