When performing high intensity strength training using proper form and a slow, controlled speed of motion additional warm up sets are almost never necessary. In most cases they provide little or no benefit while wasting time and energy that could otherwise be devoted to the “work” sets.
Most of the physical benefits of a warm up – increased blood flow to the muscles, enhanced metabolic reactions, reduced muscle viscosity, increased extensibility of connective tissue, improved conduction velocity of action potentials, etc. – are obtained during the first few repetitions of an exercise. Additionally, each exercise performed helps prepare the muscles and joints for subsequent exercises they’re involved in.
I do not warm up for my workout or any specific exercises, and do not have the people I train warm up with only a few rare exceptions. I’ve been training people this way for two decades and none have been injured as a result. Like most aspects of exercise, whether to perform a warm up or not depends on the individual and the specifics of the workout being performed.
People with some physical conditions or joint problems may find they tolerate certain exercises better or experience noticeably reduced joint discomfort if they perform a warm up set prior to exercises involving the affected joints or body areas. When this is the case only a single warm up set is required, and it should be performed with half or less of the resistance to be used for the work set to provide the previously mentioned benefits while wasting as little energy as possible.
In some of these cases they can perform certain exercises better by first performing a different exercise that involves the same joints. For example, some people’s knees tolerate exercises involving extension better if they warm them up with a knee flexion exercise, and some people whose elbows tend to lock during pulling movements find it helps to perform a pushing movement first.
Some trainers still recommend stretching as part of a warm up, however stretching prior to a workout does not prevent injury, and should not be performed since it can reduce the muscles’ ability to produce force. Stretching is highly overrated and with a few exceptions there is no need to do it at all. Regularly performing exercises for all the major muscle groups over a normal range of motion will help achieve and maintain a functional, healthy level of flexibility adequate for the majority of activities a person would participate in. If stretching is performed at all it should only be performed after the workout.
Darden, Ellington. The Nautilus Book: An Illustrated Guide to Physical Fitness The Nautilus Way. Chicago, IL: Contemporary Books, Inc., 1981
Hutchins, Ken. SuperSlow: The Ultimate Exercise Protocol, 2nd Edition. Casselberry, FL: Media Support. 1992
Enoka, Roger. Neuromechanics of Human Movement, 3rd Edition. Champaign, IL: Human Kinetics. 2002
Herbert RD, Gabriel M. Effects of stretching before and after exercise on muscle soreness and risk of injury: systematic review. BMJ 2002; 325: 468-470
Shrier I. Stretching before exercise does not reduce the risk of local muscle injury: a critical review of the clinical and basic science literature. Clin J Sports Med 1999; 9: 221-227
MacAuley, D., Best, T. M (2002). Reducing risk of injury due to exercise. BMJ 325: 451-452
Fowles, JR. Sale, DG. MacDougal, JD. Reduced strength after passive stretch of the human plantarflexors. J Appl Physiol 2000 Sep;89(3):1179-88.