A warm up is intended to raise the body temperature and prepare a player physiologically and psychologically to compete in a competitive game. Research has suggested that the optimum duration of the warm up period, before flexibility or functional activities are undertaken, should be between 15 and 20 minutes. This should consist of a gradual increase in intensity until the player is working at 70% of maximal heart rate. A warm up at this intensity has the effect of allowing an increase in the range of movement of the joints and improving aerobic performance. This means the player becomes more flexible and running efficiency improves.

A warm up produces a 2 to 3 degree rise in body temperature that can last for 45 minutes. This increase in temperature leads to beneficial changes in body tissue:

• The heating effect allows muscles and tendons to become more extensible. This makes stretching muscles and tendons easier and more effective. Research has suggested that this decreases the incidence of muscle strains.
  
  
• There is an increase in blood flow, which means that there is an increase in oxygen to muscle tissue.
  
  
• There is an increase in the temperature of the blood, which changes the partial pressure of blood gases. This means that more oxygen leaves the blood and enters muscle tissue.
  
  
• The increase in temperature causes a rise in enzyme and metabolic activity. This improves the efficiency of muscle contraction.
  
  
• By carrying out functional activities such as sprinting or kicking the ball in the later stages of the warm up, there will be an activation of neural pathways, which speeds up reaction time during a match.

In addition to the physiological effects, the warm up has the effect of preparing the player psychologically by encouraging them to focus on the physical activity to follow.