First of all, it naturally depends on the nature of the injury. Regardless of injury, you should recognise that   during sport, you will feel much less pain than normal. This will make the injury feel less severe than it really   is.

  Stop immediately if the injury feels unusual, rather than just a knock. Unusual injuries, where there is some   sharpness of pain, or restriction of movement may be the result of a muscle tear or strain.

  For such injuries, rest as fully as possible for at least 2 days, elevating the leg, and applying an ice pack for 20   minutes every 3 hours. This reduces inflammation, thereby speeding recovery.

  Physiotherapy can then help heal the injury, especially with regard minimising the knotting of scar tissue.

  During sport, your arms and legs gradually become fatigued. They communicate this to you by contracting -   tightening up. Sometimes, this can result in cramps.

  In more extreme situations, however, you may push your contracting muscles too much - as you stretch in a   run, for example. These opposing forces can and will tear the muscle, or even detach ligaments from bones.   Follow the advice above if this happens.

  These habits tend to stretch the spine and muscles at the upper back and neck regions. This places the spine   under a lot of strain, and the muscles in turn can incur minor tears. In some cases, this stretching can lead to   tension headaches.

  Whilst the back is being stretched, your front is compressed. This literally compacts the internal organs,   impairing their performance.

  Read Sitting pretty to learn how to correct your posture. It is well worth the effort.