A player experiencing hamstring strain, would feel these symptoms: a sudden sharp pain behind the leg, muscle contraction, swelling and bruises occurs, muscle tear also occurs in extreme cases.
A player sustains hamstring injury when making quick and sharp movements, like running, jumping and stretching, his or her leg to reach the ball, this causes the muscle fibres that makes up the hamstring, to over expand, thereby leading to internal bleeding, swelling and bruising.

How A Player Can Prevent Hamstring injury Occurring
For a player to avoid having hamstring injury, the player should adopt, routine warm up exercises, such as kicks, squats and lunges, during football training sessions and just few hours before commencing any football match.