Its given that stretching makes you flexible. Further to this, we're told that stretching reduces the risk of injury. This relationship isn't clear cut. This is due to the difficult nature to isolate the relationship between stretching and injury when conduction research. There are a number of factors that related to an increased risk of injury. Fitness, strength and any previous injuries are important considerations.
Much research that investigates this relationship between stretching and injuries have followed groups of athletes or military recruits. Using military recruits have their advantages - homogeneous in age, exercises and nutrition. When they are divided into a group that stretches and a group that doesn't stretch its a comparison of apples and apples. However, comparing a young male in the military as a recruit to an over 60's female personal training client when it comes to prescribing stretches is a stretch unto itself.
The Golden Level of Flexibility
This study that followed 900 recruits over two years showed fewer back injuries with an a stretching program. Whilst compared with this study showing that the most flexible and the least flexible males recruits had increased injury risks. This shows that recruits with less flexibility tended to suffer from overuse injuries, whilst their more flexible counterparts were more likely to suffer from acute injuries. This suggests that there is a minimum requirement for a certain amount of flexibility for the tasks given - but too much or not enough flexibility may lead to injury.
Improving Predictors of Injury
An Australian military study consisting of over 1500 recruits who were undergoing basic training, compared two groups - these who trained with a warm; and those training with a warm up and six lower body stretches held for 20 seconds at a time. There were no different rates of injuries between the 12 weeks of training. One of the measured fitness tests - the 20-meter shuttle run, was a better predictor of injury, leading to the suggestion that fitness training rather than flexibility training may be a better investment of your time if you're looking to reduce your risk of injury.
Other factors, from this study, suggested that factors other than a good flexibility may be more important for reducing injury. This study saw over 3500 different injuries in over 26000 subjects. The bottom-line is that stretching was found to have no effect on these injuries, but, proprioceptive training and strength training halved the risk of injury.
This leads to the appearance that once you have a reasonable level of flexibility a focus on strength and personal awareness may be more protective against injury than stretching.
Increased Injury Risks
Some studies have also reported increased injury rates with stretching. The theory from these studies postulates that sustained stretching of 90 seconds may compromise muscle function for up to 2 hours afterwards. But, not too many people will hold a stretch for 90 seconds. Whilst other research in articles around stretching and performance showed either a detrimental effect on performance or no effect at all.
From this, long and sustained stretches prior to activity probably aren't useful for increasing performance or possibly detrimental for optimal performance. A better solution for static stretching would be well before you start any exercising or directly after. The best solution would be dynamic stretching prior to exercise. Research articles show that dynamic stretching is better and much safer for your fitness journey.
To Stretch or Not to Stretch?
While there isn't much evidence that stretching reduces the risk of injury, I wouldn't tell someone not to stretch after their training. When an adequate range of motion for activity is achieved it would be best to invest in fitness, strength and proprioception work to have a less injury prone fitness journey.