It is definitely important to ensure you warm up before commencing exercise but there is no good evidence to suggest that ‘stretching your muscles’ makes much difference to your body’s capabilities and injury risk. A warm-up of 5-10 minutes of low-intensity cardiovascular exercise i.e. short jog/walk can be beneficial. Some studies have found that stretching before exercise can destabilize your muscles and may cause micro-tearing that can worsen with exercise, leading to further muscle damage and possible injury.
It is recommended that you stretch after exercise as part of a cool down to help the muscle return to its resting length. Stretching outside of exercise can also be useful to help maintain or increase flexibility which can lead to improvements in performance long term.