Running shoes that are old or worn through can lead to injury, and as a runner, there is nothing more frustrating than being sidelined by a strain or a niggle - particularly when it could have been prevented. Modern running shoes and trainers contain hi-tec shock absorption and cushioning, alongside a structure designed to make sure your foot is supported properly. All the major manufacturers invest a great deal of money in research and development, often alongside a number of athletes who help feedback on the performance of the shoe and the technology within it. Whether it's Asics renowned GEL system or the most famous of all - Nike's AIR system, for cushioning or something such as the Adidas Torsion system for foot support, some of these concepts may seem like a "nice to have", but comfort and support is critical for anyone undertaking anything from moderate exercise and upwards.
You can't always rely on the visual condition of your trainers as a guide to how worn they are. A better guide to listen to your body - if you start to feel pain in your joints or additional muscle tiredness following your exercise routine, it could mean it's time for change. As a more general rule, you should look to change running shoes every 300 to 400 miles, but this varies from runner to runner depending on weight (heavier runners should change more frequently) and running surface (treadmill runners can change less frequently).
Consider this as well... your heel must absorb up to 10 times your own body weight when exercising or running, depending on intensity and type of activity.
Your body has it's own built-in shock absorption in the form of joints, cartilage and ligaments, to try to dissipate some of this impact. Repetitive impact leads to wear, tear and pain unless your properly equiped.
In comparison, the forefoot is designed to deal with much lesser impact forces as it serves a different function - to allow you to push-off when running or sprinting. During this push-off, stability is critical.
It's important that you choose a running shoe or trainer that meets the needs of your chosen activity or sport and consider the level of intensity of what you do.