There are a million different answers to “when should you replace your running shoes,” which is why the question comes up so frequently.

The reason that there are a million different answers to this question is because there are a million different things that contribute to and impact the life of a running shoe. The following are some of the different factors that influence shoe life, but realize that they all are interrelated and there isn’t one factor alone that will determine your need for new shoes.

• Different road surfaces – running on rough terrain like a trail can put more stress on the structure of your shoe than running on smooth surfaces like a treadmill or even pavement. By examining the bottom or the edges of your shoes, you can see if the treads are breaking down (think tire treads). Looks can be deceiving though–if you primarily run on smooth surfaces your treads may look fine yet your shoes could still need replacing.

• Mileage –  Runner’s World  recommends every 300-500 miles, but there is no exact number because it depends on all of these other factors as well. Every person is different and because of differences in gait, terrain, shoe type, etc., one person may need to replace their shoes at 300 miles when you may need to do it earlier or later. Apps like   Nike+ running  can help you keep track of how many miles you’ve put on each pair.

• Shoe type – there are a variety of shoes to choose from. They run the gamut from minimalist shoes, which provide little cushion and more contact with the ground, to extra stability which keep the foot in generally the same position and provide a good deal of cushion. Shoes with less of a formal structure are more susceptible to breaking down, but that is also dependent on our next factor.

• Gait type – people tend to have one of three types of running gaits: underpronation, neutral, or overpronation. Normal, or neutral pronators, strike the ground with the outside of their heel and roll inward slightly to make full contact with the ground. Underpronators still strike the ground with their heel, but the foot does not roll inward sufficiently and most of the force is kept on the outside of the foot. Overpronators strike with the heel and then roll inward more than the normal amount. There are shoes designed for each of these gait styles and the way that you run in general plays a big part in how quickly your shoes break down.

• Midsole – the midsole of a shoe is the part between the insole and outsole and is primarily responsible for shock absorption. Since this layer is unable to be seen, special attention needs to be paid to the feel of the shoe as you are running. If you no longer feel bounce or cushioning, the midsole may have become compressed. Your body will be the best indicator of a worn midsole as you begin to experience pain or discomfort.

Ultimately, when you replace your running shoes is up to you. Pay close attention to any cues that your body may be giving you that it’s time to u pgrade. If you start to notice pain in your shins, knees, or hips, ask yourself when the last time was that you replaced your shoes. Most athletic shoe stores offer free gait analysis and shoe consultations, so make sure you take advantage of those services to find the best shoe for you!