Running is a relatively cheap way to get your cardio in and get outside — and you don't need much equipment to get started. Sure, the right outfit might make you feel like a track star in the making, but going for a run in your old basketball shorts and a T-shirt probably isn't going to hurt.
However, you might be hurting yourself by running in an old pair of sneakers that you haven't worn in ages.
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"While wearing the wrong running shoes may not directly cause an injury, it can certainly be a contributing factor among other variables, such as a runner's individual mechanics and training regimen," says Jolan Browne, PT, DPT, senior physical therapist and orthopedic specialist at NYU Langone's Orthopedic Center.
One aspect of a runner's mechanics that some shoes are designed to control is the amount your feet pronate or roll inward. "Some pronation is required for normal shock absorption as you contact the ground, but too much or too little can put a runner at risk for injury," Browne says.
What Happens if You Run in Old Shoes
Research is limited and inconclusive when it comes to actually documenting injuries due to running in old sneakers.
A small October 2009 study in the British Journal of Sports Medicine found that runners altered their stance and body positioning slightly as they unknowingly adapted their form to accommodate worn-out cushioning in older sneakers, which has long been thought to lead to greater pressure on the feet (and potential injury).
"An older running shoe that has tons of miles logged on it won't be as responsive, cushioned or supportive as a new pair," says Greg Laraia, an athletic trainer and running consultant at Custom Physical Therapy in New York.
That lack of responsiveness, cushion and support is cause for concern: Overuse injuries, such as tendonitis, can occur when you train in an older pair of shoes if the lack of cushion doesn't provide enough support when your steps land. In some runners, that can even result in over-pronation, Browne explains, which "over time can make a runner more prone to injury at the knee, foot, ankle or hip."
But a small August 2011 study in BMC Research Notes found new running shoes can cause more pressure on the feet, too, potentially because they haven't been broken in yet, according to the researchers.
Still — whether out of an abundance of caution or due to convincing marketing from footwear companies — experts generally recommend replacing your running sneakers every 300 to 500 miles, or every four to six months.
"Unfortunately, most shoes have a shelf life: Not only is it the number of miles, but the amount of time you've owned them before the foam ages," Laraia says.
Browne uses an equation to tailor that recommendation even more: Divide 75,000 by your own body weight for the number of miles you should run before replacing your kicks. For a runner who is 155 pounds, for example, that would mean new shoes about every 484 miles.
Divide 75,000 by your body weight. That's how often, in miles, you should replace your running sneakers.
How to Tell if You Need New Shoes
Even if you haven't been diligent about tracking mileage on your shoes (don't worry if you haven't — many runners forget!), there are still visual cues that indicate when it's time to replace your sneakers.
"Check the bottoms: If the tread is gone, it's probably time," Laraia says. "If you feel like you are pounding against the ground with each step — but it didn't feel like that when you got your shoes — or if you wear supportive shoes and feel your arches are getting sore, it's probably time to replace them."
Foot pain and calf tightness are also signs that your feet are working harder and absorbing more of the impact of running, he adds.
What to Know Before Buying a New Pair
If all signs point to new sneakers, consider shopping at a local running store where a specialist can help you determine how much you pronate and pick a sneaker with enough support to propel you through your workouts while avoiding injury.
"A shoe that is too firm, such as a motion-control shoe, may potentially limit pronation too much, contributing to high-force injuries such as stress fractures and shin splints," Browne says.
"On the other hand, running in a shoe that does not have enough support, such as a minimalist shoe, could lead to an excessive amount of pronation, which is associated with injuries such as patellofemoral pain (or runner's knee), plantar fasciitis and iliotibial band syndrome," she adds.
Consider the terrain you'll be running on, too. "Shoes are all designed for a specific need: to make you faster on each surface," Laraia says. Trail shoes are typically bulkier, with more protection against uneven surfaces, and are often waterproof, for example.
Try to go shopping in the late afternoon or evening, when your feet are at their largest. Doing this will help you avoid purchasing a pair that's too small, which tends to result in blisters for many runners, Browne says.
"Most running shoe stores will allow you to try your shoes for a short period to determine if they're a comfortable fit," she adds. "Running shoes should be comfortable right away and should not need a 'breaking in' period."
Help elongate the life of your running sneakers by saving them for just that: running. "Running shoes are specifically designed to be most efficient with the forward motion of running," Browne says, "while cross-training shoes have features that also permit lateral movements, such as lifting weights or doing an exercise class."
So, How Bad Is It Really to Run in Old Shoes?
Even if you're not sure how long your enthusiasm for your new exercise kick will last, you're not doing yourself any favors by running in an old pair of shoes, even if it's for a treadmill run or jog around your block.
"For a runner just starting out or returning to running, their best bet is to start with a new pair of shoes, preferably from a store where they can have a trial period to test the comfort of the shoe," Browne says.
"Starting in a new pair allows you to know exactly how much mileage you've put in so you know when to replace them, and they'll be most supportive for your feet to minimize injury as you progress in your training," she says.
In other words: Investing in a new pair of shoes can only help, and it will increase your chances of making running a long-term habit.