In an interview with Runner's World, Ben Yamuder, MS, CSCS, USAW-1, exercise physiologist at HSS, shares how the movement (or lack thereof) you do outside of your training sessions affects athletic performance.

First off, long periods of being sedentary can cause an overall increase in muscle tightness, Yamuder said. Specifically, your hip flexors—the muscles at the front of your quads that help bring your knees up toward your chest—shorten up. This, in turn, leads to less hip extension, Yamuder explained.

Hip extension is what allows your legs to go behind your body, and in running, it’s crucial to the push-off phase of your stride. “If your hip flexors are super tight, you can’t get full hip extension, which means you can’t push off fully,” Yamuder explained. This can lead to an increase in stride frequency (how many steps you take per minute), which can cause people to overstride. And that, in turn, can up your risk of issues like shin splints as well as pain and instability in the hip and knee joints, Yamuder said.

Too much sitting can also tighten up your ankles and soleus, a deep calf muscle. And when these areas get tight, your knees can end up absorbing more force from running, which can overload that joint and also decrease your running economy, Yamuder said. Tight ankles can also cause the Achilles tendon to repetitively and forcefully stretch when you run, leading to microtears and inflammation in the tendon, which can cause discomfort and pain, he added.

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