The pain you're feeling along the bottoms of your feet can be the result of anything from improper footwear to poor foot mechanics, choice of pursuit and foot injury. Each time your foot strikes down -- whether on the treadmill or the pavement -- it is bearing your body weight, which can take a toll on the bones, muscles, ligaments and other tissues that make up the feet. To determine what is causing the pain, it is best to talk to a doctor.
Your choice of footwear could be causing you pain. The support and flexibility of a shoe can help ensure that your body weight is evenly distributed across your foot so one area isn't taking the brunt of the impact. Use walking shoes for walking and running shoes for running. Also, swap out your shoes with some regularity to ensure good support.
Everyone's feet are slightly different, not only in regard to size but also in arch and pronation -- which can greatly affect the distribution of body weight on the foot while walking or running. Like selecting the proper footwear for your chosen pursuit, choose the right shoes for your feet. Instead of just buying a pair of walking shoes to walk on a treadmill, make sure the arch, sole and toe box fit properly.
The incline set on the treadmill can also cause you some discomfort. When you walk or run at an incline, the foot must adjust accordingly, which can change the way your weight distributes across the soles. This change in weight distribution can irritate and subsequently strain the ligaments, tendons and other tissues within the foot. Try knocking the incline down to a more even setting.
If you're overweight or obese, walking or running on a treadmill may not be the best option for exercise. Remember that your foot is bearing the entirety of your weight. In fact, jogging can put a force on the feet equivalent to as much as three to four times your body weight, according to the American Academy of Podiatric Sports Medicine. This isn't to say you should stop exercising, but you may need to choose an activity that is lower in impact, such as biking or swimming.
Walking or running on a treadmill can just as easily lead to a foot injury, as walking or running on any other surface. Even if you're a healthy weight, the repetitive impact can eventually lead to an injury. Plantar fasciitis is one of the more common. It is a condition in which the band of tissue that runs along the bottoms of your feet becomes irritated and inflamed. The repetitive stress can also lead to stress fractures, which are small cracks that develop in the bone and lead to pain. Other conditions that affect the feet include bursitis, tendinitis, sesamoiditis and neuromas.