The incline of a treadmill is the angle of the ramp that you walk on, this is another setting on the treadmill that can be adjusted to make it more or less difficult. A higher incline simulates walking up a hill, a lower incline is more similar to travelling across flat ground.
Increasing the incline to make the exercise more intense mitigates the problems of increasing the speed which can cause greater impact stress on the joints and possibly challenge the individual beyond their current capabilities.
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