The question of whether running on a treadmill is harder than running outside is a common debate. While the subjective feeling of difficulty can vary significantly between the two environments, objective physiological data reveals distinct differences in the energy required to maintain the same pace. The core of the comparison lies in the physical and mechanical factors that make an indoor run at a flat grade less taxing than an outdoor run at the same speed. Understanding these biomechanical and metabolic distinctions allows a runner to accurately match their effort regardless of the running surface.
How Treadmill Movement Differs From Running Outdoors
The mechanics of running on a moving belt present two main physical differences compared to running outdoors. The first is the complete absence of air resistance, which the body must continuously overcome when moving forward outside. Outdoor running requires energy to combat wind drag, an effect that becomes increasingly significant as speed increases. Since the runner remains stationary relative to the air on a treadmill, that energy expenditure is eliminated.
The second primary difference is the nature of the push-off and propulsion phase of the stride. On a treadmill, the moving belt assists in pulling the foot backward, meaning the runner does not need to generate as much forward momentum as they would on solid ground. Studies indicate that treadmill running results in decreased peak propulsive forces. This reduction in push-off effort contributes to a lower overall energy cost at the same speed.
Measuring Energy Expenditure
Physiological metrics like oxygen consumption (\(\text{VO}_2\)) and heart rate help settle the debate over difficulty. \(\text{VO}_2\) is a measure of the body’s energy expenditure. Studies have historically shown that at the same speed, a treadmill run at a zero percent incline requires less \(\text{VO}_2\) than outdoor running. This difference is largely attributed to the lack of wind resistance indoors.
The difference in energy cost translates to marginal differences in calorie expenditure, with outdoor running requiring approximately 3 to 7 percent more energy at moderate to fast paces. Interestingly, a large 2019 meta-analysis revealed that oxygen consumption can be equal between outdoor running and treadmill running at a zero percent incline, even at speeds as fast as a six-minute mile. However, the analysis noted that perceived exertion and heart rate follow a U-shaped curve. At slower paces, the treadmill feels easier, but at faster paces, it may elicit a higher heart rate and greater perceived effort compared to the same speed outdoors.
Adjusting Treadmill Settings for Comparable Effort
To replicate the energy demands of an outdoor run, a simple adjustment can be made to the treadmill’s settings. The widely accepted recommendation is to set the treadmill to a 1 percent incline. This small elevation compensates for the lack of air resistance and the mechanical assistance of the belt, simulating the energy cost of running on a level surface outdoors.
The 1 percent incline rule originated from a 1996 study, which found this grade produced the same oxygen consumption as overground running at paces faster than 7:10 per mile. For slower paces, the difference in energy expenditure is often negligible, and no incline adjustment may be necessary. However, at higher speeds, a 1 to 2 percent incline is required to offset the lack of wind resistance, which becomes more pronounced the faster a person runs.
To further simulate the outdoor experience, runners can use a fan aimed at their body to create a cooling airflow. This airflow helps combat the heat buildup that occurs in stationary indoor environments, which can otherwise increase perceived exertion. Maintaining a consistent pace indoors can sometimes create a different type of muscular fatigue than running on varied outdoor terrain. Incorporating incline changes throughout a run can help mimic the subtle variations of the natural environment and reduce the risk of overuse injury.