The researchers at Kent State University assessed how common smartphone use - texting and talking - interfere with treadmill exercise.
They found that when individuals use their smartphones during exercise for texting or talking, it causes a reduction in exercise intensity.
"Exercising at a lower intensity has been found to reduce the health benefits of exercise and fitness improvements over time," said researcher Jacob Barkley.
The results of the study and the widespread use of smartphones during exercise help explain the results of a previous study conducted at Kent State by the same researchers, which found a negative relationship between smartphone use and cardiorespiratory fitness.
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The researchers assessed the effects of common smartphone functions, such as music, talking and texting, and a control condition where the students had no access to their smartphones.
During each session, average treadmill speed, heart rate and enjoyment were all assessed. The study demonstrated that relative to the no smartphone condition, the three smartphone functions - music, talking and texting - differentially affect exercise behaviour.
Using the phone exclusively for listening to music increased the average treadmill speed, heart rate and enjoyment of a bout of exercise.
"It appears as if listening to music and, to a lesser extent, talking may have benefits on the duration and/or frequency of exercise due to their ability to increase enjoyment," researcher Andrew Lepp said.
"However, if an individual's opportunity for exercise is constrained by time, then it appears best to avoid talking on a smartphone during planned exercise," Lepp said.
"Most people indicate that their lack of exercise is due to time constraints," Barkley said.
"When this is the case, this study suggests that a smartphone should not be used for talking or texting as both may potentially reduce fitness.