"If you haven't been exercising for a while, you need to start slow and develop a fitness base," Marcas Bamman, a professor in the University of Alabama at Birmingham and director of the Center for Exercise Medicine, said in a statement. "Doing too much too fast is a recipe for injury."
Bamman said most people begin to see positive benefits from exercise within about a month, and many early benefits are psychological: People report they feel more alert and have more energy.
"People engaged in strength training don't typically see a change in muscle mass over the first six weeks of an exercise program," Bamman said. "The effect in those first weeks is to help the nervous system better activate existing muscles so they work more efficiently and completely."
Following a slow and low start, a mix of aerobic exercise and strength training is ideal, aerobic exercise -- activities like running, cycling or swimming -- three-to-four days a week, between 20 and 40 minutes per day, Bamman said.
Intensity can also help determine the proper duration of a workout.
"If your exercise intensity is high, the 20-minute timeframe is right for you," Bamman said. "If your intensity is low or moderate, you'll need to exercise for the full 40 minutes."