POPSUGAR discusses what people should know about building muscle with High-intensity Interval Training (HIIT) workouts according to HSS exercise physiologist Aaron Karp, MS, ATC, CSCS.

In order to build muscle, HIIT workouts should focus on strength training as opposed to just doing calisthenics, such as jumping jacks, explained Karp. Moves such as squats, lunges, and single-leg deadlifts "in a series will lead to muscle fatigue, and at the end of the day, that's how you build muscle," he said.

The most important variable to focus on is the work-to-rest ratio, cited Karp. "As far as specific numbers, it's really what you're able to do.” If people are unconditioned or new to training, Karp suggested to begin with a 1:2 or 1:1 work-to-rest ratio. For example, performing each exercise for 20 seconds, followed 40 seconds of rest, or performing each exercise for 20 seconds, followed by 20 seconds of rest. In addition to focusing on your work-to-rest ratio, Karp said it's imperative to have compound exercises, which are multijoint movements that work large groups of muscle at once, in your routine as opposed to isolated movements such as bicep curls. "You want to primarily be doing compound movements," he noted. If you want to keep isolated movements like curls in your workouts, he recommended programming them in a way that allows you to do them to slightly rest without taking a full break.

According to Karp, it will take a minimum 4-6 weeks to start seeing results. Someone who is unconditioned may experience neurological changes within the first two weeks, "meaning your body is not going to look any different, but the exercises feel better or you feel like you're more coordinated," he said. As far as physical changes (e.g., muscle size or the amount of body fat), it will take a minimum of 4-6 weeks and sometimes closer to 6-8 weeks, Karp cited. Though, how long it will take to see results varies from person to person based on variables such as genetics and fitness level.

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