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She is a runner, her body is acclimated to steady cardio. I don't think sprints are going to be a bad thing for her. She's doing this to change things up a little bit.
Scientific Studies Show HIIT Works
For years, we've been told low-intensity aerobic exercise is the best method for ridding the body of excess fat. I'm here to tell you that's not the case. You knew deep down, anyhow, that busting your butt burned off more fat than an exercise that allowed you to read at the same time, didn't you? Well, research shows our instincts were right.
The reason this low-effort theory of aerobic exercise came about is a study showed low-intensity exercise burns a greater percentage of calories from fat as opposed to carbs; however, when high-intensity exercise is practiced, the research showed the percentage of fuel from carbs is increased, while the amount of fat utilized is greater than or equal to that burned during low-intensity exercise.
In research, HIIT has been shown to burn adipose tissue more effectively than low-intensity exercise—up to 50% more efficiently! In other words, HIIT speeds up your metabolism and keeps it revved up for some time after your workout. The bottom line is HIIT training burns a greater number of total calories than low-intensity training, and more calories burned equals more fat lost. What I'm suggesting is you forget about the "calories burned" readout on the stairstepper or Lifecycle; if you practice HIIT training, the majority of calories burned will come after your workout!
Another reason low-intensity training became popular is the average, where's-my-remote-control American has no tolerance for high-intensity training. But, of course, if you're a weight trainer, high intensity is a part of life.
The short duration of the workouts associated with interval training prevent the body from entering the catabolic state that can happen with extended steady-rate cardio. This most often occurs when activities are prolonged and the body starts breaking down muscle tissue to use as fuel. Because it may also increase the production of many anabolic hormones, HIIT is the perfect method for losing fat while retaining muscle mass.
High intensity interval training has not only been shown to be superior to other forms of training, but it's also a way to escape the tedium of long, boring cardio sessions. Many of us do not have the time for such nonsense, while those that do often find themselves weary of the mindless task. HIIT provides a greater challenge, requiring greater resolve and concentration to complete.
REFERENCE: http://www.musclemedia.com/training/hiit.asp
