You may not want to be as fleet of foot as the world-class sprinters who competed in the Summer Olympics, but you wouldn’t mind it if your fat-loss plan results in a lean body like theirs, right? Would sprinting as part of your overall training regimen give you the desired results?
To find out, editors in the Oxygen office tapped the expertise of exercise physiologist John Babraj, Ph.D., of Abertay University in Dundee, Scotland. Here, he provides five considerations for sprint-centric cardio workouts.
#1. Looking to use sprints to lose body fat? Babraj suggests a 50-meter sprint, ideally on an uphill grade of about 10 percent. “This should take about 15 seconds to run,” he says. “It’s important that you do sprints on a suitable gradient to ensure you get a high enough intensity.”
#2. To be efficient and remain injury-free, proper mechanics matter. Many factors go into a perfect running stride, but according to Babraj, two of the most common errors take place at the heel and hip. “One is the heel striking first rather than the forefoot or midfoot,” he says. “Another is poor hip extension — your hip needs to propel the leg backward after contact with the ground. Poor hip extension will limit the stride length and prevent you from running at your true optimal pace.”
#3. How long you sprint is not as important as you’d think. “There does not appear to be any difference in physiological adaptation regardless of the length of the sprints,” Babraj reports. “We have studied 15-second and 30-second sprint durations and saw no difference in the adaptation [when using] the same work-to-rest ratio.” However, psychologically speaking, 30 seconds may be better. Theoretically, Babraj says, “It allows you to acclimatize to working under very low fuel conditions.”
#4. For short-duration power, incorporate longer rest periods. “The optimal work-to-rest ratio [time spent sprinting versus resting] depends on the adaptation you are trying to get,” Babraj explains. “A 1:12 ratio will help you develop better power adaptations.” That would equate to 30-second sprints with about six minutes of rest in between. “If you’re looking to build both endurance and power, then either a 1:6 or 1:8 ratio would work best,” he adds.
#5. To enhance your endurance and maximum oxygen consumption (for more calories burned), rest less between sprints. “Something like a 1:3 work-to-rest ratio produces greater endurance adaptations than longer recovery does,” Babraj states. For instance, after a 15-second sprint, rest 45 seconds and repeat six to eight times, depending on your current fitness level.
To Eat or Not to Eat, That Is the Question
Sprinting obviously helps burn body fat, but can sprinting on an empty stomach burn even more?
It seems logical that fasted cardio would work for fat loss: When you fast (as you do when sleeping), your circulating blood sugar and glycogen stores fall and insulin levels drop, which should theoretically help the fat-burning process because insulin suppresses fat metabolism. Therefore, doing cardio on an empty stomach means your body needs another source of fuel. Unfortunately, muscle tissue is more readily broken down than fat in the absence of glycogen, which means fasted cardio could potentially be catabolic. In fact, one study showed that protein breakdown could actually double during fasted cardio.
Yet, hard research on this subject is equivocal. On the pro side are studies like one showing that participants burned 20 percent more fat by fasting before cardio than by eating beforehand. On the con side are studies like one in which participants who ate showed an increase in oxygen consumption and fat usage during the workout and in the 12 to 24 hours afterward versus those who fasted.
A better way of looking at cardio and fat loss is to consider your training week as a whole. Fat loss really comes down to calories in versus calories out and, of course, sound nutritional decisions that fuel your body, build your muscles and stoke your metabolism. Activities with a high metabolic deficit produce an insane afterburn that can last 48 to 72 hours and burn twice the fat of a steady-state workout in half the time. Do two to four high-intensity workouts per week in conjunction with a solid strength-training program and you’re on the road out of Fat City.
So why not do high-intensity cardio in a fasted state to burn more fat? Because your body will break down lots of fat during the session but not fast enough to use all those free-ranging fatty acids, so they simply get repackaged into your fat cells. As for the afterburn: For high-intensity intervals to be effective, they should be done all-out. Training on an empty stomach may mean reduced intensity.
Where does that leave you? With a matter of preference. Some people prefer to work out on an empty stomach, while others feel like they’re going to bonk without a little breakfast. And while fasted cardio is not inherently bad for you, it probably isn’t your best option for losing body fat.
—By Lara McGlashan, MFA, CPT