Shorter Sprints, More Reps Lead to Better Feelings and Calories Burned

Accumulating evidence demonstrates that physical inactivity is linked to many chronic diseases. Despite this information, most people still fail to achieve adequate amounts of physical activity. Although the reason for this general lack of physical activity is complex, the most commonly cited barrier is lack of time. This prevailing response has resulted in a large interest in interval training programs that use higher intensity exercise to reduce the overall time requirement of each exercise session.

Sprint Interval Training: does duration matter?

One of the many forms of high-intensity interval training or HIIT is sprint interval training (SIT) and its classic form involving repeated 30-second bursts of “all-out” exercise separated by short 4-minute recovery periods. Although SIT provides an attractive time-efficient method to boost health and fitness, the strenuous nature of this exercise protocol has raised questions about its applicability to the general public. Interestingly, several studies have demonstrated that the physiological adaptations to SIT (e.g., aerobic fitness) are not compromised when the traditional 30-second sprint duration is reduced to 20, 15, and even 10 seconds. This finding suggests that the metabolic stimulus underlying the health and performance benefits to SIT is contained within the initial portion of an all-out sprint. We then posed the question: “Could shorter sprint durations also be more enjoyable and boost motivation of exercisers?”

In our two recently published companion papers in Applied Physiology, Nutrition, and Metabolism, we studied both the physiological and psychological responses to modified SIT protocols in an effort to determine if we could maintain the potent benefits of SIT while improving the perceptions of this intense but beneficial form of training. We recruited eight recreationally active male participants who completed three different SIT protocols with varying durations of sprinting (5, 15, and 30 seconds) and recovery (40, 120, and 240 seconds) and varying numbers of bouts (24, 8, and 4 bouts). Total exercise time (18 minutes) and sprinting time (2 minutes) were kept identical by changing the number of bouts, the sprinting duration, and the recovery period following each sprint (Table 1). We measured the number of calories expended by participants, both during the exercise session itself and during a 3-hour period after exercise, as well as fat metabolized post-exercise. We also surveyed participants throughout the exercise session to measure several psychological variables including affect (pleasure), enjoyment, confidence, and the participants’ intentions to perform the type of exercise in which they had participated.

Table 1. Classic SIT protocol and two modified protocols. Total session was 18 min.

In Part I of our study we found that shorter sprint durations (5-15 seconds) performed for a greater number of repetitions (8-24 bouts) actually resulted in a greater number of calories expended compared to the traditional 30-second SIT protocol, despite no difference in total sprinting time (2 minutes). In fact, the caloric expenditure with our modified SIT protocol involving twenty-four 5-second all-out sprints was comparable to ~20 min of a moderately paced jog (~210 kcal), despite 90% less exercise time. We also found that our modified SIT protocol involving eight 15-second all-out sprints resulted in similar energy expenditure and fat utilization over the 3-hour post-exercise period as the traditional SIT protocol.

Tell me how you really feel

In Part II of our study we found that the participants perceived the classic 30-second sprint duration as more unpleasant than the shorter 15- and 5-second duration sprints. Perhaps even more importantly, their affect (pleasure) had returned to near pre-exercise values 30 minutes after the two modified SIT protocols (5 and 15 second sprints), which was not the case with the classic SIT protocol. This rapid return to pre-exercise affect probably contributed to the greater intentions to perform modified protocols, greater perceived enjoyment of modified SIT, and greater confidence in their ability to perform modified SIT.Taken together, our results suggest that the classic 30-second SIT protocol can be modified by performing shorter sprints with reduced recovery periods for a greater number of repetitions without attenuating the boost to metabolism. Given that the modified SIT protocols improved the participants’ pleasure, enjoyment, and confidence in performing the activity as well as their intentions to perform this type of training in the future, modified protocols might enable increased uptake and adherence of this exercise regime by the general population.