Reinforcing Value of Interval and Continuous Exercise Above and Below the Ventilatory Threshold

Abstract

PURPOSE: Activities that children engage in at high rates are reinforcing to them. Observational studies show that during play children engage in ≥15 sec intermittent bouts of activity, much like interval training. These bouts comprise 95% of total physical activity whereas bouts >30 sec account for only 5%. Children may not be motivated to adhere to exercise prescriptions that are based on extended periods of continuous activity. Engaging in brief bouts may be driven by a child's physiology, which is ideally suited for performing short bouts of activity, especially when the intensity is above the ventilatory threshold (VT). However, the reinforcing value of continuous versus interval exercise at intensities above and below VT has not been determined. METHODS: Boys (n = 7 age 10±1.29) performed a progressive exercise test on a cycle ergometer to determine VO2 peak and VT. Children returned to the laboratory and sampled interval and continuous exercise on a cycle ergometer at an intensity that was either 20% <VT on one day and 5% >VT on another day. Sampling and intensity orders were counterbalanced. The exercise protocols were matched for energy expenditure. For the continuous protocol children pedaled at a fixed workload for 5 min and for the interval protocol they performed 20 sec of work followed by 20 sec of active recovery for 5 min. After sampling the exercise protocols children played a computer game to assess the reinforcing value of interval or continuous exercise by having the children work to earn access to either interval or continuous exercise in 2-min blocks for a total of 10 minutes. The amount of time children chose to perform interval or continuous exercise above and below VT was compared. RESULTS: One-way ANOVA showed children worked for a greater amount of interval exercise at intensities >VT (8.29±1.38 min) than interval exercise <VT (4.86±2.55 min) (p<0.04). Children also worked for a greater amount of interval time (8.29±1.38 min) than continuous time (1.31±1.38 min) when exercising >VT (P=0.001). However, there was no preference between interval (4.86±2.55 min) or continuous (5.14±2.55 min) exercise <VT (P=0.89). CONCLUSIONS: Children chose to be active in interval rather than continuous exercise >VT. Additionally, >VT children chose a greater amount of interval than continuous exercise. Below VT there was no preference for interval or continuous exercise. However, children broke the 10 min <VT period into a 5 min period and a series of 20 sec intervals demonstrating that 10 min of continuous physical activity is not reinforcing even at low intensities. Supported by NIH grant RO1 HD042766 to JNR and a Gatorade Sports Science Institute grant to JEB.