Physiological and Metabolic Responses to Low-Volume Sprint Interval Exercises: Influence of Sprint Duration and Repetitions

Purpose: This study aimed to determine physiological and metabolic responses to two different sprint interval exercises (SIE) matched for total sprint duration and sprint-rest ratio. Methods: After having measured peak oxygen uptake (VO2peak), 14 healthy males (27.1 +/- 4.8 yr, 169.6 +/- 6.0 cm, 64.5 +/- 8.4 kg, VO2peak: 47.2 +/- 7.7 mL<middle dot>kg-1<middle dot>min-1) performed four 10-s sprints with 80-s recovery (SIE10) and two 20-s sprints with 160-s recovery (SIE20) on different occasions in a counterbalanced crossover manner. Pulmonary VO2 and changes in tissue oxygenation index (triangle TOI) at vastus lateralis (VL) and rectus femoris (RF) were measured during the SIE. Furthermore, T2-weighted magnetic resonance imaging was taken immediately before and after the SIE to determine the activation levels of VL, RF, vastus medialis, vastus intermedius, adductor magnus, biceps femoris long head, semitendinosus, and semimembranosus at 50% of right thigh length. Results: In SIE10, increases in VO(2 )and triangle TOI at VL and RF plateaued after the second sprint, whereas session-averaged triangle TOI was greater in SIE20 than SIE10 in both muscles (VL: 20.9 +/- 7.4 vs 14.2% +/- 5.9%, RF: 22.8 +/- 9.3 vs 12.9% +/- 6.6%, P = 0.00). Although both SIE significantly increased T2 values in all eight muscles, those magnitudes were similar between the conditions (SIE10 vs SIE20: 5%-16% vs 8%-16%). Conclusions: This study showed blunted responses of whole-body (VO2) and peripheral (triangle TOI) oxidative responses with successive sprints (sprint 1 < sprints 2-4) in SIE10, suggesting that increasing sprint repetitions does not necessarily induce greater oxidative metabolism or stimulus. Moreover, greater peripheral oxygen extraction (triangle TOI) was achieved with SIE20, whereas %changes of T2 indicates that the thigh muscles were similarly activated between the SIE conditions.