Purpose: This study aimed to determine physiological and metabolic responses to two different sprint interval exercises (SIE) matched for total sprint duration and sprint-to-rest ratio.
Methods: After having measured peak oxygen uptake (V̇O 2peak ), fourteen healthy males (27.1 ± 4.8 years, 169.6 ± 6.0 cm, 64.5 ± 8.4 kg, V̇O 2peak : 47.2 ± 7.7 ml·kg -1 ·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 V̇O 2 and changes in tissue oxygenation index (∆TOI) at vastus lateralis (VL) and rectus femoris (RF) were measured during the SIEs. Furthermore, T2-weighted MRI was taken immediately before and after the SIEs 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 V̇O 2 and ∆TOI at VL and RF plateaued after the second sprint, whereas session-averaged ∆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). While both SIEs 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 (V̇O 2 ) and peripheral (∆TOI) oxidative responses with successive sprints (sprint 1 < sprint 2-4) in SIE10, suggesting that increasing sprint repetitions does not necessarily induce greater oxidative metabolism or stimulus. Moreover, greater peripheral oxygen extraction (∆TOI) was achieved with SIE20, whereas %changes of T2 indicates that the thigh muscles were similarly activated between the SIE conditions.
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