Physiological responses at critical running speed during continuous and intermittent exhaustion tests

Summary Objectives The main aim of this study was to compare the time to exhaustion and physiological responses at critical speed during continuous and intermittent running in order to provide support to use critical speed during interval training sessions. Equipments and Method Nine male runners and triathletes completed an incremental treadmill test, two track performances of 800 m and 2400 m, in order to determine the critical speed, and finally two randomized tests until exhaustion, one continuous and other intermittent running (consisting of 4 min running and 1 min of passive recovery) performed at critical speed. Results The mean critical speed was 14.8 ± 2.0 km.h −1 . The continuous and intermittent times to exhaustion were 19.3 ± 6.4 min and 37.9 ± 14.6 min respectively. The blood lactate response during continuous running showed an increase in values of about 9.0 ± 0.8 mmol.l −1 at the end of the exercise. On the other hand, only one of the 9 subjects analyzed, showed an abrupt increase in blood lactate during the intermittent running. The heart rate, perceived exertion and blood lactate measured at the end of both exhaustion tests were not significantly different compared with the incremental treadmill test values. Conclusions Our findings support the use of critical speed when looking for a blood lactate steady state during endurance interval sessions using a 4:1 ratio.