Recovery kinetics of oxygen uptake following severe-intensity exercise in runners

Background. This investigation sought to characterise the oxygen uptake ((V)over dotO(2)) off-transient kinetics from severe exercise and to clarify discrepancies between on- and off-transient kinetics for (V)over dotO(2) seen in humans. Methods. Eleven competitive endurance athletes underwent treadmill running until exhaustion at work-rates corresponding to the speed that elicited similar to95% of maximal <(V)over dot>O-2. Gas exchange variables were determined breath-by-breath. Computerised non-linear regression techniques were used to fit the <(V)over dot>O-2 on- and off-transient kinetics. A 3-exponential model described the <(V)over dot>O-2 on-transient. <(V)over dot>O-2 off-transient was analysed to each response time course using 3 different models: a single-exponential model for the entire period and 2 3-exponential models where exponential terms starting either together after a common time delay or after independent time delays. Results. Both 3-exponential models provided an excellent fit (r(2)>0.90) to the off-transient data. Compared with on-transient, <(V)over dot>O-2 off-transient kinetics was associated with a slower primary phase (time constant: 16+/-4 vs 34+/-13 sec, p<0.01) but was similar both in time delay and amplitude. Conclusions. These data indicate that there is no general symmetry between the exercise and recovery kinetics for <(V)over dot>O-2 because the response-of the primary phase of <(V)over dot>O-2 off-transient resolves to a greater time constant, reflecting altered tissue metabolism. However, the mechanism(s) for the slow component is slow both in developing and to recover within the severe exercise domain.