Kinetics of Phosphocreatine and Deoxyhemoglobin in Children and Adults During High-Intensity Exercise

Abstract

PURPOSE: To compare the kinetic response of PCr and deoxyhemoglobin (HHb) during high-intensity exercise in boys and men. METHODS: Seven men (25 ± 5 y) and six boys (13 ± 1 y) exercised using the right leg on a quadriceps ergometer within a 1.5 T MR scanner. Following habituation sessions each participant completed an incremental test to exhaustion. After a minimum 48 hours recovery, participants completed two to four constant work rate bouts on separate days. Exercise bouts consisted of 2 min rest and 7 min exercise, at an intensity equivalent to 20% of the difference between the workload at the intracellular Pi/PCr threshold and the maximal workload. Changes in HHb were determined every 1 s using a NIRS probe which was secured over the vastus lateralis muscle. 31P spectra were collected every 6 s using a 6 cm surface coil positioned beneath the right quadriceps muscle. The breakdown of PCr at the onset of exercise was modelled using a single-exponential function until the onset of the PCr slow component (SC). This point was identified using an iterative fitting window - where the time constant diverged from a plateau, a PCr SC was deemed to emerge. HHb profiles for participants were modelled as above, although a delay term was included in the model to account for the transient decrease in HHb at the onset of exercise. Data are reported as mean ± standard deviation and 95 % confidence intervals (CI) are presented. PCr and HHb data from two boys and HHb data from one adult were excluded from the analysis. RESULTS: The fundamental time constant (boys: 31 ± 11 s, CI 5 s; men: 45 ± 19 s, CI 6 s; P = 0.23), fundamental amplitude (boys: 43 ± 10%; men: 36 ± 8%, P = 0.24), and the SC amplitude (boys: 6 ± 1%; men: 12 ± 12%, P = 0.33) for PCr were similar in both groups. The net change in PCr was similar (men: 52 ± 16%; boys: 51 ± 9%, P = 0.93), as was the time constant for HHb increase in boys and men (19 ± 2 s and 22 ± 2 s, P = 0.55) respectively, with a delay of 5 ± 2 s. Responses over the last three minutes of exercise for HHB showed considerable inter-individual variation. CONCLUSIONS: Results from this study showed no differences in the breakdown of PCr or in the balance between the delivery and utilisation of O2 at the onset of highintensity exercise between boys and men. However, a trend of an age-related slowing of the breakdown of PCr at the onset of exercise warrants further investigation.