Marios Hadjicharalambous, Liam P Kilduff and Yannis P Pitsiladis
Department of Life & Health Sciences, University of Nicosia, Nicosia, Cyprus, Integrative and Systems Biology, Faculty of Biomedical and Life Sciences (IBLS), University of Glasgow, Glasgow, UK and School of Human Sciences, University of Wales, Swansea, UK
Background: The present experiment examined the responses of peripheral modulators and indices of brain serotonin (5-HT) and dopamine (DA) function and their association with perception of effort during prolonged exercise in the heat after creatine (Cr) supplementation.
Methods: Twenty one endurance-trained males performed, in a double-blind fashion, two constant-load exercise tests to exhaustion at 63 ± 5% O2 max in the heat (ambient temperature: 30.3 ± 0.5 °C, relative humidity: 70 ± 2%) before and after 7 days of Cr (20 g·d-1 Cr + 140 g·d-1 glucose polymer) or placebo (Plc) (160 g·d-1 glucose polymer) supplementation.
Results: 3-way interaction has shown that Cr supplementation reduced rectal temperature, heart rate, ratings of perceived leg fatigue (P < 0.05), plasma free-tryptophan (Trp) (P < 0.01) and free-Trp:tyrosine ratio (P < 0.01) but did not influence the ratio of free-Trp:large neutral amino acids or contribute in improving endurance performance (Plc group, n = 10: 50.4 ± 8.4 min vs. 51.2 ± 8.0 min, P > 0.05; Cr group, n = 11: 47.0 ± 4.7 min vs. 49.7 ± 7.5 min, P > 0.05). However, after dividing the participants into “responders” and “non-responders” to Cr, based on their intramuscular Cr uptake, performance was higher in the “responders” relative to “non-responders” group (51.7 ± 7.4 min vs.47.3 ± 4.9 min, p < 0.05).
Conclusion: although Cr influenced key modulators of brain 5-HT and DA function and reduced
various thermophysiological parameters which all may have contributed to the reduced effort
perception during exercise in the heat, performance was improved only in the “responders” to Cr
supplementation. The present results may also suggest the demanding of the pre-experimental
identification of the participants into “responders” and “non-responders” to Cr supplementation
before performing the main experimentation. Otherwise, the possibility of the type II error may be
It is well established that perception of effort is elevated and exercise performance is markedly impaired in hot environments but the precise mechanism(s) of fatigue have yet to be determined. The previous observation of the maintenance of carbohydrate reserves at exhaustion in conjunction with no impairment in the capacity of skeletal muscle to generate force during exercise with hyperthermia may preclude peripheral factors as the main cause of fatigue during exercise in the heat. Previously, Nielsen and colleagues proposed a core temperature increase to approximately 39.6°C as a critical factor in the reduction of central neural motivation and exercise performance. In subsequent studies, it was proposed that fatigue during exercise in the heat may occur at a critical brain temperature, through a reduction in cerebral blood flow and brain glucose levels. However, the exact mechanism(s) for these hyperthermia-induced effects and/or how thermal stress may affect brain neurotransmission during exercise in the heat are presently unknown.