Does raising morning rectal temperature to evening levels offset the diurnal variation in muscle force production? Pullinger, S.A., Robinson, W., Robertson, C., Waterhouse, J.M. & Edwards, B.J. Research Institute of Sport & Exercise Sciences, Liverpool John Moores University. INTRODUCTION RESULTS Muscle force production and power output in active males, independent of the group of muscle (grip, leg, back, etc.) or speed of contraction has reported to be higher in the evening than the morning (Reilly & Waterhouse, 2009). * * WARM-UP (a) (b) Figure 4. Mean (+SD) of rectal temperature pre warm-up (a) and post warm-up (b). *Denotes Trec significantly higher (P < 0.05) in the afternoon (17:30 h) than both morning sessions (07:30 h). * * Denotes Trec significantly higher (P < 0.05) in the afternoon (17:30 h) than the morning (07:30 h), but no different than the extended active warm-up session in the morning (07:30 h). Figure 1a. The circadian rhythm in left grip strength. Figure 1b. The circadian rhythm in leg strength. Diurnal variation is attributed to motivational, peripheral and central factors and a higher core temperature present in the evening (Edwards et al., 2008; Hayes et al., 2010). * Figure 5. Mean (+SD) of intramuscular temperature post warm-up. * * Denotes Trec significantly higher (P < 0.05) in the afternoon (17:30 h) than the morning (07:30 h), but no different than the extended active warm-up session in the morning (07:30 h). Figure 2. The circadian rhythm of rectal temperature (Trec). Therefore, the purpose of this study was to investigate whether increasing morning rectal temperature to evening levels, by active warm-up, leads to an increase in muscle force production or power output to evening levels (using a muscle lab velocity transducer). Secondly, the study also looked at establishing whether the muscle lab force velocity transducer displays diurnal variation. Table 2. Mean (± SD) values for all the strength variables displaying circadian variation measured under the three experimental conditions. † Denotes values to be significantly higher (P < 0.05) in the evening (17:30 h) than the morning (07:30 h)., irrespective of whether a warm-up was performed. * Denotes morning warm-up values to be no different to morning (07:30 h) values. METHODOLOGY Ten healthy active males were chosen to take part in this study. Two preliminary familiarization sessions took place to ensure participants were fully accustomed to the full testing procedures to minimize the learning effects. The participants then completed three test conditions, each separated by a minimum of 48 h and administered in a counterbalanced order. Table 1. Anthropometric data of subjects. Characteristic Mean SD Age (yrs) 21 1.3 Height (cm) 176.5 5.9 Mass (kg) 82.8 15.7 DISCUSSION Two major findings emerged from the present study: Diurnal variation was only present in a number of strength measures recorded through the muscle lab velocity transducer. An increase in Trec in the morning to evening levels following an active warm-up did not display alteration of the normal diurnal variation present in muscle strength. In conclusion, the present study confirms, that there was no evidence that increasing Trec significantly increased muscle strength, thus suggesting muscle strength is not completely mediated by core temperature variation, but possibly by a number of different factors. REFERENCES Edwards B, Waterhouse J, Reilly T. (2008). Chronobiology Int, 25, 115-132. Hayes L, Bickerstaff GF, Baker JS. (2010).. Chronobiology Int, 27, 675-705. Reilly T, Waterhouse J. (2009). European J App Phys, 106, 321-332. Figure 3. Schematic overview for the three test conditions. *Blue filled bars indicate time-points for intramuscular temperature.