THE EFFECTS OF CIRCADIAN RHYTHMICITY OF SALIVARY CORTISOL AND TESTOSTERONE ON MAXIMAL ISOMETRIC FORCE, MAXIMAL DYNAMIC FORCE, AND POWER OUTPUT

Teo, W, McGuigan, MR, and Newton, MJ. The effects of circadian rhythmicity of salivary cortisol and testosterone on maximal isometric force, maximal dynamic force, and power output. J Strength Cond Res 25(6): 1538-1545, 2011-The study investigated the effects of circadian rhythm of cortisol (C) and testosterone (T) on maximal force production (F-peak) and power output (P-peak). Twenty male university students (mean age = 23.8 +/- 3.6 years, height = 177.5 +/- 6.4 cm, weight = 78.9 +/- 11.2 kg) performed 4 time-of-day testing sessions consisting of countermovement jumps (CMJs), squat jumps (SJ), isometric midthigh pulls (IMTPs), and a 1-repetition maximum (1RM) squat. Saliva samples were collected at 0800, 1200, 1600, and 2000 hours to assess T and C levels on each testing day. Session rate-of-perceived exertion (RPE) scores were collected after each session. The results showed that F-peak and P-peak presented a clear circadian rhythm in CMJ and IMTP but not in SJ. One repetition maximum squat did not display a clear circadian rhythm. Session RPE scores collected at 0800 and 2000 hours were significantly (p <= 0.05) higher than those obtained at 1200 and 1600 hours. Salivary T and C displayed a clear circadian rhythm with highest values at 0800 hours and lowest at 2000 hours; however, no significant correlation was found between T and C with F-peak and P-peak. A very strong correlation was found between T-aural with F-peak of CMJ and IMTP and P-peak of CMJ (r = 0.86, r = 0.84 and r = 0.8, p <= 0.001). The study showed the existence of a circadian rhythm in F-peak and P-peak in CMJ and IMTP. The evidence suggests that strength and power training or testing should be scheduled later during the day. The use of T-aural seemed to be a more effective indicator of physical performance than hormonal measures, and the use of session RPE should also be closely monitored because it may present a circadian rhythm.