Do rhythms exist in elbow flexor torque, oral temperature and muscle thickness during normal waking hours?

The purpose of the current study was to examine the influence of “time” on isometric elbow flexion torque, body temperature and muscle size without interrupting the sleep wake cycle in college aged males. Two hours following the participants normal wake time, oral temperature was measured, followed by muscle thickness of the upper and lower body using ultrasound, as well as elbow flexor torque via a maximal voluntary contraction (MVC). Measurements were repeated every 2 h for 12 h (Time points 1-7). To examine the repeatability of the rhythm, participants returned and completed the same procedures as before within 14 days of their first circadian visit (Circadian visit 2). There was no time × day interaction for body temperature (p = 0.29), nor were there main effects for time (p = 0.15) or day (p = 0.74). For MVC, there was no time × day interaction (p = 0.93) or main effect for day (p = 0.50), however, there was a main effect for time (p = 0.01). MVC at time points 1 (86.4 ± 6.4 Nm) and 2 (87.1 ± 6.2 Nm) was greater than time points 4 (84.2 ± 6.6 Nm) and 6 (83.4 ± 6.8 Nm, p < 0.05). Additionally, time point 5 MVC was greater than time point 4. For upper body muscle thickness, there was no time × day interaction (p = 0.34), nor was there a main effect for day (p = 0.38), or time (p = 0.06). For lower body muscle thickness, there was no time × day interaction (p = 0.57), nor was there a main effect for day (p = 0.75), or time (p = 0.13). Cosinor analyses revealed no group level rhythms for oral temperature, muscle thickness or strength (p > 0.05), however, there were some individual rhythms noted for muscle thickness and strength. Results suggest that, when accounting for an individuals normal wake time, circadian rhythms of strength, temperature and muscle thickness are not apparent in most individuals.