Is the diurnal variation in muscle force output detected/detectable when multi-joint movements are analysed using the musclelab force–velocity encoder?

ABSTRACT

We have investigated the magnitude of diurnal variation in back squat and bench press performance using the MuscleLab force velocity transducer. Thirty resistance-trained males (mean ± SD: age 21.7 ± 1.4 years; body mass 80.5 ± 4.5 kg; height 1.79 ± 0.06 m) underwent two sessions at different times of day: morning (M, 07:30 h) and evening (E, 17:30 h). Each session included a period when rectal temperature (T_rec) was measured at rest, a 5-min standardized 150 W warm-up on a cycle ergometer, then defined programme of bench press (at 20, 40 and 60 kg) and back squat (at 30, 50 and 70 kg) exercises. A linear encoder was attached to an Olympic bar used for the exercises and average force (AF), peak velocity (PV) and time-to-peak velocity (tPV) were measured (MuscleLab software; MuscleLab Technology, Langesund, Norway) during the concentric phase of the movements. Values for T_rec at rest were higher in the evening compared to morning values (0.48°C, P < 0.0005). Daily variations were apparent for both bench press and back squat performance for AF (1.9 and 2.5%), PV (8.3 and 12.7%) and tPV (−16.6 and −9.8%; where a negative number indicates a decrease in the variable from morning to evening). There was a main effect for load where AF and tPV increased and PV decreased from the lightest load to the heaviest for both bench press and back squat (47.1 and 80.2%; 31.7 and 57.7%; −42.1 and −73.9%; P < 0.0005 where a negative number indicates a decrease in the variable with increasing load). An interaction was found only for tPV, such that the tPV occurs earlier in the evening than the morning at the highest loads (60 and 70 kg) for both bench press and back squat, respectively (mean difference of 0.32 and 0.62 s). In summary, diurnal variation in back squat and bench press was shown; and the tPV in complex multi-joint movements occurs earlier during the concentric phase of exercise when back squat or bench press is performed in the evening compared to the morning. This difference can be detected using a low cost, portable and widely available commercial instrument and enables translation of past laboratory/tightly controlled experimental research in to main-stream coaching practice.

KEYWORDS:

• Strength
• upper body
• lower body
• time of day
• rectal temperature

Acknowledgments

We would like to thank the participants who participated in the study. We would also like to acknowledge the intellectual input of Professor James M. Waterhouse who sadly passed away in October 2016.

Declaration of interest

The authors report no conflicts of interest, the linear encoder was bought from internal funds and there is no link between our laboratory and chronobiology group and the MuscleLab Company. The authors are responsible for the content and writing of this article.

Practical implications

The present study supports the established notion of an evening preference (or optimal time of day) for strength training, but due to the diurnal nature of the current investigation it is not possible to quantify how much of this is due to the endogenous component of circadian rhythmicity. That said, as the majority of athletes will live and train in typical diurnal conditions such that they sleep at night and wake and are active in the day time, the findings of this study support the idea strength training should be undertaken in the early to late evening (17:00 to 19:00 h). Specifically where maximal or near-maximal external loads are involved or a requirement for producing PV outputs (explosive power work, sprinting, or plyometric training, for example).

Participants could achieve maximal rates of velocity in less time during the heavier lifts in both lower and upper body exercise in the evening. Although the mechanisms regarding development of RFD remain largely unknown (Rodriguez-Rossell et al. 2017), this evening superiority in RFD provides an insight as to how greater peak velocities and power outputs in the evening are achieved. Research investigating the potential time-of-day specificity to training as previously attempted by other authors (such as Sedliak et al. 2008), but now employing linear encoders (measuring muscle force output, RFD and PV) and conducting upper and lower body tasks such as in this study are warranted.

Past laboratory-based and tightly controlled experimental research can now be translated into main-stream coaching practice. New and emerging intervention and methods that look to use either chronobiotics such as bright light or exercise, to directly shift the body clock or dawn simulators to improve mood (Thompson et al. 2014) or investigate the causal relationship between performance and T_rec or muscle temperature – to ultimately increase morning gross muscular performance and aid athletes that have to compete or undergo training in the morning could be tested in a real world context using MuscleLab (linear encoder).