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Acute short-term dim light exposure can lower muscle strength endurance

Background: Since it has been shown that spending 18 h under dim light conditions can result in reduced handgrip endurance, it was questioned whether or not a shorter exposure to dim light (i.e., 1 h) would have similar influence upon muscular endurance. Therefore this study compared the number of weighted knee extension lifts that could be done after spending 1 h in either dim or bright light.

Methods: Participants (5 women, 11 men, college students 19–26 years) performed knee extension lifts to exhaustion with a load approximating 40% of their body weight. The lifts to exhaustion were measured immediately following 1 h of exposure to each of the following three conditions: dark (DL), room light (RL) and room light plus 5 mg melatonin (RLM). A minimum of 48 h separated each condition, and all participants started the exposures in a rested fed condition.

Results: Average (±SD) number of knee extension lifts for RL (62.0 ± 22.0) was significantly (p < 0.05) greater than DL (51.4 ± 14.7) and RLM (57.8 ± 22.9). The number of RLM knee extension lifts was not significantly different from DL. Exposure to 1 h of dim light immediately prior to activity can result in a reduction in thigh muscle endurance. The decline in performance to short-term dim light exposure was similar to that found following longer-term exposure.

Conclusion: It appears that light intensity can influence muscle endurance, however, at this time this effect cannot be directly related to endogenous melatonin production.

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Alex’s Notes: Only recently has there been an exponential increase in the acknowledgement of the influence that our circadian rhythm has on us. Even so, our understanding of it is lacking. To help further the data-base of knowledge on our biological clocks, the current study aimed to investigate how muscular endurance in the legs would be affected by light exposure and melatonin.

Eleven male and five female college students went into a lab on three separate occasions (separated by at least 48 hours) to perform knee extensions to failure with 40% of their bodyweight. All the lifts were performed with a 2-second cadence and under normal lighting conditions. However, each of the three conditions exposed the participants to different stimuli for one hour immediately beforehand.

  • The dark condition (DL) was an hour of quiet sitting in the dark (<50 lux)
  • The normal light condition (RL) was an hour of sitting in normal room lighting
  • The melatonin condition (RLM) was an hour of sitting in normal room lighting with consumption of 5mg of melatonin

An hour was chosen as the time for the conditions because that is how long it takes melatonin to peak after ingestion, and has also been shown to be the time after which it becomes increasingly difficult to stay awake under dark conditions (probably from endogenous melatonin). Thus, during the endurance session that immediately follows, serum melatonin levels would be greatest. Aside from all the above, the college students were asked to maintain the same daily sleeping, dietary, and exercise habits, as well as eat the exact same meal at the exact same time of day 1-2 hours before each visit. Finally, while sitting for an hour the participants were kept awake and occupied via conversation with each other or the researchers.

When the whole thing was said and done, RL averaged a significantly greater number of lifts until fatigue compared to DL and RLM. The researchers state the difference was 14% and 4%, respectively, but when I plug the average number of lifts into my calculator I get 20% and 11%, respectively. Either way, the RL clearly performed the best, and despite the RLM doing slightly better than the DL this difference was not significant. Heart rate (HR) was not different between conditions, but both mean arterial pressure (MAP) and blood glucose (BG) levels were; the former being lower in the RLM and the latter being higher in the RL.

So what caused the reduction in endurance?

This may very well be a mind over matter difference. Melatonin’s effects are most noticeable on mental performance, and studies have found that its effects to reduce auditory vigilance, reaction time, and self-reported vigor, as well as increase self-reported fatigue, confusion, & sleepiness. It stands to reason that melatonin (both exogenous and endogenous) simply dulled the drive to lift to failure. The non-significant difference in performance between RLM and DL certainly supports this notion.

Whatever the cause, it seems prudent to ensure you are awake before hitting the gym.

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