TLDR;
In general, the daily period when a person will achieve their heighest maximum grip strength is between 6 to 8 p.m. However this does not mean you shouldn't measure your grip strength at other times, only that it's a variable you need to pay attention to.
Why is time of day important?
Studies have shown a ~5% change in grip strength output depending on the time of day1 so if you do want to make comparable grip strength measurements on a daily/weekly/monthly basis, it is important that you conduct the tests at roughly the same time using a similar daily routine. While the maximum recorded strength for a person is usually between 6 amd 8 p.m., this does not mean that it is the only valid time to measure grip strength.
What drives this daily fluctuation in grip strength?
It appears that it is due to the processes internal to the daily circadian rhythm of the muscle cells themselves.2 Studies have explored but have not found any direct correlation with core body temperatures or ability of neural motor recruitement to influence this daily fluctuation.
The idea of the circadian rhythm of muscle cells themselves being the main factor of daily strength variation has proposed mechanisms of action based on studies of the circadian rhythm of various human and animal tissues, however, it has not been studied directly and its prominance is based in part on the fact that the studies looking into the direct effect of core body temperature and ability of neural muscle recruitment were not able to explain the daily variation in strength.*
Are large population grip strength surveys corrected for the time of day?
No, or at least we have not seen it thus far. Surveys usually take place between 8 a.m. and 8 p.m., so some people’s grip strength is at their true daily max while for others it may be as much as ~5% less. If you’re using our grip calculator and you are measuring your true daily grip strength max (as mentioned, it's generally around 6 to 8 p.m.) then we would recommend reducing your strength measurement by 2.5% for a more realistic comparison with population averages.
References
- Jasper I, Haussler A, Baur B, Marquardt C, Hermsdorfer J. Circadian variations in the kinematics of handwriting and grip strength. Chronobiol Int. 2009;26(3):576-594. doi:10.1080/07420520902896590
- Time of Day and Muscle Strength: A Circadian Output? Collin M. Douglas, Stuart J. Hesketh, and Karyn A. Esser Physiology 2021 36:1, 44-51 doi:10.1152/physiol.00030.2020
* Daily variation in strength can be studied with several different tests, not only the hand grip strength tests but other maximum isometric strength tests of different muscle groups. For example, elbow extensor/flexors or the knee extensors/flexor muscle groups. There appears to be a similar time of day phenomenom across the different muscle groups which leads us to believe that depending on the testing protocol the relative measures from different muscle groups are somewhat comparable.
